CHAPTER 9 NUTRITION

The components of food and drink that provide for growth, replacement, and energy are
called nutrients. Not all components of food are nutrients. Some components of food and drinks,
such as those that provide flavor, color, or aroma, enhance our pleasure in the food but are not
themselves nutrients.

For food to be used in our bodies, it must be absorbed through the intestinal walls into the
bloodstream or lymph system.
✓ Some nutrients, such as vitamins, minerals, glucose, and amino acids, can be absorbed
directly. Others, such as starch, fats, and proteins, must first be broken down into smaller
components before they can be absorbed. This breakdown process is called digestion.
A healthy body needs the proper intake of all nutrients. However, nutrient requirements vary from
one person to another. For example, more energy is needed to maintain the body temperature of
an adult than that of a child. For this reason, nutritional requirements are usually given per
kilogram of body weight.
✓ Furthermore, the energy requirements of a physically active individual are greater than
those of a person in a sedentary occupation. Therefore, when average values are given,
as in Dietary Reference Intakes (DRI) and in the former guidelines called
Recommended Daily Allowances (RDA), one should be aware of the wide range that
these average values represent.
✓ The public interest in nutrition and diet changes with time and geography. Seventy or
eighty years ago, the main nutritional interest of most Americans was getting enough food
to eat and avoiding diseases caused by vitamin deficiencies, such as scurvy or beriberi.
This issue is still the main concern of the large majority of the world’s population. In
affluent societies such as the industrialized nations, however, today’s nutritional message
is no longer “eat more,” but rather “eat less and discriminate more in your selection of
food.” Dieting to reduce body weight is a constant effort in a sizable percentage of the
American population.
Many people discriminate in their selection of food to avoid cholesterol and saturated fatty acids
to reduce the risk of heart attacks. Along with such discriminatory curtailment diets came many
faddish diets.
✓ Diet faddism is an exaggerated belief in the effects of nutrition upon health and disease.
This phenomenon is not new; it has been prevalent for many years. Many times it is driven
by visionary beliefs, but backed by little science.
 In the nineteenth century, Dr. Kellogg (of cornflakes fame) recommended a largely vegetarian
diet based on his belief that meat produces sexual excess. Eventually his religious fervor withered
and his brother made a commercial success of his inventions of grain-based food.
Another fad is raw food, which bans any application of heat higher than 118°F to food. Heat,
these faddists maintain, depletes the nutritional value of proteins and vitamins and concentrates
pesticides in food. Obviously raw food diet is vegetarian, as it excludes meat and meat products.
A recommended food is rarely as good, and a condemned food is rarely as bad, as faddists
claim. Each food contains a large variety of nutrients.
✓ For example, a typical breakfast cereal lists the following items as its ingredients: milled
corn, sugar, salt, malt flavoring, and vitamins A, B, C, and D, plus flavorings and
preservatives. U.S. consumer laws require that most packaged food be labeled in a
uniform manner to show the nutritional values of the food. Figure 22.1 shows a typical
label of the type found on almost every can, bottle, or box of food that we buy.
✓ Such labels must list the percentages of Daily Values for four key vitamins and minerals:
vitamins A and C, calcium, and iron. If other vitamins or minerals have been added, or if
the product makes a nutritional claim about other nutrients, their values must be shown
as well.
✓ The percent daily values on the labels are based on a daily intake of 2000 Cal. For anyone
who eats more than that amount, the actual percentage figures would be lower (and
higher for those who eat less).
✓ Note that each label specifies the serving size; the percentages are based on that portion,
not on the contents of the entire package. The section at the bottom of the label is exactly
the same on all labels, no matter what the food; it shows the daily amounts of nutrients
recommended by the government, based on consumption of either 2000 or 2500 Cal.
✓ Some food packages are allowed to carry shorter labels, either because they have only
a few nutrients or because the package has limited label space.
✓ The uniform labels make it much easier for consumers to know exactly what they are
eating.
In 1992, the U.S. Department of Agriculture (USDA) issued a set of guidelines regarding
what constitutes a healthy diet, depicted in the form of a pyramid (Figure 22.2). These guidelines
considered the basis of a healthy diet to be the foods richest in starch (bread, rice, and so on),
plus lots of fruits and vegetables (which are rich in vitamins and minerals).
Protein rich foods (meat, fish, dairy products) were to be consumed more sparingly, and fats,
oils, and sweets were not considered necessary at all. The pyramid shape demonstrated the
relative importance of each type of food group, with the most important forming the base and the
least important or unnecessary appearing at the top.
This pictorial description has been used in many textbooks and taught in schools to children
of all ages since its initial publication. However, the USDA has recently revised the information
and appearance of the food pyramid. Chemical Connections 22A discusses the most recent
version of the pyramid.
An important non-nutrient in some foods is fiber, which generally
consists of the indigestible portion of vegetables and grains.
Lettuce, cabbage, celery, whole wheat, brown rice, peas, and
beans are all high in fiber. Chemically, fiber is made of cellulose,
which, as we saw in Section 12.5C, cannot be digested by
humans. Although we cannot digest it, fiber is necessary for
proper operation of the digestive system; without it, constipation
may result. In more serious cases, a diet lacking sufficient fiber
may lead to colon cancer. The DRI recommendation is to ingest
35 g/day for men age 50 years and younger, and 25 g/day for
women of the same age.

