[Frontiers in Bioscience, Elite, 8, 143-169, January 1, 2016]

Functional amino acids in fish health and welfare

Synne M. Andersen1,2, Rune Waagbø1, Marit Espe1

1
National Institute of Nutrition and Seafood Research (NIFES), PO BOX 2029 Nordnes, 5817 Bergen,
Norway, 2Current address: EWOS AS, Tollbodalmenningen 1B, 5803 Bergen, Norway

TABLE OF CONTENTS

1. Abstract
2. Introduction
3. Amino acids
3.1. Arginine
3.2. Glutamine and glutamate
3.3. Tryptophan
3.4. Histidine
3.5. Sulfur amino acids
3.6. Branched chain amino acids
4. Metabolic perspectives
5. Summary
6. Acknowledgement
7. References
1. ABSTRACT

Protein is the most expensive part of fish diets generally higher in carnivorous than herbivore fish. In
and supplies amino acids (AA) for energy, growth, salmonid diets, protein makes up 35-55% of the diet,
protein synthesis and as substrates for key metabolic with highest inclusion levels at early life stages (2). Most
pathways. Functional AA is a term used to describe AA of the AA are protein bound but can also be supplied in
that are involved in cellular processes apart from protein the form of crystalline AA to fulfill the AA requirement,
synthesis. A deficiency, or imbalance, in functional as regulated by national legislation of feed additives,
AA may impair body metabolism and homeostasis. especially when using alternative protein sources (3).
Recent years have seen an increased interest in AA The National Research Council (NRC) published their
to increase disease resistance, immune response, latest recommendations for AA requirements in fish and
reproduction, behavior and more. This has led to a boost shrimp in 2011 and takes into consideration different
of commercially available functional fish feeds that aim requirement between species and developmental stages
to optimize fish performance and quality of the product. (Table 1). The NRC data are based on the literature
This review aim to collect recent findings of functional AA available at the time, on requirement studies performed
and of how they may improve fish health and welfare. It in these species. However, it has some main drawbacks;
will focus on functional properties of some of the most I - several of these studies were performed with diets with
studied AA, namely arginine, glutamine, glutamate, a high fishmeal inclusion, while fish diets today generally
tryptophan, sulfur amino acids (methionine, cysteine have a high plant protein inclusion. Plant protein diets
and taurine), histidine and branched chain amino acids. have been demonstrated to reduce feed intake, growth
Where information is not available in fish, we will point and protein utilization (4), and requirement of some AA
towards functions known in animals and humans, with appears to be higher when fish are fed a plant protein
possible translational functions to fish. based diet (5). This could be related to differences in
non-protein nitrogen compounds, as soy bean meal
2. INTRODUCTION for example is low in taurine, and a minimum fishmeal
inclusion of 5% is still required for Atlantic salmon
Dietary amino acids (AA) are crucial for fish as (Salmo salar) (6). II - The studies often use growth as the
energy substrates, for endogenous protein synthesis parameter to determine requirement, while overlooking
and to regulate metabolic pathways. More than half the metabolic need. The metabolic requirement often
of the AA consumed by fish may be deposited into surpasses the need for growth, especially during stressful
body protein, and the requirement of essential amino and challenging conditions (7). For instance, histidine
acids (EAA) corresponds to the AA tissue content (1). requirement to support optimal ocular health is higher
Protein is a significant component of fish diets, and is than the requirement for growth in Atlantic salmon (8).

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Table 1. NRC requirements of digestible EAA in studied AA and their effects on health, disease and
% of diet dry matter. Estimated using a factorial metabolism in fish (Table 2). The AA reviewed herein
includes arginine, glutamate, glutamine, tryptophan,
model for different weights of Atlantic salmon fed histidine, sulfur amino acids (SAA; methionine, cysteine
diets with 4.7.8 Mcal DE From (2). and taurine) and branched chained AA (BCAA; leucine,
Amino acid Weight class isoleucine and valine). Notably, other AA including
0.2.‑20 g 20‑500 g 500‑1500 g >1500 g glycine, lysine, threonine and aromatic AA are also
% diet DM involved in metabolic pathways but is not the topic for
this review. As salmonid species dominate northern
Arginine 1.7.9 1.8.2 1.7.0 1.4.6
European aquaculture, this review will focus on the
Histidine 0.8.0 0.8.0 0.7.5 0.6.4 information available in salmonid species, but will also
Leucine 2.3.1 2.3.1 2.1.4 1.8.2 include general knowledge from studies in other fish
species when relevant or when this is not available in
Isoleucine 1.3.2 1.3.2 1.2.2 1.0.4 salmonids. It will not discuss the requirements of each
Lysine 2.5.5 2.5.4 2.3.5 2.0.0 AA as this has been extensively discussed elsewhere (2).
Met+Cys (TSAA) 1.2.8 1.3.0 1.2.1 1.0.3
3. FUNCTIONAL AMINO ACIDS
Phe+Tyr (AAA) 2.7.1 2.6.8 2.4.6 2.0.9

Threonine 1.5.5 1.6.0 1.5.1 1.3.0 3.1. Arginine

Arginine is an EAA in fish, while in humans
Trypthophan 0.3.5 0.3.7 0.3.5 0.3.0 arginine is considered a conditionally EAA, as
Valine 1.7.5 1.7.9 1.6.7 1.4.4 glutamate, glutamine and proline can be converted to
citrulline via the intermediate pyrroline-5-carboxylate
TSAA: total sulfur amino acids, AAA: aromatic amino acids
(P5C) in the enterocytes (Figure 1). Citrulline is then
converted into arginine in the kidneys of healthy
III - The fish is not always pair fed, leading to differences in individuals and transported to the liver where it is
feed intake between dietary treatments. Thus, the effects metabolized trough the urea cycle and used for
observed may be due to palatability of the feed rather production of polyamines and creatine (11). Citrulline
than a growth stimulating/inhibiting effect. IV) Finally, the synthesis from glutamine have been indicated in
type of growth is not always reported. The main aim in channel catfish (12) but whether this is the case for all
aquaculture is as high muscle growth as possible, while fish species as well as the location of this production
visceral mass is a byproduct. Increased deposition of fat and how, where and at what rate this citrulline can be
around the viscera might have negative health effects on converted to arginine in fish still needs to be elucidated.
the fish, as is the case in humans (9). There have been indications of interconversion between
arginine and glutamate in channel catfish (Ictalurus
AA have traditionally been classified as essential puntatus) (13,14), where arginine supplementation
(EAA) or nonessential (NEAA) relating to whether the increased plasma levels of free citrulline, glutamate
organism can produce the AA endogenously from the and glutamine and supplementing glutamine reduced
dietary NEAA. Recently, the term functional AA (FAA) dietary arginine requirement. In Atlantic salmon
have received more attention relating to AA that modulate however, arginine reduced citrulline concentrations in
key metabolic pathways thus affecting immune response, plasma and muscle without affecting glutamine (15),
health, reproduction, cell signaling, animal welfare and indicating specie differences in arginine metabolism.
more (10). In addition, AA classified as NEAA such as Chicken and cats are unable to produce arginine
glutamine, glutamate and proline have been demonstrated endogenously as they lack the enzyme P5C synthase
to have functional properties in both fish and mammalian and this is also believed to be the reason for a lack of
metabolism, suggesting that fish have requirement also arginine synthesis in fish. Production of citrulline takes
for NEAA to obtain maximum performance. In the case for place in the mitochondria and requires carbamoyl
NEAA, both the dietary content of the AA and its substrates phosphate, which in fish is synthesized by carbamoyl
are of importance. Cysteine and tyrosine for instance, phosphate synthase III (CPSIII) requiring glutamine, not
can be synthesized endogenously from methionine and ammonia, as a substrate (16). No hepatic activity were
phenylalanine respectively, both of which are EAA and observed of CPSIII or ornithine carbamoyl transferase
need to be supplied through the diet. Thus, a deficiency (OCT) in rainbow trout further explaining the low de
of these AA may occur due to limited dietary supply of novo synthesis of arginine (17). CPSIII and OCT activity
the AA and/or its precursor that may again affect growth, is shown to be higher in early developmental stages of
metabolism and health. zebrafish (Danio reiro) (18), which has been linked to
a higher need for ammonia detoxification through the
Moreover, all AA have some degree of functional urea cycle in early life stages. Citrulline production from
properties, but this review will focus on some of the most ornithine may thus be limited in most adult fish, limiting

