https://longreads.com/2024/04/18/crohns-life-without-eating/
During digestion, physiological responses are triggered in the brain by the vagus nerve, contributing to the feeling of fullness. Hormonal signals also act on the brain: leptin, a hormone produced by fat cells, sends signals to the hypothalamus to inhibit hunger. In patients on TPN or enteral nutrition, leptin does increase after infusions, but it doesn’t appear to be well correlated with decreased hunger.
While the hormones and neural signals are crucial to satiation, so is the sensory experience that takes place during the first phase of digestion—the cephalic phase—which begins at the sight of food. The pleasure that we take during this phase appears to be important to feeling satisfied. Monkeys on TPN continued to eat real food even when their caloric needs were met. Studies in healthy humans found that people on TPN reported being as hungry as those injected with only lactated Ringer’s, a solution designed to replenish electrolytes and fluid rather than calories. I asked an on-call gastroenterologist once what I could do for the hunger.
Chewing food, even without swallowing, helps to activate the cephalic phase, triggering a partial sense of satiation.
The 36 participants were underfed until they lost 25% of their body weight. As the experiment progressed, Ancel Keys, the nutritionist running the study, noticed odd psychological effects. The participants became increasingly focused on food, collecting recipes, and taking down pin-ups of women to hang pictures of food. One even decided he would change careers and become a chef. After the study, most participants gorged themselves long after their weight returned to normal.