The largest part of our food supply goes to provide

energy for our bodies. We learned that this energy comes from the oxidation of carbohydrates,
fats, and proteins. The energy derived from food is usually measured in calories.
One nutritional calorie (Cal) equals 1000 cal or 1 kcal. Thus, when we say that the average
daily nutritional requirement for a young adult male is 3000 Cal, we mean the same
amount of energy needed to raise the temperature of 3000 kg of water by 1°C or of 30 kg
(64 lb) of water by 100°C.
o A young adult female needs 2100 Cal/day. These are peak requirements—children
and older people, on average, require less energy.
Keep in mind that these energy requirements apply to active people. For bodies
completely at rest, the corresponding energy requirement for young adult males is 1800
Cal/day, and that for females is 1300 Cal/day. The requirement for a resting body is called
the basal caloric requirement.
An imbalance between the caloric requirement of the body and the caloric intake creates
health problems.
Chronic caloric starvation exists in many parts of the world where people simply do not
have enough food to eat because of prolonged drought, the devastation of war, natural
disasters, or overpopulation. Famine particularly affects infants and children.
Chronic starvation, called marasmus, increases infant mortality by as much as 50%. It
results in arrested growth, muscle wasting, anemia, and general weakness. Even if
starvation is later alleviated, it leaves permanent damage, insufficient body growth, and
lowered resistance to disease.
At the other end of the caloric spectrum is excessive caloric intake. It results in obesity, or the
accumulation of body fat. Obesity is becoming an epidemic in the U.S. population, with important
consequences: It increases the risk of hypertension, cardiovascular disease, and diabetes.
Obesity is defined by the National Institutes of Health as applying to a person who has a
body mass index (BMI) of 30 or greater. The BMI is a measure of body fat based on height
and weight that applies to both adult men and women.
 For example, a person 70 inches tall is normal (BMI less than 25) if he or she weighs 174
lb or less. A person of the same height is overweight if he or she weighs more than 174 lb
but less than 209 lb; an individual is obese if he or she weighs more than 209 lb. More
than 200 million Americans are overweight or obese.
Reducing diets aim at decreasing caloric intake without sacrificing any essential nutrients.
A combination of exercise and lower caloric intake can eliminate obesity, but usually these diets
must achieve their goal over an extended period. Crash diets give the illusion of quick weight loss,
but most of this decrease is due to loss of water, which can be regained very quickly. To reduce
obesity, we must lose body fat, not water. Achieving this goal takes a lot of effort, because fats
contain so much energy. A pound of body fat is equivalent to 3500 Cal. Thus, to lose 10 lb, it is
necessary either to consume 35,000 fewer Cal, which can be achieved if one reduces caloric
intake by 350 Cal every day for 100 days (or by 700 Cal daily for 50 days) or uses up, through
exercise, the same number of food calories.