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Figure 1. Arginine metabolites and interactions with methionine, proline, glutamine and BCAA metabolism are as described in mammals. Amino acids
are highlighted in blue. Production of creatine is via multiple steps also consuming SAM and glycine. Notably, not all of these steps are well described
in fish and may take place in different tissues and compartments. NO – nitric oxide, NOS – NO synthase, OCT – ornithine carbamoyltransferase, ASS –
argininosuccinate synthase, ASL – argininosuccinate lyase, OAT – ornithine aminotransferase, P5C – pyrroline-5-carboxylate, P5CR – P5C reductase,
P5CS – P5C synthase, P5CD – P5C dehydrogenase, PO – proline oxidase, CP – carbamoyl phosphate, CPSIII – CP synthase III, ODC – ornithine
decarboxylase, MTA – 5’methylthioadenosine, MAT – methionine adenosyl transferase, SAM – S-adenosylmethionine, SAMdc – SAM decarboxylase,
dcSAM – decarboxylated SAM, SAH–S-adenosylhomocysteine, SMO – spermine oxidase, SSAT – spermidine/spermine acetyltransferase, APAO –
acetylated polyamine oxidase.

Table 2. Some functional properties of amino acids reviewed herein and their metabolites in fish
Amino acid Metabolite (if applicable) Function Specie Reference
Arginine NO Increased resistance to Edwardsiella ictaluri Channel catfish (28)
Increased disease resistance during external stress Senegalese sole (31)

Polyamines Spermine induce intestinal maturation Sea bass (48)

Trypthophan 5‑HT Reduced aggressive behavior Rainbow trout (72)

Melatonin Enhanced innate immune response in vivo Gilthead seabream (80)

Glutamine GABA Affecting secretion of pituitary hormones Rainbow trout (59)

Direct Improve intestinal structure and activate intestinal enzymes Red drum (46)
Improve immune response Channel catfish
Protect against oxidative damage (29, 65) (63)

Methionine Direct Stimulates protein synthesis Rainbow trout (143)

Taurine Reduce green liver syndrome Red sea bream (182)
Increase cell survival Atlantic salmon (158)
Improve digestibility and growth Rainbow trout (146)

Histidine Prevent cataracts Atlantic salmon (8)

BCAA Leucine Stimulate protein synthesis Indian major carp (170)

HMB Increase disease resistance Rainbow trout (179, 180)

NO: nitric oxide, 5‑HT: serotonin, GABA: gamma‑aminobutyric acid, BCAA: branched chain amino acids, HMB: beta‑hydroxy beta‑methylbutyrate acid

the potential of glutamine as a substrate for arginine. Arginine is involved in several cellular metabolic
Plasma urea is suggested as a better requirement pathways, including the urea cycle and synthesis of
parameter than growth in fish, as it increases once the creatine, nitric oxide (NO) and polyamines (Figure 1). The
requirement is met (19). enzyme arginase catalyzes the production of ornithine and

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Functional amino acids in fish nutrition

Figure 2. AA also affects metabolism by interacting with several different cellular signaling pathways, and the different AA may have synergistic and/or
opposing effects. The outcome of imbalanced diets affects multiple signaling pathways and depends on both concentration and ratios and location of
the AA.

urea from arginine. Two isoforms of arginase are known: in mammals, NO has also been linked to cardiac function
arginase-1 and arginase-2. In mammals, arginase-1 is in fish, as inhibiting iNOS in juvenile salmon leads to
known to be primarily expressed in the cytosol of hepatic initial increase in heart rate followed by a decreased
cells, while arginase-2 is expressed in the mitochondria heart rate (24). NO further has a role in neurological
of several tissues (20). Both isoforms are also expressed function and embryological development. Detection of
in Atlantic salmon, where arginase-1 was also found nNOS positive neurons in masu salmon (Oncorhynchus
in muscle tissue, though arginase-2 was the dominant masou), indicates that NO is a key modulator of
isoform in the muscle (15). Arginase activity varies somato-, viscerosensory, and visceromotor systems of
between fish species, but is generally highest in liver the medulla (25). In the gills of Atlantic salmon NOS is
and kidney, and was not detected at all in white muscle colocalized with Na+, K+ATPase, indicating a role of NO
of several species including Atlantic salmon (21). Three in ion transport (26). NOS require NADPH as a cofactor,
isoforms of the enzyme nitric oxide synthase (NOS), and increased concentrations of reduced NADPH was
which produces NO and citrulline from arginine is present observed in the gills during smoltification, suggesting a
in fish: inducible (iNOS), neuronal (nNOS) and endothelial role for NO in attenuation of increased Na+,K+ATPase
(eNOS) NOS (22). While production of NO from nNOS activity following seawater transfer. Through NO, arginine
and eNOS is relatively low and stable, NO production stimulates mitochondrial biogenesis and function, thus
from iNOS upon stimulation from endotoxins, cytokines regulating energy metabolism (27). NO stimulates
or nutrients can be significant. NOS has to compete for expression of peroxisome proliferator activator receptor
its substrate arginine with arginase. Increased activity of (PPAR)-gamma coactivator1-alpha (PGC-1-alpha), which
arginase or a deficiency of arginine availability will lead to further stimulates PPAR-alpha, increasing mitochondrial
uncoupling of NOS leading NOS to produce superoxide biogenesis and metabolism (Figure 2).
instead of NO. Superoxide further stimulates arginase
activity and inhibits NOS, creating cellular oxidative Effects of arginine on both the innate and
stress that has been linked to endothelial dysfunction adaptive immune response have been demonstrated
and related cardiovascular diseases in mammals (23). in fish, where arginine may act through NO to combat
NO is a potent vasodilator, and can increase blood pathogens, through polyamines, directly by affecting
flow to peripheral organs thus allowing for increased gene expression or by regulating nutrient availability for
uptake of oxygen and nutrients. In the endothelium, immune cells by endocrine control. Increased inclusion of
NO diffuses into neighboring smooth muscle cells and arginine in channel catfish diets has shown to correlate
activates guanylyl cyclase, increasing intracellular cGMP with survival when exposed to the bacteria Edwardsiella
concentrations, relaxing the muscle tissue. NO thus ictaluria (28). This was linked to increased NO production
allows for increased uptake and oxidation of nutrients. As in activated macrophages after increased plasma

