Encyclopedia of Health and Medicine

178 Nutrition and Diet
The hunger center resides within the HYPOTHAL-
AMUS, a structure of the midbrain that integrates
neurologic and hormonal activity to maintain
essential body functions. The hunger center
responds primarily to the level of GLUCOSE (the
form of sugar that is the primary fuel for the
body’s cells) in the BLOOD, activating the appetite
center when the blood glucose level drops. The
hunger center’s activation triggers a cascade of
response from the hypothalamus that includes
sending nerve signals to the appetite center and
the cerebral cortex to stimulate the desire to eat,
hormonal signals to the gastrointestinal tract to
begin releasing DIGESTIVE HORMONES (such as gastrin,
secretin, cholecystokinin, and pepsin), and
neurohormonal signals that result in an increase
of acetylcholine, a NEUROTRANSMITTER that facilitates
smooth MUSCLE contraction, in the gastrointestinal
tract. These events establish a cycle that
continues until blood glucose levels rise. Because
the hunger center responds to neurohormonal signals
related to basic survival, neither it nor its
influence on the appetite center is within conscious
control.
The satiety center also resides in the hypothalamus
near the hunger center. It responds to nerve
signals from the hunger center and from the
appetite center. As food enters the SMALL INTESTINE
for the main phase of digestion, the small intestine
releases peptide YY, a HORMONE that signals the
satiety center. The satiety center in turn sends
nerve messages to the hunger center and to the
appetite center, signaling that the body no longer
needs to consume food. Concurrently the balance
of digestive hormones begins to shift, further signaling
the satiety center as well as slowing the signals
going to the appetite center.
Appetite Response
Appetite is a powerful mechanism intended to
bring food (energy) into the body. Though aspects
of appetite represent areas of conscious control,
appetite response is not simply an issue of
willpower or of survival. Some people eat a small
amount, feel satisfied, and stop eating. Other people
eat large amounts of food and do not feel satisfied,
even when they begin to feel physically
uncomfortable because they have eaten more
than enough to fill their gastrointestinal tracts.
Appetite appears to be a short-term feature of
energy management designed to meet the body’s
daily energy needs; how appetite correlates with
the body’s available stores of surplus energy (in
the form of body fat) remains a mystery.
There is some evidence that continued exposure
to the smells of food without eating may signal
both the satiety center and the appetite center
that the body is consuming enough food, even
when a person is only smelling, not eating, food.
However, manipulating the appetite is not so easy.
Establishing EATING HABITS that provide adequate
CALORIE and nutrition intake helps maintain balance
among the appetite, hunger, and satiety centers.
This is particularly important for WEIGHT LOSS
AND WEIGHT MANAGEMENT as well as for overall
health maintenance.
See also DIGESTIVE ENZYMES; METABOLISM; OBESITY
AND HEALTH; STARVATION.