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PHYSIOLOGICAL BASIS OF EATING

• Eating is ingesting food to provide for all humans and animals nutritional needs,
particularly for energy and growth
• People commonly have two or three meals a day at regular times
• Having three well balanced meals will then account to some 1800 to 2000 kcal
which is the average requirement for a regular person
• Leading nutritionists believe that instead of indulging oneself in three large meals
each day, it is much healthier and easier on the metabolism to eat five smaller
meals each day
• Emotional eating is the tendency to eat in response to negative emotions.
Empirical studies have indicated that anxiety leads to decreased food consumption
in people with the normal weight and increased food consumption in the obese

FEEDING CENTERS IN BRAIN

• Group of cells in the lateral hypothalamus that when stimulated cause a sensation
of hunger
• The lateral hypothalamus or lateral hypothalamic area is a part of the
hypothalamus concerned with the hunger
• Damage to this area can cause reduced food intake
• Stimulating the lateral hypothalamus causes a desire to eat while stimulating the
ventromedial hypothalamus causes a desire to stop eating
• Lateral hypothalamus is known as hunger center and ventromedial hypothalamus
is known as the satiety center

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DUAL CENTER HYPOTHESIS

• Hypothalamus contains two centers responsible for physiological mechanisms of

feeding, one responsible for hunger and the other for satiety. This hypothesis came
to be known as the dual center hypothesis or dual control theory of feeding
• The stomach exhibits peristaltic movements. Normally when the stomach is empty
it produces vigorous peristaltic movements and it signals to the hunger center of
CNS, the feel of hunger is given by LH center
• When the stomach is filled with the food, the stomach wall stretches, the
stretching activity activates stretch receptors of stomach wall. This stretching of
stomach will be given to the CNS giving the feeling of satiety by VMH center
• They two areas of brain are involved with the control of eating hence feeding is
under the control of dual mechanism by the brain
• Lesioning of lateral hypothalamus leads to Aphagia or lack of eating and
stimulation of lateral hypothalamus makes the individual feel hungrier even after
eating
• Lesioning of ventromedial hypothalamus leads to Hyperphagia or overeating and
stimulation ventromedial hypothalamus would produce the feeling of having had
food too much and try to avoid further consumption of food, even if it continued to
taste good

HUMAN DIGESTIVE SYSTEM

• Human digestive tract includes a receiving organ, a region of conduction and

storage, a region of internal trituration and early digestion, a region of water
absorption and the feces formation
• The receiving organ is mouth; the ducks of the salivary glands open into the mouth
cavity
• The region of conduction and storage is esophagus
• The stomach is the region of internal trituration and early digestion

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• The region of stomach into which esophagus opens is called cardiac stomach
followed by fundus and the posterior part of the stomach opens into intestine and
is called pyloric region. The wall of stomach is composed of circular, longitudinal
and diagonal muscles. The internal lining of the stomach or mucosa contains
gastric glands
• The region of final digestion and absorption is the small intestine
• Small intestine has two parts, duodenum is very short, the bile duct and pancreatic
duct open into it. The reminder of the small intestine is highly coiled and vascular
and is concerned with the absorption of digested food. The epithelial lining of the
intestine is provided with the several finger-like projections called villi which
increases the surface area for absorption
• The region of water absorption and the feces formation is the large intestine or
colon
• Colon leads to rectum which opens out by anus

EATING SIGNALS

• Physiological mechanisms that control starting and stopping a meal is called as on

and off eating signals
• Hormones such as Cholecystokinin (CCK), Bombesin (gastrin releasing peptide
hormone), Neurotensin (found in enteroendocrine cells of small intestine),
Anorectin (reduce food intake), Calcitonin (calcium regulation), Enterostatin
(inhibit fat intake), Leptin (satiety hormone made by adipose cells), corticotrophin
(releasing hormone have shown to suppress food intake), Ghrelin (from
gastrointestinal tract is a stimulant of hunger) are found to control these processes.
• The feeling of hunger could be triggered by the thought and smell of food, sight of
a plate, or hearing someone talk about food
• The signals from the stomach are initiated by the release by the peptide hormone
ghrelin that increases appetite by signaling to the brain