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arginine (28). Further in vivo and in vitro experiments (ROS) as APAO and SMO releases hydrogen peroxide.
in channel catfish confirmed this positive effect on the However, as APAO is located in the peroxisomes and
immune system, as arginine supplementation improved SMO in the cytosol, SMO is expected to be the main
macrophage killing and phagocytosis abilities (29,30). producer of cytotoxic ROS (37). Arginine is expected to
Moreover, arginine increased hematocrit, hemoglobin, induce SSAT, thus causing limited oxidative stress. On
erythrocyte count and lysozyme activity, as well as the contrary, arginine supplementation has been shown
enhanced native T-cells and B-lymphocytes proliferation to induce antioxidant capacity in pigs (38) and increased
after mitogenic exposure (29,30). Costas et al (31) found lysozyme activity in Japanese flounder (Paralichthys
that dietary arginine supplementation to Senegalese olivaceus) (39). How and if arginine can protect against
sole (Solea senegalensis) increased the respiratory oxidative stress in fish, and at what doses, needs to be
burst after mitogenic exposure, and this correlated with further examined. A role for PA in the innate immune
increased NO production in headkidney leucocytes. In a system has been postulated, as in vitro supplementation
later paper, they found that plasma cortisol levels were of PA increased expression of immune associated genes
reduced in stressed turbot (Scophthalmus maximus) in gilthead seabream leucocytes and that putrescine,
after arginine supplementation (32). Both arginine and but not spermidine or spermine, increased phagocytic
lipopolysaccharide (LPS) exposure induce NO production ability (40).
from iNOS in headkidney macrophages (33,34). Substrate
availability of arginine may thus be of importance in order A lipid-reducing effect of arginine has been
to produce a sufficient immune response when required. observed in diet induced obese or diabetic mammals (27),
This highlights the importance of sufficient dietary supply where it reduced white fat mass while maintaining lean
of arginine during exposure to pathogens as plasma muscle mass. Generally, dietary arginine upregulated
arginine is known to decrease in fish during stress (7). expression of genes for lipolysis and decreased genes
Arginine increased abundance of phosphorylated for lipogenesis in the adipose tissue, while increasing
p38MAPK in Atlantic salmon headkidney and liver cells lipogenesis and protein synthesis in the muscle.
in vitro contrary to the effects of LPS, suggesting anti- This effect has been linked to activation of SSAT and
inflammatory effects of arginine (35). Notably, small increased turnover of PA (41). SSAT and MAT consumes
concentrations of NO can protect the cell from apoptosis acetyl-CoA and ATP, respectively, making PA turnover
and pathogens by activating heat shock proteins and an energy consuming process. Activation of SSAT will
inducing macrophage activity. However, overproduction deplete cellular concentrations of acetyl-CoA, further
of NO is toxic to the cell and can induce apoptosis through decreasing malonyl-CoA concentrations and thus
DNA damage or activating endoplasmic reticulum stress release the inhibitory effect of malonyl-CoA on carnitine
pathway (20). palmitoyl transferase-1 (CPT-1), the rate limiting enzyme
for transportation of long chained fatty acids (LCFA)
Polyamines (PA) are present in all eukaryotic into the mitochondria for ß-oxidation (41). Increased PA
cells and are essential for cell growth and differentiation. turnover and expression of CPT-1 was observed in the
Putrescine is produced from ornithine by ornithine liver of juvenile Atlantic salmon fed a high arginine diet,
decarboxylase (ODC), which is further synthesized into but not in the muscle or adipose tissue (42). An increased
the PA spermidine and spermine. As they are positively capacity of beta-oxidation in the liver could improve their
charged at physiological pH they can bind to negatively metabolic health and avoid fat accumulation in the liver.
charged RNA, DNA, proteins and phospholipids, affecting NO also upregulates CPT-1 expression, increasing
gene expression, cell signaling and cell membrane mitochondrial oxidation of LCFA. The effect of arginine
stability. Activity of ODC is almost absent in muscle of on lipid oxidation in fish is still uncertain. Lall et al (43)
Atlantic salmon (36) suggesting liver or intestine as the showed that a deficiency of arginine in Atlantic salmon
main sites for PA production. Also, arginase activity rather smolts increased lipid retention, while no effect was
than ODC activity appears to be regulated by arginine observed from supplementing surplus arginine. Similarly,
availability in fish, suggesting that arginase rather no effect of arginine on lipid deposition was observed
than ODC is the rate-limiting step for PA production in in later studies in Atlantic salmon (15,44). Arginine
fish (5,36). Spermidine and spermine synthase consumes increased expression of mammalian target of rapamyacin
decarboxylated s-adenosyl methionine (dcSAM), which (mTOR) in muscle and hepatopancreas of Jian carp
is made from methionine in and ATP-dependent process (Caprinus carpio var Jian), suggesting increased protein
(Figure 1, see section 3.5.). The PA can then be acetylated synthesis (45). In addition, dietary arginine increased
by spermidine, spermine acetyltransferase (SSAT), activity of several intestinal enzymes and changed
enabling them to be transported out of the cell or to be composition of the intestinal microbiota. This is in line
oxidized back to the shorter PA by acetylated polyamine with observations in red drum and hybrid striped bass
oxidase (APAO). Spermine may also be oxidized to (Morone chrysops x Morone saxatilis) where arginine
spermidine by spermine oxidase (SMO). Increased improved intestinal morphology (46,47), suggesting a
turnover of PA also has the potential to induce oxidative potential role for arginine to improve intestinal health.
stress through production of reactive oxygen species These effects may be mediated through PA, as spermine

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supplementation improved intestinal maturation in energy substrates in fish, through deamination and
sea bass larvae (dicentrarchus labrax), by activating transamination reactions. Most of the dietary glutamine
pancreatic enzymes and increasing activity of brush and glutamate (more than 60%) is known to be
border membrane enzymes (48). catabolized by the intestinal mucosa in mammals (55),
which is probably also the case for fish. Thus, dietary
Creatine is synthesized from arginine, glycine glutamine and glutamate may never pass the intestine
and SAM in a two-step process producing creatine and in fish. Glutamate can be decarboxylated to gamma-
ornithine and is used as a storage molecule for energy aminobutyrate (GABA) and both glutamate and GABA
in the muscle in the form of phosphocreatine. Creatine act as neurotransmitters found in high concentrations
is continuously broken down and excreted as creatinine in the fish brain (56). GABA is considered to be the
in the urine, and thus needs to be continuously replaced. main inhibitory neurotransmitter, while glutamate is
Little is known about how dietary arginine can influence the main excitatory one (57). GABA is demonstrated
creatine in fish, but Chen et al (45) showed that creatine to stimulate secretion of lutenizing hormone (LH)
kinase activity increased in the intestine of jian carp after from the pituitary of Atlantic croaker (Micropogonias
arginine supplementation, while in Atlantic salmon no undulatus) via the GABAA receptor (58), and in rainbow
effect was observed on plasma creatinine due to dietary trout GABA stimulated secretion of both LH and of
arginine (15). If supplemented arginine could be directed follicular stimulating hormone (FSH) (59). However, in
towards creatine production this has the potential to both cases this effect appeared to be dependent on the
increase muscle mass, though as mentioned this is also reproductive stage of the fish, as GABA stimulated LH
dependent on sufficient supply of glycine and the methyl in regressed fish, while it inhibited secretion in mature
donor SAM. fish (58,59). The actions of GABA on LH release appears
to be mediated through both activation of gonadotropin
Arginine is involved in endocrine functions in releasing hormone, by direct stimulation and through
fish, and is indeed a more potent stimulator for insulin inhibition of dopamine (57). Glutamate itself may also
release than glucose in rainbow trout (Oncorhynchus stimulate secretion of LH and growth hormone through
mykiss) (49). In coho salmon (Oncorhynchus specific glutamate receptors (56,57). GABA is known
kisutch), arginine injections increased plasma insulin to affect muscular tone in humans, and evidence in
concentrations in a dose dependent matter concomitantly chinook salmon Oncorhynchus tshawytscha supports
decreasing plasma glucose, indicating increased stimulatory effect of GABA on locomotor acitivity
glucose uptake (50). A similar effect was observed in mediated through dopaminergic and seretonergic
isolated liver cells from Atlantic salmon, where cells pathways (60).
from salmon fed a high arginine diet appeared to have
a higher uptake of glucose (51). Rainbow trout fed diets Glutamate and glutamine play a crucial role in
supplemented with arginine for two months had a higher intestinal health of fish, by modulating intestinal structure,
plasma insulin and a higher weight gain, but this was protecting against oxidative damage and acting as energy
associated with increased food consumption and FCR substrate for the enterocytes. Glutamine supplementation
(50). Arginine further stimulates release of glucagon increased growth and intestinal structure in red drum
and GLP-1, and arginine injections to feeding rainbow (Sciaenops ocellatus) (46), Jian carp (61) and hybrid
trout resulted in decreased plasma fatty acids and liver striped bass (47), as well as increasing growth and
glycogen (49). Arginine is known to stimulate glucose differentiation of carp enterocytes in vitro (62). The
uptake and oxidation in mammals through NO and positive growth effect can be explained by a stimulatory
activation of 5’activated protein kinase (AMPK), which effect on protein synthesis as glutamine increased
activates translocation of glucose transporter-4 (GLUT- intestinal protein content (61) and in vitro protein
4) to the cell membrane as well as inactivating acetyl retention (62). Furthermore, glutamine supplementation
Co-A carboxylase (ACC) (52). NO may also increase upregulated alkaline phosphatase and Na+K+ATPase
GLUT-4 translocation through cGMP, rather than activity in enterocytes in vivo and in vitro, demonstrating
insulin signaling (53,54). Thus, arginine has potential increased nutrient uptake and cell differentiation.
to increase glucose utilization is fish, though further Glutamine may also protect the enterocytes against
research is required in this area. oxidative damage, as glutamine supplementation to
hydrogen peroxide exposed enterocytes reversed the
3.2. Glutamate and glutamine increase in lactic acid dehydrogenase activity, lipid
While glutamate is a NEAA in fish, glutamine peroxidation and protein oxidation, while restoring ROS
is generally considered a conditionally EAA. Glutamate induced decreased activity of intestinal enzymes (63).
may be synthesized endogenously from α-ketoglutarate Glutamine also restored the ROS induced decline in
and BCAA, and glutamine is produced from glutamate enzymes involved in oxidative repair and increased the
by the ATP-dependent enzyme glutamine synthase reduced to oxidized glutathione ratio (GSH:GSSG) (63).
(GS). Glutaminase may hydrolyze glutamine back to Glutamate is together with cysteine and glycine essential
glutamate, releasing ammonia. Both AA are important to synthesize the antioxidant glutathione.

