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| How Your Brain Signals Your Body's Need for Food |
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.