Carbohydrates are the major source of energy in the diet. They also furnish important compounds
for the synthesis of cell components. The main dietary carbohydrates are the polysaccharide
starch, the disaccharides lactose and sucrose, and the monosaccharides glucose and fructose.
Before the body can absorb carbohydrates, it must break down di-, oligo-, and
polysaccharides into monosaccharides, because only monosaccharides can pass into the
bloodstream. The monosaccharide units are connected to each other by glycosidic bonds.
Glycosidic bonds are cleaved by hydrolysis. In the body, this hydrolysis is catalyzed by acids and
by enzymes. When a metabolic need arises, storage polysaccharides—amylose, amylopectin,
and glycogen—are hydrolyzed to yield glucose and maltose. This hydrolysis is aided by a number
of enzymes:
 In acid-catalyzed hydrolysis, storage polysaccharides are cut at random points. At body
temperature, acid catalysis is slower than the enzyme- catalyzed hydrolysis.
The digestion (hydrolysis) of starch and glycogen in our food supply starts in the mouth,
where a-amylase is one of the main components of saliva. Hydrochloric acid in the stomach and
other hydrolytic enzymes in the intestinal tract hydrolyze starch and glycogen to produce mono-
and disaccharides (D-glucose and maltose).
D-glucose enters the bloodstream and is carried to the cells to be utilized. For this reason,
D-glucose is often called blood sugar. In healthy people, little or none of this sugar ends up in the
urine except for short periods of time (binge eating). In diabetes mellitus, however, glucose is not
completely metabolized and does appear in the urine. As a consequence, it is necessary to test
the urine of diabetic patients for the presence of glucose.
The latest DRI guideline, issued by the National Academy of Sciences in 2002,
recommends a minimum carbohydrate intake of 130 g/day. Most fruits or starchy vegetables are
allowed. For the longer term, an additional 5 g of carbohydrates per day is added in the form of
fruits. This restriction induces ketosis, the production of ketone bodies that may create muscle
weakness and kidney problems.
Many fad diets exist today. The Atkins diet was preceded by the Zone Diet and the Sugar
Buster’s Diet, both of which attempted to limit carbhydrate intake. Another diet suggests that you
match the foods you eat to your ABO blood type. To date, little scientific evidence supports any
of these approaches, although some aspects of many diets have merit.

Fats are the most concentrated source of energy. About 98% of the lipids in our diet are fats and
oils (triglycerides); the remaining 2% consist of complex lipids and cholesterol.
The lipids in the food we eat must be hydrolyzed into smaller components before they can
be absorbed into the blood or lymph system through the intestinal walls. The enzymes that
promote this hydrolysis are located in the small intestine and are called lipases. However,
because lipids are insoluble in the aqueous environment of the gastrointestinal tract, they must
be dispersed into fine colloidal particles before the enzymes can act on them
Bile salts perform this important function. Bile salts are manufactured in the liver from
cholesterol and stored in the gallbladder. From there, they are secreted through the bile ducts into
the intestine. Lipases act on the emulsion produced by bile salts and dietary fats, breaking the
fats into glycerol and fatty acids and the complex lipids into fatty acids, alcohols (glycerol, choline,
ethanolamine, sphingosine), and carbohydrates. These hydrolysis products are then absorbed
through the intestinal walls.
Only two fatty acids are essential in higher animals, including humans: linolenic and
linoleic acids. Nutritionists occasionally list arachidonic acid as an essential fatty acid. In reality,
our bodies can synthesize arachidonic acid from linoleic acid.
Although the proteins in our diet can be used for energy, their main use is to furnish amino acids
from which the body synthesizes its own proteins.
The digestion of dietary proteins begins with cooking, which denatures proteins.
(Denatured proteins are hydrolyzed more easily by hydrochloric acid in the stomach and by
digestive enzymes than are native proteins.) Stomach acid contains about 0.5% HCl. This HCl
both denatures the proteins and hydrolyzes the peptide bonds randomly. Pepsin, the proteolytic
enzyme of stomach juice, hydrolyzes peptide bonds on the amino side of the aromatic amino
acids: tryptophan, phenylalanine, and tyrosine (see Figure 22.4).
Most protein digestion occurs in the small intestine. There, the enzyme chymotrypsin
hydrolyzes internal peptide bonds at the same amino acids as does pepsin, except it does so on
the other side, leaving these amino Most protein digestion occurs in the small intestine. There,
the enzyme chymotrypsin hydrolyzes internal peptide bonds at the same amino acids as does
pepsin, except it does so on the other side, leaving these amino.