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regulate DNA and RNA synthesis and affect lymphocyte

proliferation. Nucleotides is commercially used as an
immune-stimulant in Atlantic salmon feed, as dietary
supplementation is known to increase antibody titers in
Atlantic salmon after pathogen exposure (66). Whether
dietary glutamine may stimulate the immune response
in fish through endogenous nucleotide synthesis is still
unknown.

Glutamine plays a key role in nitrogen

detoxification, as the enzyme GS binds ammonia to
Figure 3. Key steps in tryptophan metabolism. Production of 5-HT is high glutamate forming glutamine. This glutamine may then
during the day (light), while melatonin secretion and activity of AANAT enter the urea cycle as described in section 3.1. As
is high during the night (dark). Multiple steps not shown are required to
CPSIII activity is present only in the muscle, not in the
convert kynurenine into nicotinic acid. TH – tryptophan hydroxylase, 5-HTP
– 5-hydroxytryptophan, 5-HT – serotonin, 5-HIAA – 5-hydroxyindoleacetic liver of rainbow trout (17), this pathway may be essential
acid, AAAD – aromatic amino acid decarboxylase, MAO – monoamine for elimination of ammonia produced in the muscle during
oxidase, AANAT – arylkylamine N-acetyltransferase, HIOMT – exercise. Indeed, urea secretion increased rapidly after
hydroxyindole-0-methyltrasnferase, IDO – indole 2,3-dioxygenase.
exercise without affecting ammonia excretion in juvenile
trout and increased muscle activity of GS was observed
The role of glutamine in immune response after 4 days exposure to high-speed swimming (67). GS
appears to be specie specific. Glutamine has been is also found in high concentrations in the brain (56,64),
proposed to aid the immune response by modulating where it plays a pivotal role in ammonia detoxification.
the NO response of macrophages and act as energy Ammonia exposure increased brain activity and
substrate for leucocytes. In general plasma levels of expression of GS in rainbow trout (68), an important
glutamine and glutamate drops after exposure to stress adaptation to cope with changes in environmental pH.
or pathogens, increasing the demand of glutamate
for energy (64). GS is expressed in the brain, muscle, Finally, glutamate is used as a flavor enhancer in
intestine, liver and kidney of snapper (Pagrus auratus) the form of monosodium glutamate (MSG) in human and
but is absent form lymphoid organs (64). Maintaining animal diets, where glutamate and GABA stimulate feed
sufficient plasma glutamine concentrations during disease intake through orexigenic neurons in the hypothalamus.
is thus important to supply energy for the leucocytes. In Limited work has been conducted on the role of glutamate
mammals, glutamine is essential for proliferation of T to affect hunger and palatability in fish. Dietary glutamate
and B cell lymphocytes, an effect also observed in fish, supplementation affected fillet quality in Atlantic
as dietary glutamine increased lymphocyte proliferation salmon (69), without affecting growth or FCR. This was
in channel catfish head-kidney and spleen, but not in observed in association with decreased hepatosomatic
peripheral lymphocytes, after vaccination (65). The index and fat accumulation in the liver of salmon (69).
proportion of IgM+ cells also increased in glutamine Further, microarray analysis of gene expression in white
supplemented head-kidney tissue after vaccination. muscle demonstrated that glutamate affected several key
Furthermore, in vitro glutamine supplementation metabolic pathways.
increased non-specific T and B cell proliferation in
channel catfish lymphocytes (29). Glutamine further 3.3. Tryptophan
increased superoxide anion and neutrophil oxidative Tryptophan is the precursor for the
radical production in kidney macrophages from red neurotransmitter serotonin (5-hydroxytryptamine, 5-HT,
drum (46), and improved macrophage superoxide Figure 3), which implications for fish welfare have
anion production and lysozyme activity in hybrid striped recently been well reviewed (70). 5-HT is unable to pass
bass (47). In channel catfish macrophages on the the blood-brain barrier and synthesis is thus dependent
other hand, no effect was observed from glutamine on tryptophan uptake into the brain. Dietary tryptophan
supplementation on phagocytosis or bacteriocidal activity has shown to correlate with tryptophan concentrations
after pathogen exposure (29). Glutamine is demonstrated in the brain and with 5-HTergic activity in fish (71,72).
to regulate cytokine production, expression of immune Supplementing dietary tryptophan has shown to reduce
related genes and inhibit apoptosis in mammals (10), aggressive behavior in rainbow trout (72), Atlantic
while these interactions is yet to be investigated in fish. cod (71) and fighting fish (Betta splendens) (73), through
Synergistic effects of glutamine and arginine on the calmative effects of 5-HT. Indeed, inhibitors of monamine
immune response have been examined in several fish oxidase (MAO inhibitors, Figure 3) is a common strategy
species (29,46,47,65), with various results, showing both to treat depression and anxiety in humans.
synergistic and inhibitory effects.
5-HT is further the precursor for the hormone
Glutamine is a precursor for synthesis of melatonin produced in the pineal gland. Melatonin is
purine and pyrimidine nucleotides, and could thus involved in day rhythm regulation, reproduction, immune

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and Atlantic halibut (Hippoglossus hippoglossus) (76).

Continuous light regime is readily applied in commercial
aquaculture in order to maximize growth, though the
exact mechanisms behind is not clearly understood.

