Nutrition Science and Everyday Application - beta v 0.1

378 ALICE CALLAHAN, PHD, HEATHER LEONARD, MED, RDN, AND TAMBERLY POWELL, MS, RDN
Figure 7.6. The hypothalamus, shown in blue, is about the size of an almond and serves as the
hunger center of the brain, receiving signals from the gastrointestinal tract, adipose tissue, and
blood and signaling hunger and satiety.
Hunger is the physiological need to eat. When the stomach is empty, it contracts and
starts to grumble and growl. The stomach’s mechanical movements relay neural signals to
the hypothalamus. (Of course, the stomach also contracts when it’s full and hard at work
digesting food, but we can’t hear these movements as well because the stomach’s contents
muffle the noise.) The stomach is also the main organ that produces and secretes the
“hunger hormone,” ghrelin, the only gut hormone found to increase hunger. Ghrelin levels
are high before a meal and fall quickly once nutrients are absorbed. 2
Appetite is the psychological desire to eat. Satiety is the sensation of feeling full. After you
eat a meal, the stomach stretches and sends a neural signal to the brain stimulating the
sensation of satiety and relaying the message to stop eating. There are many hormones
that are associated with satiety, and various organs secrete these hormones, including
the gastrointestinal tract, pancreas, and adipose tissue. Cholecystokinin (CCK) is an example
of one of these satiety hormones and is secreted in response to nutrients in the gut,
especially fat and protein. In addition to inhibiting food intake, CCK stimulates pancreatic
secretions, gall bladder contractions, and intestinal motility—all of which aid in the digestion
of nutrients. 2
Fat tissue also plays a role in regulating food intake. Fat tissue is the primary organ that
produces the hormone leptin, and as fat stores increase, more leptin is produced. Higher
levels of leptin communicate to the satiety center in the hypothalamus that the body is in
positive energy balance. Leptin acts on the brain to suppress hunger and increase energy
expenditure. The discovery of leptin’s functions sparked a craze in the research world and
in the diet pill industry, as it was hypothesized that if you give leptin to a person who is
overweight, they will decrease their food intake. In several clinical trials, it was found that
people who are overweight or obese are actually resistant to the hormone, meaning their