Mechanism of Regulation of Food Intake
Under normal physiological
conditions, appetite and food intake are well balanced and continues in a
cyclic manner. Feeding
center and satiety center of hypothalamus are responsible for the regulation of
appetite and food intake.
These centers are regulated by the following mechanisms:
i. Glucostatic mechanism
ii. Lipostatic mechanism
iii. Peptide mechanism
iv. Hormonal mechanism
v. Thermostatic mechanism.
Glucostatic Mechanism
Cells of satiety center
function as glucostats or glucose receptors, which are
stimulated by increased blood glucose level.
While taking food, blood
glucose level increases. Slowly the glucostats are stimulated and satiety
center is activated. At one stage, it develops the feeling of ‘fullness’. Now,
the satiety center inhibits the feeding
center and stops the food
intake. After few hours of food intake, the blood glucose level decreases and satiety
center becomes inactive. So, the feeding center is no longer inhibited. Now it becomes
active and increases the appetite and induces food intake. After taking food,
once again blood glucose level increases and the cycle is repeated. However,
glucostats do not give response to very high level of glucose in blood (hyperglycemia). So, in
conditions like diabetes, hyperglycemia fails to stimulate the satiety center.
The satiety center does not inhibit the feeding center, so the frequency of
food intake increases (polyphagia).
Lipostatic Mechanism
Leptin is a
peptide secreted by adipocytes (cells of adipose tissue). It plays an
important role in controlling the food intake and adipose tissue volume. When
the volume of adipose tissues increases,
adipocytes secrete and
release a large quantity of leptin into the blood. While circulating through
brain,
leptin crosses the
blood-brain barrier and enters hypothalamus. In hypothalamus, leptin inhibits
the feeding center, resulting in loss of appetite and stoppage of food intake.
It is suggested that the cells present in bloodbrain barrier contain many
receptor-like proteins, which are responsible for the transport of leptin
across the barrier.
Mode of
action of leptin
Leptin
acts through some specific neuropeptides inhypothalamus, such as:
a. Neuropeptide Y: It is
secreted in small intestine, medulla and hypothalamus. Normally, this peptide
stimulates the food
intake. But, leptin inhibits neuropeptide Y, leading to stoppage of food
intake.
b. Pro-opiomelanocortin (POMC): It is secreted from
anterior pituitary. It is also secreted from
hypothalamus, lungs, GI
tract and placenta. Normally, it inhibits food intake. Leptin stimulates
the secretion of POMC.
Leptin receptor
Many
leptin receptors are identified. However, leptin acts via ‘LepRb’, which is
the only active receptor present in many
nuclei of hypothalamus.
Peptide Mechanism
Some peptides regulate the
food intake either by stimulating or inhibiting the feeding center, directly or
indirectly. The important
one among the peptides is ghrelin.
Ghrelin is
secreted in stomach during fasting. It directly stimulates the feeding center
and increases the appetite and food intake. Besides ghrelin, several other
peptides are involved in the regulation of
food intake.
Peptides, which increase
the food intake:
a. Ghrelin
b. Neuropeptide Y.
Peptides, which decrease
the food intake:
a. Leptin
b. Peptide YY.
Hormonal Mechanism
Some endocrine hormones
and GI hormones inhibit the food intake by acting through hypothalamus.
Hormones which inhibit the
food intake:
a. Somatostatin
b.
Oxytocin
c. Glucagon
d. Pancreatic polypeptide
e. Cholecystokinin.
Thermostatic
Mechanism
Food intake is inversely
proportional to body temperature. So in fever, the food
intake is decreased. Exact
mechanism of this fact is
not known. It is suggested that the preoptic thermoreceptors (see
above) may act via feeding center. The cytokines are also suggested to play a
role in decreasing the appetite during fever.
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