Furthermore, dietary tryptophan may alleviate

stress, as dietary tryptophan attenuated stress
induced anorexia in brown trout (Salmo trutta) (83)
and counteracted stress induced increase of plasma
cortisol in rainbow trout (84). In Atlantic salmon fed
increasing inclusion of tryptophan, cortisol secretion
was suppressed at a basal state while during stress an
acute stimulatory and long-term inhibitory effect was
observed along with altered dopamine concentrations
in the hypothalmaus (85). In mammals, most of the
tryptophan is metabolized along the kynurenine pathway,
Figure 4. Schematic illustration of the histidine dipeptide metabolism.
Anserine can be synthesised by incorporating β-alanine to 1-methyl which is used for production of niacin and for regulating
histidine or by methylation of carnosine in the cells. Excess histidine the immune response (86). Little information is available
is degraded to urocanate by the enzyme histididase in the liver. about this pathways in fish, though Ng et al showed that
ANS – anserinase, MT – methyl transferase, CS – carnsosine synthetase,
carn NMT – carnosine N-methyl transferase. tryptophan was a poor precursor for niacin in channel
catfish (87). Dietary tryptophan is shown to advance
maturation in male and females ayu (Plecoglossus
function and act as an antioxidant (74). Secretion of altivelis) (88) and the intermediate tryptophan metabolite
melatonin is inversely related to light intensity in Atlantic l-kynurenine is identified as a sex attractant in female
salmon, where secretion is high at night while it is masu salmon (89), suggesting that there is some activity
suppressed by light during the day (75). Concentrations of the kynurenine pathway also in fish.
are also slightly higher in the summer at high
temperatures and melatonin is thought to signal timing of A deficiency of tryptophan results in scoliosis
developmental, reproductive and behavioral changes in and an imbalance in mineral metabolism in rainbow
fish. A role for melatonin to affect reproduction in seasonal trout (90) and some salmonids (91), and is thus crucial
breeders by either inhibiting or stimulating gonadal for maintaining fish health and welfare. As discussed
function have been suggested, as light manipulation has above, further supplementing tryptophan beyond the
successfully been used to induce sexual maturation (76). requirement for growth could improve fish behavior,
Maitra et al recently reviewed melatonin’s effect on disease resistance and alleviate stress. Tryptophan is
reproduction in carp (77). Melatonin may assert its effect thus important to avoid production loss due to aggressive
directly on peripheral organs or through modulating behavior and cannibalism. Still, further research is
the hypothalamo-pituitary axis, as melatonin is shown required to elucidate the underlining mechanisms.
to increase release of growth hormone and decrease
release of prolactin from the pituitary gland in rainbow 3.4. Histidine
trout in a dose dependent matter (78). Seasonal changes Histidine is classified among the EAA for fish.
in immune-competence and disease outbreaks have also Chemically, the histidine molecule has a functional
been linked to diurnal and seasonal changes of melatonin positively charged imidazole group (therefore also
secretion in fish (79). Injecting melatonin to gilthead named an imidazole) that can act as an ampholyte.
seabream (Sparus aurata) enhanced gene expression of Besides having important functions in the catalytic
virus and lymphocyte markers and increased respiratory sites of many proteins (enzymes), histidine is also an
burst, cytotoxic and peroxidase activity of head-kidney important component for histamine formation (signalling),
leucocytes (80), indicating a role for melatonin in the the synthesis of purines, as well as being a precursor
innate immune defense of fish. Effects of melatonin may for many functional imidazoles in fish (carnosine,
vary dependent on specie, age, photoperiod, temperature anserine, N-acetyl histidine (NAH); Figure 4). Functional
and dosing and may act trough release of other hormones aspects of histidine was firstly focused in fish nutrition
and these areas all requires further studies. The melatonin after recurrent occurrence of high incidences of the eye
system can be manipulated by changing light regime disorder cataract in farmed Atlantic salmon in Europe
or injecting melatonin, affecting reproduction, immune in the late nineties (92,93). Cataract formation was
response, feeding and behavior in fish. Continuous connected to periods of rapid growth in Atlantic salmon,
light is shown to halt melatonin secretion and affect and was observed after blood meal was omitted from the
cortisol and expression of clock genes in Atlantic salmon salmon feeds in the late nineties due to a potential risk for
smolt (81). Light regimes have successfully been applied transmitting Bovine Spongiform Encephalopathy (BSE).
for endogenous control of reproduction in both trout (82) Blood meal (haemoglobin) is especially rich in histidine,

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Functional amino acids in fish nutrition

interacted and provoked by several farming conditions

and fish genetics (104). The latter is probably mediated
through genetic differences in histidine metabolism (101).
Recently, a higher requirement of histidine to prevent
cataract development in triploid Atlantic salmon versus
diploids were demonstrated (105). The reason for this
difference in histidine metabolism with ploidity is not
presently known.

Quantitative amounts of free histidine is used

for cell protein synthesis, for synthesis of imidazole
derivatives and for purine synthesis. Minor amounts are
used for histamine synthesis in immune cells and paracrine
signalling in the stomach, after a decarboxylation. From
the requirement studies, the well regulated protein
synthesis is of priority for the cellular free histidine. The
synthesis of histidine dipeptides and modified histidine
Figure 5. Photo of an Atlantic salmon eye with a complete mature
compounds seem to be of second priority, with the
cataract (Photo by Prof. Ellen Bjerkås, reproduced by kind permission aim to trap the imidazoles at high concentrations in
of CAB Reviews). the cells for physiological purposes (like pH buffering,
osmolyte function, antioxidation etc). Thus, the imidazole
and the marginal dietary histidine supply was identified compounds have been used as markers of the histidine
as a causative factor for cataract development in Atlantic status in salmonids (8). Interestingly, the repertoire of
salmon (92). Recent research on this topic has explored imidazoles change between fish species, as well as
novel functions of histidine and related imidazole between organs in the individual fish species (106).
compounds in osmoregulation (94), pH buffering (95,96), Typically, salmonids have high concentrations of anserine
anti-oxidation (97,98), metal chelation (99) and anti- in the trunk muscle, while high concentrations of NAH is
glycation of proteins in salmonids. Histidine has the ability present in the lens and heart (8,101). NAH is synthesized
to bind to and modulate the absorption of zinc, copper in the cells from acetyl-CoA and histidine by the enzyme
and iron (99), and may thereby affect the distribution histidine N-acetyl-transferase. NAH have been identified
and excretion of essential elements in the fish. The most in the lens, heart and brain of salmonids (107). NAH is
striking and acute pathology at suboptimal histidine hydrolysed by anserinase (108), found in the extracellular
nutrition is observed as cataract development in the fluids and blood (109).
salmon lens (Figure 5).
Anserine is synthesized from 1-methyl-histidine
The histidine requirement for growth has been (1-MHis) and β-alanine by carnosine synthase or through
estimated to 7 g histidine/kg feed or 15 g histidine/kg the formation from carnosine, followed by methylation of
of crude protein in salmonids (100), while freshwater carnosine (carnosine–N-methyl-transferase, Figure 4).
species show variation in requirements between 4 g Carnosine is synthetized by binding of β-alanine to
histidine/kg feed in channel catfish (Ictalurus punctatus) histidine. The low levels of carnosine and 1-MHis and
to 10 g histidine/kg feed (carp and tilapia species), high concentrations of anserine in the salmonid muscle
or 15 to 21 g histidine of crude protein. Most of these cells show that carnosine and 1MHis are intermediate
studies are from the nineties, while a comment from compounds to anserine (8,101,103). While blood and
the recent NRC (2) is that these requirement estimates fluids carnosinase activity hydrolyse carnosine back to
do not support optimal ocular health in fast growing histidine and β-alanine, the histidine moiety of anserine
Atlantic salmon and salmon undergoing parr smolt cannot be recycled to histidine. Significant levels of
transformation (smoltification). Breck et al (101) showed anserine is found in marine and animal products, while
that supplementation with dietary histidine at levels far it is nearly absent in novel plant sources used in fish
above the requirement for growth (~18 g histidine/kg feeds (110). Muscle anserine seems, however to be
feed) significantly reduced the prevalence of cataracts in homeostatic regulated in rainbow trout and independent
salmon smolt. Studying histidine turnover in the salmon on dietary levels of anserine (from marine hydrolysates)
lens, Breck et al (102) suggested that histidine not only is between 0.3. to 5.0. g/kg and with 22 g histidine/kg
essential for lens protein synthesis, but also constitutes crude protein (110). However, recent data indicate large
an important osmolyte in the chemical form of NAH. The differences in histidine metabolism (and requirement)
cataract alleviation of excess dietary histidine has later between rainbow trout and Atlantic salmon (8). Excess
been confirmed by Remø et al (8) at 13.4. g histidine/ histidine is degraded by the enzyme histidase (histidine
kg for Atlantic salmon smolt and at 12.8. g histidine/kg in ammonia lyase, HAL) to urocanate and further to
adult salmon (103). The cataract formation is, however glutamate. While an imbalanced AA composition in the