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• When the glucose levels of the cell drop the body starts to produce the feeling of
hunger
• The body also simulates eating by detecting a drop in cellular lipid level
• Both brain and liver monitor the level of metabolic flow fuels; the brain checks for
glucoprivation on its side of blood brain barrier while the liver monitors the rest of
the body for both lipoprivation and glucoprivation
• There are two peptides in the hypothalamus that produce hunger which are
melanin concentrating hormone (MCH) and orexin.
• MCH play a greater role in hunger and orexin play a role in controlling
relationship between eating and sleeping.
• Other peptide of hypothalamus that induce eating is a Agouti Related Protein
(ARP) and that which stimulates satiety is leptin. Leptin targets the receptors on
arcuate nucleus and suppresses the secretion of MCH and orexin.
• The arcuate nucleus also contains two more peptides that suppress hunger which
are Cocaine and Amphetamine Regulated Transcript (CART) and α Melanocyte
Stimulating Hormone (α MSH)
• The two neurotransmitters namely noradrenalin and serotonin from the medial
hypothalamus play an important role in eating behavior. Noradrenaline stimulates
intake of carbohydrate and serotonin inhibit it
• Electrical stimulation of areas like amygdala, hippocampus, thalamus and frontal
cortex also enhances eating
• The short-term signals of satiety arise from head, the stomach, the intestine and
the liver. The stomach contains receptors to allow us to know when we are full.
The intestine and also contain receptors that send satiety signals to brain. The
hormone CCK secreted by duodenum controls the rate of emptying of the stomach
and known as satiety signal to the brain
• The long-term signals of satiety come from adipose tissue. Adipose tissue secretes
the hormone leptin which suppresses appetite. During hunger leptin decreases

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which causes the release of ghrelin which initiates hunger. When leptin level rises,
they bind receptors in an ARC (protein), suppresses the release of neuropeptide Y
which prevent appetite by the production of orexin that binds lateral hypothalamus

LIPOSTATIC HYPOTHESIS

• According to this hypothesis, adipose tissue produces a humoral signal that is

proportional to the amount of fat and act on hypothalamus to decrease food intake
and increase energy output
• Hormone leptin act on hypothalamus in this way
• The fat or lipid is stored in adipose tissue mostly lying beneath the skin
• The body fat is normally maintained at a relatively constant level
• Everyone’s body has a set point for body fat and deviation from the setpoint leads
to compensatory adjustments in food intake

THERMO STATIC HYPOTHESIS

• According to this hypothesis, a decrease in body temperature below a given set

point stimulates appetite, whereas an increase above the set point inhibits appetite

OBESITY

• Overweight and obesity are defined as abnormal or excessive fat accumulation

that may impair health
• Body mass index or BMI is a simple index of weight-for-height that is commonly
used to classify overweight and obesity in adults. It is defined as a person’s weight
in kilograms divided by the square of its height in meters (kg/m2)
• BMI = m/h2

where m and h are the subject’s weight and height respectively

• For most adults, a BMI of:

➢ 18.5 to 24.9 means you are a healthy weight

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➢ 25 to 29.9 means you are overweight

➢ 30 to 39.9 means you are obese
➢ 40 or above means you are severely obese

Effects of obesity on health

• Excessive body weight is associated with various diseases and conditions,

particularly cardiovascular diseases, diabetes mellitus type 2, obstructive sleep
apnea, certain types of cancer, osteoarthritis and asthma
• Obesity reduces life expectancy by six to seven years

Causes of obesity

1.Consuming too many calories

• People eat much more food than in previous generations.

• Many people are not physically active, so lots of the calories they consume end up
being stored in their body as fat

2.Leading a sedentary lifestyle

• With the arrival of modern appliances, people are commonly leading a much more
sedentary lifestyle compared to their previous generations
• Physical activity has an effect on how your hormones work and hormones have an
effect on how your body deals with food
• Several studies have shown that physical activity has a beneficial effect on insulin
levels keeping them stable but due to unstable insulin levels weight gain is
becoming a serious problem
• Adults should do at least 150 minutes of moderate intensity aerobic activity every
week

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3.Poor diet

• Obesity develops gradually overtime as a result of poor diet and lifestyle choices
such as eating large amounts of processed or fast food that is high in fat and sugar
• Drinking too much alcohol contains a lot of calories and they end up overweight
• Eating out a lot and foods higher in fat and sugar and drinking too many sugary
drinks including soft drinks and fruit juice results in obesity
• If you have low self esteem or feel depressed you may eat to make yourself, feel
better
• Unhealthy eating habits tend to run in families. You may learn bad eating habits
from your parents when you’re young and continue them into adulthood

4.Genetics

• Like many other medical conditions, obesity is the result of an interplay between
genetic and environmental factors
• Polymorphisms in various genes controlling appetite and metabolism predispose
to obesity when sufficient food energy is present
• Obesity is a major feature in several syndromes such as Prader-Willi syndrome,
Bardet-Biedl syndrome, Cohen syndrome and MOMO syndrome
• Different people exposed to the same environment have different risks of obesity
due to their underlying genetics
• The thrifty gene hypothesis postulates that due to dietary scarcity during human
evolution people are prone to obesity

