How Your Brain Signals Your Body's Need for Food

By Jane Kirby, RD and The American Dietetic Association from Dieting For Dummies, 2nd Edition

You may think that hunger is all in your stomach and that dieting is all in your head. But nutrition experts know that hunger is regulated by a complex system of chemicals that send signals between your brain and your body.

The cells in the hypothalamus communicate with cells in other parts of the brain to coordinate the release and uptake of chemicals that help regulate how much and what you eat. Food triggers the brain to turn the desire to eat into the act of eating. How a food smells, what it looks like, and how you remember it tasting excite chemicals within your brain.

The breakdown products of foods — amino acids from protein, fatty acids from fat, and glucose from carbohydrates — regulate hormones such as insulin, which affect the process at a cellular level. They send messages to the brain telling it that fuel is needed.

When the body needs nourishment, neurotransmitters are released. One neurotransmitter called Neuropeptide Y (NPY) is important in sending messages to various parts of the brain.

Scientists have recently identified two chemicals — ghrelin and leptin — circulating in the blood that communicate with NPY.

• Ghrelin and glucose: According to the theory, low levels of glycogen and low blood sugar levels stimulate a spike in ghrelin and NPY’s activity in the hypothalamus. As NPY is stimulated, your desire for sweet and starchy foods goes up. And when ghrelin rises, so does appetite.
  While you sleep, your glycogen and blood sugar stores are used up, causing the brain to release NPY. Skipping breakfast increases NPY levels so that by afternoon, you’re set up for a carbohydrate binge. This craving for carbs is not the result of a lack of willpower; it’s an innate biological urge at work.
• The leptin link: After eating, leptin levels increase and inhibit the firing of NPY, so you feel full. If it has been a while since you’ve eaten, your blood levels of glucose are low and therefore leptin is low, and ghrelin is high.
  The circulating levels of ghrelin peak at different times depending on when you have your heaviest meal. People who eat big lunches show ghrelin peaks at a different time than people whose main meal is at night.

In addition, these processes are at work:

• The galanin-fat connection: Galanin is released when fat stores need filling up. In the evening, galanin levels tend to rise, which may be nature’s way of making sure that people have enough calories to last them through the night.
• CCK: When you eat, food enters and fills your stomach and then travels to the intestinal tract. As the food is digested and the body’s cells are fed, a chemical called cholecystokinin (CCK) is released, turning on feelings of fullness and turning off the appetite.
  Researchers think that certain conditions, such as anorexia and bulimia, may affect many appetite-control body chemicals, including CCK. In bulimics, researchers think that either the CCK mechanism doesn’t work properly or the body’s chemical systems become so desensitized that the person eats huge quantities of food quicker than the brain is able to signal satisfaction and fullness.
  The opposite effect may occur in anorexics — the CCK mechanism is so oversensitized that they feel full after only a few bites of food. When bulimics and anorexics start eating normally, their CCK systems usually normalize.