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diet to rats increased the catabolism of histidine (111), The chemical structure allows the imidazole
Remø et al (8) could not demonstrate changes in the compounds to act as pH buffers. Histidine and anserine
transcriptional level of HAL in the liver of Atlantic salmon constitute important buffer components in the fish
smolt fed dietary histidine levels up to 18 g/kg. muscle, and are vital for the continuous anaerobic energy
production during burst swimming activity (126-128).
N-acetyl histidine was suggested to be an Histidine and anserine, and the muscular buffer capacity
osmolyte in the fish lens (109,112,113) and a dysfunction of masu salmon differ in parr and smolts and seem to relate
of the lens osmoregulation was related to cataract to the bust swimming ability (129). In juvenile Yellowtale
formation. Ex vivo lens culture studies in hypoosmotic (Seriola quinqueradiata), the tissue buffer capacity
media demonstrated that NAH was rapidly released from correlated to the level of histidine and anserine (130).
lenses to maintain the water homeostasis to prevent lens Climatic changes imply periodic critical high water
swelling and subsequently lens fibre lysis and rupture temperatures and low water oxygen saturation, which
of the lens capsule (112). Baslow (114) suggested that may increase the anaerobic metabolism and challenge
NAH forms a part of an intercompartmental biochemical the muscle buffer capacity in farmed salmon (8). While the
cycle, with the aim to transport water out of the lens cells. buffering capacity of histidine decreases with increasing
The cycle appears to be gradient driven and depend on temperature, the buffering capacity of anserine does not
a high standing concentration of NAH. Once released appear to be sensitive to changes in temperature in the
from the lens, NAH is hydrolysed by anserinase and physiological pH range of pH=6.5.-7.5. (96). Like muscle
histidine re-transported into the lens cells. For the Atlantic anserine, Remø et al (8) showed that NAH in heart tissue
salmon, this mechanism may be especially important in of Atlantic salmon also increase after sea transfer, and
the period after seawater transfer, which is an osmotically seem to reach tissue saturation at moderate dietary
challenging period for Atlantic salmon smolts (115,116). histidine concentrations. Histidine compounds exert
The high NAH status in the lens depend on dietary protective mechanism including regulation of intracellular
histidine, and low NAH concentrations has been directly pH (131) and antioxidant functions (120) in rodent and
related to the severity of cataract development in Atlantic mammalian hearts. In Atlantic salmon heart, anserine
salmon (8,101,103). Several studies have shown a and carnosine were not detected, while NAH could
correlation between rapid growth rates and cataract possess an analogue buffering function (8, 98).
development (115,117,118). Waagbø et al (103) showed
a higher prevalence of cataracts in adult Atlantic salmon 3.5. Sulfur amino acids
after a natural increase in water temperatures between Methionine is an EAA and can be used to
12 and 18.5.°C. The increased growth in these studies synthesize cysteine, which together with methionine
implies less free histidine available for NAH synthesis constitutes the sulfur AA (SAA). Methionine is in addition
and optimal lens osmoregulation. to participating in protein synthesis the most common
methyl donor in the body (132). Before methionine can
Several studies have implied that histidine and transfer its methyl-group it has to be activated by ATP,
histidine derivatives functions as anti-oxidants or can producing S-adenosylmethionine (SAM, Figure 6).
mitigate the impact of oxidative stress (119-121), low S-adenosylhomocysteine (SAH), produced when
density lipoprotein oxidation (122), oxidative DNA damage methionine donates its methyl group to methyl acceptors
and experimentally induced oxidation of liposomes (99), within the body, is unstable and quickly converted
diabetes induced oxidation (123) and heart ischemia (124). to homocysteine. Homocysteine is a branch point in
The antioxidant activity is connected to the imidazole ring sulfur metabolism, as it can be re-methylated or trans-
structure in histidine and histidine compounds, with ability methylated back to methionine or be trans-sulfurated
to scavenge hydroxyl radicals and singlet oxygen (97-99). to cystathionine. Cystathionine may be metabolized
The imidazole dipeptide N-acetyl-carnosine (NAC) has to cysteine of which may participate as a constituent
been shown to protect against oxidative damages in both of glutathione or it may be metabolized to taurine.
canine and human lenses, and eye remedies containing Re-methylation of homocysteine to methionine occurs in
NAC has been suggested as a treatment for cataracts almost all body compartments by the enzyme methionine
in humans (125). Since neither carnosine nor anserine synthase (MS) that needs folic acid as the methyl donor.
have been detected in salmon lenses (116), the high While trans-methylation of homocysteine to methionine
concentrations of NAH has been suggested to cover the by the enzyme betaine homocysteine methyltransferase
role as an antioxidant in the salmon lens. Remø et al (98) (BHMT) needs betaine as the methyl donor. BHMT
showed that the concentration of NAH was significantly is reported to be present in gastrointestinal, liver and
reduced in oxidatively stressed salmon lenses and in kidney only (132,133). Betaine needed for the trans-
culture, and based on this study it was suggested that methylation arrives from food or it may be synthetized
NAH has a role as antioxidant in the Atlantic salmon lens. from choline by two successive reactions taking act in
The integrated antioxidative defence system in the lens the mitochondria (134). In addition to being a methyl
also appeared to be influenced on a transcriptional level donor, SAM may be de-carboxylated producing the
by histidine enrichment of the media. aminopropyl group used in synthesis of the PA spermine

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Functional amino acids in fish nutrition

Figure 6. Schematic Figure of methionine pathways producing cysteine and taurine appearing in salmonids. MAT – methionine adenosyl transferase,
SAM- s-adenosyl methionine, SAH – s-adenosyl homocysteine, MS – methionine synthetase, BHMT – betaine homocysteine transferase,
CBS – cysthathionine beta-synthase, CO – choline oxygenase, CDH – choline dehydrogenase.

and spermidine (Figure 1, section 3.1.). SAM also During the last two decades most of the animal protein
donates methyl groups to phosphatidyl ethanolamine used in formulated aquaculture diets have been replaced
methyltransferase (PEMT) in the endogenous synthesis of with plant proteins of which may affect the requirement
phosphatidylcholine (PC), a major constituent of very low for precursor AA for other metabolite synthesis especially
density lipoprotein (VLDL) assembling and thus transport SAA as taurine is low or absent in plant proteins.
of lipids from liver to peripheral organs. It also donates
methyl groups for the synthesis of carnitine necessary In animal models, AA directly stimulate protein
in CPT-1 synthesis (135). SAM also is the methyl donor synthesis in an insulin independent manner (reviewed
in the synthesis of creatine (135) and used in DNA and by L. J. C. van Loon (139)). Likewise, salmon juveniles
RNA methylation that may have significant effects on fed methionine deficient diets had reduced protein gain
metabolism and phenotypes (136). Excess methionine and growth (140,141) opposite to what was observed
is transsulfurated to cystathionine and cysteine, the in adult Atlantic salmon where growth was unaffected
precursors for taurine and glutathionine or degraded to by methionine intake (142). Additionally, methionine
propionyl-CoA entering the Krebs cycle, or excreted in limitation reduced concentrations of PA in both liver and
the urine as taurine. Considering the number of metabolic muscle tissues that may contribute to the reduced growth
pathways requiring methyl groups, it is not surprising that and protein accretion (140). Thus, it seems like during
methionine availability have the potential to affect the shorter periods of time the juvenile salmon were able
metabolic health in animals, fish included, both when to keep their liver sulfur metabolites in a physiological
present in surplus or deficient concentrations (137,138). normal concentration, but seemingly started to deplete