5.Medical and psychiatric illness

• In some cases, underlying medical conditions may contribute to weight gain

• These include an underactive thyroid gland and Cushing’s syndrome

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• Certain medicines including some corticosteroids, medications for epilepsy and

diabetes and some medications used to treat mental illness like antidepressants and
medicines for schizophrenia can contribute to weight gain
• Weight gain can sometimes be a side effect of stopping smoking
• The risk of overweight and obesity is higher in patients with psychiatric disorders
• Certain medications may cause weight gain or changes in body composition which
include insulin, sulfonylureas, thiazolidinediones, typical antipsychotics,
antidepressants, steroids, certain anticonvulsants and some forms of hormonal
contraception.

6.Not sleeping enough

• Researchers found out that if you do not sleep enough your risk of becoming obese
doubles
• The risk applies to both adults and children
• Sleep deprivation significantly increased obesity risk in both groups

7.Social determinants

• Among developed countries, levels of adult obesity and percentage of teenage

children who are overweight are correlated with income inequality
• Attitude towards body weight held by people in one’s life may also play a role in
obesity
• Stress and perceived low social status appear to increase the risk of obesity
• Smoking has a significant effect on an individual’s weight
• Obesity is also associated with cognitive deficits

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Management of obesity

• The main treatment for obesity consists of dieting and physical exercise
• The most effective treatment for obesity is bariatric surgery however due to its
cost and the risk of complications researchers are searching for other effective yet
less invasive treatments

1.Dieting

• Diets to promote weight loss are generally divided into 4 categories: low fat, low
carbohydrate, low calorie and very low calorie
• These diets are not recommended for general use as they are associated with
adverse side effects such as loss of lean muscle mass, increased risks of gout and
electrolyte imbalances
• People attempting these diets must be monitored closely by a physician to prevent
complications

2.Exercise

• With use muscles consume energy derived from both fat and glycogen
• Due to the large size of leg muscles walking, running and cycling are the most
effective means of exercise to reduce body fat
• To maintain health, the American Heart Association recommends a minimum of
30 minutes of moderate exercise at least five days a week
• High levels of physical activity seem to be necessary to maintain weight loss

3.Weight loss programs

• They often promote lifestyle changes and diet modification

• This may involve eating smaller meals, cutting down on certain types of foods and
making a conscious effort to exercise more

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• These programs also enable people to connect with a group of others who are
attempting to loss weight, thus the participants can form mutually motivating and
encouraging relationships

4.Medication

• The two most commonly used medications to treat obesity are orlistat (Xenical)
and sibutramine (Meridia)
• Orlistat reduces intestinal fat absorption by inhibiting pancreatic lipase and
sibutramine acts in the brain to inhibit deactivation of the neurotransmitter
norepinephrine, serotonin and dopamine therefore decreasing appetite
• Weight loss with these drugs is modest
• The use of these drugs is not recommended due to potential side effects

5.Surgery

• Bariatric surgery or weight loss surgery is the use of surgical intervention in the
treatment of obesity
• Surgery is only recommended for severely obese people who have a BMI greater
than 40 and who have failed to lose weight following dietary modification and the
pharmacological treatment
• Weight loss surgery relies on various principles: the two most common
approaches are reducing the volume of stomach which produces an earlier sense of
satiation and reducing the length of bowel that comes into contact with food which
directly reduces absorption
• Complications from weight loss surgery are frequent
• Surgery for severe obesity is associated with long term weight loss and decreased
overall mortality

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SPECIFIC HUNGER

• A type of hunger that is satisfied by specific dietary requirements such as vitamins

and minerals
• Many animals vary their food intake according to the nutritive value of the
products of digestion
• The simplest mechanism is the direct detection of substance in the food as is the
case with sodium
• In a free natural environment, there are few complete food stuffs for any particular
species
• Animals are therefore faced with the task of identifying and ingesting a variety of
foods which will constitute a nutritionally complete diet
• They must also be able to identify and discriminate those foods which may be
safely ingested from among poisonous or inedible foods
• There is very little strong evidence for specific appetite in humans.
• However, it has been demonstrated that humans have the ability to taste calcium
and indirect evidence supports the idea that patients on kidney dialysis who
develop hypocalcemia prefer cheese with the greater amounts of calcium added
• Exercise also increases the preference for salt
• Addison’s disease is known to induce a specific craving for sodium although other
diseases causing hyponatremia may not induce the same response
• Extreme sodium depletion in human volunteers has been demonstrated to increase
the desire for high salt foods

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