The human body is incapable of synthesizing ten of the amino acids in sufficient
quantities’ needed to make proteins. These ten essential amino acids must be
obtained from our food. The body hydrolyzes food proteins into their amino acid
constituents and then puts the amino acids together again to make body proteins.
For proper nutrition, the human diet should contain about 20% protein.
A dietary protein that contains all of the essential amino acids is called a complete
protein. Casein, the protein of milk, is a complete protein, as are most other animal proteins—
those found in meat, fish, and eggs. People who eat adequate quantities of meat, fish, eggs, and
dairy products get all the amino acids they need to keep healthy. About 50 g/day of complete
proteins constitutes an adequate quantity.
An important animal protein that is not complete is gelatin, which is made by denaturing
collagen. Gelatin lacks tryptophan and is low in several other amino acids, including isoleucine
and methionine. Many people on quick-reducing diets consume “liquid protein.” This substance
is simply denatured and partially hydrolyzed collagen (gelatin). Therefore, if it is the only protein
source in the diet, some essential amino acids will be lacking.
 Most plant proteins are incomplete. For example, corn protein lacks lysine and tryptophan;
rice protein lacks lysine and threonine; wheat protein lacks lysine; and legumes are low in
methionine and cysteine. Even soy protein, one of the best plant proteins, is very low in
methionine. Adequate amino acid nutrition is possible with a vegetarian diet, but only if a wide
range of vegetables is eaten. Protein complementation is one such diet. In protein
complementation, two or more foods complement the others’ deficiencies. For example, grains
and legumes complement each other, with grains being low in lysine but high in methionine. Over
time, such protein complementation in vegetarian diets became the staple in many parts of the
world—corn tortillas and beans in Central and South America, rice and lentils in India, and rice
and tofu in China and Japan. In many developing countries, protein deficiency diseases are
widespread because the people get their protein mostly from plants. Among these diseases is
kwashiorkor, whose symptoms include a swollen stomach, skin discoloration, and retarded
growth.
Proteins are inherently different from carbohydrates and fats when it comes to their
relationship to the diet. Unlike the other two fuel sources, proteins have no storage form. If you
eat a lot of carbohydrate, you will store glucose in the form of glycogen. If you eat a lot of anything,
you will store fat. However, if you eat a lot of protein (more than required for your needs), there is
no place to store extra protein. Protein in excess will be metabolized to other substances, such
as fat. For this reason, you must consume adequate protein every day. This requirement is
especially critical in athletes and growing children. If an athlete works out intensely one day but
eats incomplete protein, he or she cannot repair the damaged muscles. The fact that the athlete
may have eaten an excess of a complete protein the day before will not help.

Vitamins and minerals are essential for good nutrition. Animals maintained on diets that contain
sufficient carbohydrates, fats, and proteins and provided with an ample water supply cannot
survive on these alone; they also need the essential organic components called vitamins and the
inorganic ions called minerals.
Many vitamins, especially those in the B group, act as coenzymes and inorganic ions as
cofactors in enzyme-catalyzed reactions (Table 22.1). Table 22.2 lists the structures, dietary
sources, and functions of the vitamins and minerals. Deficiencies in vitamins and minerals lead
to many nutritionally controllable diseases.
The recent trend in vitamin appreciation is connected to their general role rather than to
any specific action they have against a particular disease. For example, today the role of vitamin
C in prevention of scurvy is barely mentioned but it is hailed as an important antioxidant. Similarly,
other antioxidant vitamins or vitamin precursors dominate the medical literature. As an example,
it has been shown that consumption of carotenoids (other than b-carotene) and vitamins E and C
contributes significantly to respiratory health. The most important of the three is vitamin E.
Furthermore, the loss of vitamin C during hemodialysis contributes significantly to oxidative
damage in patients, leading to accelerated atherosclerosis.