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their muscle stores and reduce the lean protein growth. in mammalian models (154,155). Taurine administration
During longer periods this may be detrimental to the in plant protein based diets fed to Atlantic salmon not
liver sulfur metabolism and health of the fish. In rainbow only reduced the whole body lipid to protein ratio but
trout methionine deficiency was reported to affect protein also increased the concentration of PA (147). Likewise,
synthesis through its deactivation of mTOR signaling taurine administration reduced the TAG accumulation
cascades (143-145) in a similar way as described in in obese rodents (156). Recently it was reported that
mammalian species, while muscle proteasome related taurine administration reduced genes associated
genes increase. Similarly, juvenile Atlantic salmon reduced with inflammation and lipogenesis and increased the
muscle protein deposition and IGF-1 gene expression lipolytic genes in liver contributing to reduced severity
after fed a low methionine diet for eight weeks (141). of non-alcoholic fatty liver disease (NAFLD) in mice
However, surplus methionine supplementation had no models (155). Opposite to this, methionine restrictions
effect on protein turnover in rainbow trout (143). reduced liver de novo lipid and cholesterol synthesis in
rainbow trout fed high carbohydrate diets (157).
In adult rainbow trout it was reported that
methionine supplementation above established Methionine derived metabolites as taurine and
requirement reduced the relative weight of viscera (146), glutathione function as antioxidants within the body. In
but growth was un-affected, while taurine supplementation mammals, the transsulfuration, and thus taurine and
had no impact on viscera mass. Juvenile salmon fed glutathione status, have significant impact on oxidative
diets supplemented with taurine did not improve growth, and inflammatory status. Methionine limitations did
but reduced the total body lipid content (147) and as however not affect the amount of total glutathione
such taurine may have a beneficial effect on health. The in juvenile salmon (140). In liver cells isolated from
differences obtained in taurine supplementation might be salmon, taurine supplementation improved viability and
due to the higher diet concentrations used by Gaylord decrease activation of kinases cascade protein signaling
et al (146) as compared to Espe et al (147). cell death (158). These observations are probably
linked to the anti-oxidative effects of taurine (reviewed
Deficiencies of SAA, choline and betaine have by G. Atmaca (159). Taurine and especially its halides
been associated with metabolic stress in salmonids. reduced pro-inflammatory cytokines and interleukins in
When Atlantic salmon are fed low methionine diets, the adipose tissues (138) and as such has the capacity to
PC was reduced in liver, but supplementation of choline to modulate the inflammatory response. Taurine depletion
these diets increase the concentration of phospholipids in may induce oxidative and inflammatory stress, which
liver of salmonids (148). Opposite to this adult salmon fed is closely associated with the metabolic syndrome, and
low methionine diets, without choline supplementation, may reduce cell viability (158). Similar cell models with
accumulated liver TAG, which was restored to normal low, adequate and surplus methionine supplementation
values when adequate methionine levels were added to showed that methionine limitation reduced viability, but
the diet (142,149). However, liver PC was not affected in surplus methionine has no beneficial effect on viability.
the adult salmon. Liver lipid accumulation is associated Supplementation of betaine to these media had no
with increased metabolic stress, energy depletion, beneficial effects on cell survival (160). In adipose
cytokine activation and inflammation in rodent models tissues, taurine administration improves the inflammatory
and human beings (150,151). Development of fatty liver responses through decreased production of pro-
may also be detrimental to fish health. Juvenile salmon inflammatory cytokines in diet induced obese mice (161).
fed low methionine diets had increased gene expression Likewise, taurine administration decreased inflammatory
of the cytokine TNF-alpha, but gene expression of pro- markers and lipid peroxidation in obese women (162),
inflammatory interleukins were not affected (140,148). implying that taurine may have beneficial effects
It is believed that liver TAG accumulation following low improving the metabolic health in obese models. During
methionine and choline diets is due to reduced availability the later years more and more of the marine ingredients
of PC for assembly of VLDL and thus transport of TAG have been replaced by plant ingredients concomitantly
from the liver to peripheral organs like the muscle (152). the salmon has grown more obese with increased liver
Therefore, deficiency of methionine during longer periods TAG and plasma lipids (153,163). Thus probably research
of time may be detrimental to fish health, but so far no on this will increase our knowledge of how AA delivery,
one has proved that salmons actually develop the and especially so SAA delivery might improve health
metabolic syndrome, even though the increased liver status by decreasing this central lipid accumulation and
lipid accumulation (149) and increased viscera mass is thus reducing the pro-inflammatory signaling cascades
reported also in salmonids (140,142,147,149). Salmon as affected by SAA will increase significantly in the years
accumulate lipids in viscera when fed high plant oil and to come. Chronic liver disease such as cirrhosis results in
protein diets (153) as compared to groups fed diets that are increased plasma methionine due to impaired conversion
based on marine oils and proteins. The increased visceral to SAM and most often is associated with decreased
fat mass may increase inflammation and contribute to a cysteine and taurine as well as the impaired synthesis
reduced synthesis of lean protein in salmon as observed of glutathione (164). Methionine deficiency increased

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Functional amino acids in fish nutrition

liver SAM in juveniles (140) and decreased liver SAM in sirtuins in fish models. BCAA supplementation during
adult salmon (142,149) thus altered methylation capacity liver diseases are beneficial in mammalian models as
may affect health and robustness of the fish. Interactions they reduce muscle proteolysis, increase glutamine
between SAA and other AA should be addressed in production and protein (169)synthesis, but their beneficial
fish models, as inflammatory and oxidative stress may effects strongly depends on the type of liver disease as
be detrimental. As circulating free AA are known to be reviewed elsewhere (170). As most literature on fish
affected in fish exposed to external stress (7,70) this will deals with requirement of BCAA to maximize growth and
probably also be the case during metabolically induced accretion (171,172), the following will focus on BCAA and
stress. their effects on metabolism in animals in general and link
this to fish metabolism improving metabolic health.
Methylation of the DNA accept methyl groups
from SAM. This may affect the expression of genes, BCAA and primarily leucine increase pathways
and modulate the phenotypes, although this has been involved in muscle protein synthesis through both insulin
studied in different mammalian models (reviewed by dependent as well as insulin independent pathways.
Wang et al (136)) little information in salmonids exist. Leucine directly activate the mTOR in skeletal muscle
Both hypo- and hyper-methylation of DNA is linked and stimulate downstream phosphorylation of p70S6
to development of obesity and cancer in several kinase and 4E-BP1 signaling RNA translation and protein
models (136), but this has to our best knowledge not synthesis, although this signaling is transient (164).
been addressed in either salmon or rainbow trout. In Furthermore, BCAA attenuates muscle wasting through
conclusion, deficiency and distribution of methionine and interactions with ubiquitine proteasome pathway (164,173)
sulfur metabolites have a significant impact on metabolism of which may involve the protein kinase Akt/PKB known
of which resembles the mechanisms described in to phosphorylate the transcription factor forkhead box
mammalian models. When low in SAA, both growth class-O (FoxO) that signals downstream to two major
and type of growth especially protein growth is affected ubiquitine ligases the atrogin-1 and muscle RING finger
and so is inflammation and oxidation. SAA restrictions protein (MuRF-1) (164). These pathways also regulates
induce metabolic changes similar to those described in protein synthesis in salmonids (144,145). Recently
the metabolic syndrome described in mammalian. Until Lansard et al (174) reported that leucine participated
today, several of the mechanisms behind these metabolic in activation of the mTOR pathway in rainbow trout
changes are still largely unknown in mammalian models, hepatocytes. There still is little literature available on
and even less is known in fish. protein turnover as affected by BCAA in fish. As BCAA
and especially leucine supplementation is linked to lean
3.6. Branched chain amino acids growth, future studies might bring more insight into the
Leucine, isoleucine and valine constitute the possibility of BCAA to maximize lean growth in fish.
BCAA. As in mammalian species, all three of these are
EAA for fish. BCAA are essential for protein synthesis, Recently using a metabolomics approach it was
but also an important regulator of protein degradation. found that BCAA and their metabolites are more strongly
Unlike the degradation of the other AA, degradation associated with obesity and type 2 diabetes than are
of BCAA starts in extra hepatic tissues producing fatty acids (FA) and FA derived metabolites (reviewed
glutamate and branched chain keto-acids before being by C. B. Newgard (175)). However, BCAA requires
transported to the liver for further degradation. These AA the background of a high fat diet to promote insulin
and their degradation metabolites are associated with resistance (IR) associated with the metabolic syndrome
maintenance of the lean body mass during prolonged in mammalian models (175). Metabolome studies has
exercise in both man and mammalian models (reviewed proved that BCAA and AAA also had the strongest
by Molfino et al (165)). Also in salmon the BCAA are association with diabetes development in obese
linked to maintenance of lean body mass during exercise men (136). Adipose tissue metabolism seems heavily
as for example forced swimming (166). In man, nutritional involved in the development of metabolic syndrome
disturbance in BCAA are associated with liver diseases. in mammalian models. Adipose metabolism release
In chronic liver diseases as cirrhosis, serum BCAA BCAA and catabolic metabolites thereof (175). Adipose
concentration is low concomitantly the aromatic AA metabolism probably will be focused in the years to come
(AAA) are elevated (167). In mammalian models, BCAA and make us able to understand the metabolic cross talks
supplementation is linked to life span, mitochondrial between lipids and AA’s in development of metabolic
biogenesis and defense against ROS (168). This is dysfunction and development of the metabolic syndrome
linked to activation of sirtuins, a family of deacetylation in different models fish included. The International
proteins that regulates key metabolic pathways by Study of Macro/Micronutrients and blood pressure
regulating PGC1-alpha, FoxO, p53, AMPK, eNOS and demonstrated that higher BCAA intake was associated
several other enzymes. Thus, BCAA availability is linked with a reduced prevalence of obesity in middle-aged
to metabolic health and welfare in mammalian models. individual from East Asian and Western countries (176).
To our knowledge, no work has been published about Alterations in the AA profile is associated with obesity,

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Functional amino acids in fish nutrition

Figure 7. A - Effects of an unbalanced AA composition on fish metabolism, growth and inflammation. An unbalanced diet increases central deposition
of lipids and increase TAG accumulation in liver and relative liver weight. This has been associated with increased expression of inflammation markers
and increased oxidative stress. This has the potential to induce oxidative stress in mammals. A diet marginal in methionine will increase proteolysis and/
or reduce protein accretion in the muscle in order to supply the liver with sufficient AA. Thus, an unbalanced diet will favor visceral growth rather than
muscle growth. B – A balanced diet will limit inflammation and oxidative stress. Sulfur amino acids (SAA) inhibit TAG formation in the liver, while arginine
and leucine stimulate protein synthesis in the muscle.

and especially BCAA are linked with obesity and IR in 4. METABOLIC PERSPECTIVES
rodent models (177). Today no clear mechanism has
emerged explaining the role of BCAA in obesity, but there In humans, deficiencies of SAA, choline or
is a consensus that the altered BCAA signature is a risk betaine or surplus BCAA have been associated with
factor for developing IR and diabetes 2 in these models. metabolic stress commonly referred to as the metabolic
Rats fed high fat diets supplemented with BCAA reduced syndrome (175). The metabolic syndrome is associated
their mesenteric lipid depot and had reduced plasma and with obesity, central fat accumulation, inflammation,
liver TAG (178). Indicating that BCAA administration may fatty liver (NAFLD) and IR, and is often the cause of
modulate metabolic disorders as describe in the metabolic prolonged excess energy intake (136). In fish, negative
syndrome and obesity in these models (167,179). In fish health effects of high-energy consumption have not
the effects of BCAA on reducing liver lipid accumulation been reported. However, AA deficiency or imbalance
and improve health status has to our best knowledge not has showed negative health effects similar to those
been addressed. The concentration of circulating free observed with metabolic syndrome in humans. When the
AA, BCAA included, are affected by stress in fish (7,70) diet is limited in one AA, deposition patterns of protein,
and these alterations may contribute to reduced lipid and lipids and glycogen are affected in Atlantic salmon (182).
increased protein accretion, but any beneficial effects for Similarly, when the AA composition is unbalanced, AA
the health and welfare of the salmonids still needs to be and nutrient uptake, growth and deposition patterns
proven. are altered (6,36) (Figure 7A). Methionine deficiency
causes increased hepatic TAG accumulation in Atlantic
Administration of beta-hydroxyl-beta-methyl- salmon (142,149). Further methionine may protect
butyrate (HMB), a metabolite of leucine catabolism, against oxidative stress by being the precursor for the
enhanced the immune response and survival in fish antioxidants taurine and gluthationine. Thus, deficiency
following bacterial infections (180,181). Beneficial effects may increase ROS. Methionine is also needed for the
of BCAA administration are not focused in fish nutrition as production of PA, through the aminopropyl donor dcSAM,
most articles in salmonid nutrition and BCAA describes and in Atlantic salmon methionine deficiency had greater
the requirement, not actually looking into possible impact on PA production than did arginine (42,140).
functional properties of BCAA supplementation. Given Arginine supplementation have shown promising to
that BCAA influence growth, health and metabolism reduce adipose tissue, reduce plasma glucose and
significantly in mammalian models, it is likely that this improve insulin sensitivity in mammals. While arginine
area of research will increase in the years to come in stimulates insulin secretion and reduces plasma glucose
fish nutrition also. in salmonids (50,51), the exact effects on lipid metabolism

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Functional amino acids in fish nutrition

is not established. Generally, dietary AA composition mechanisms and signaling between body compartments
has minor effect on liver AA, indicating an importance in order to fully take advantage of this information when
of maintaining liver AA homeostasis. When the diet is designing fish diets and optimizing fish health and
insufficient of one or more AA, increased proteolysis is welfare.
observed in muscle, in order to supply the liver and likely
the heart and brain (143). The muscle might act as a 6. ACKNOWLEDGEMENT
storage of AA for more vital organs, and will then be the
first tissue affected when fed an imbalanced AA diet. This work was carried out and financed by
NIFES, Norway.
Inflammation in liver and adipose tissue is
associated with metabolic syndrome in humans, with 7. REFERENCES
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methionine, hepatic TNF-a expression increased and PA
knowledge and future needs. Aquaculture
metabolism was affected in liver and muscle indicating
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taurine protected against apoptosis (158) while glutamine 2. NRC: Nutrient Requirements of Fish and
protected against ROS (63) in fish models. Arginine Shrimp. The National academies Press,
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Abbreviations: AA, amino acids, EAA, essential

amino acids, FAA, functional amino acids, SAA,
sulfur amino acids, BCAA, branched chain amino
acids, NO, nitric oxide, NOS, NO synthase, GS,
glutamine synthase, PA, polyamines, ODC,
ornithine decarboxylase, SSAT, spermidine/
spermine-N1-acetyltransferase, GABA, gamma-
aminobutyrate, ROS, reactive oxygen species,
5-HT, serotonin, SAM, S-adenosylmethionine,
SAH, S-adenosylhomocysteine, BHMT,
betaine homocysteine methyltransferase,
PC, phosphatidylcholine, PEMT, phosphatidyl
ethanolamine methyltransferase, LCFA, long chain
fatty acids, IR, insulin resistance, CPT-1, carnitine
palmitoyl transferase-1, VLDL, very low density
lipoprotein, TAG, triacylglycerol, mTOR, mammalian
target of rapamycin, HMB, beta-hydroxyl-beta-
methyl-butyrate, NAH, N-acetyl histidine

Key Words: Amino acids, Nutrition, Functional

amino acids, Fish, Atlantic salmon, Protein, Review

Send correspondence to: Marit Espe,

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