How does metabolism work?

Though it may seem we have endless reasons to eat (to celebrate holidays, to beat stress, to critique Super Bowl commercials), there's only one real reason why we need food: for energy. That energy allows our organs to function, our muscles to move, and our bodies to keep warm. And to a large extent, our brains help control how we convert food to energy. To help understand it, I'll break the metabolic path down to two phases of metabolism—the process that your body goes through to use energy.

Digestive Phase: Your hypothalamus (one of the brain's key command centers for your body) orchestrates this phase of metabolism by receiving signals all throughout your body about whether you're hungry or not—so that your body can use energy to power itself.

Here's how: Your body has a short-term reservoir for energy in the form of glycogen, a carbohydrate primarily stored in your liver and muscles. After eating, when you have glucose (sugar) and insulin (the hormone produced in the pancreas to transport glucose), your body uses all of the glucose it needs for immediate fuel, but takes the rest and stores it as glycogen. If your blood glucose level falls, your pancreas stops releasing insulin—and then releases another G substance, glucagon, which converts the stored energy (glycogen) to sugar (glucose). So the effect is that when your intestinal gas tank empties of sugar (i.e., when our ancestors were fasting between bison hunts), your body is still able to supply crucial energy to your central nervous system by converting glycogen to glucose.

Fasting Phase: When you're sleeping or go long periods without eating, your body needs to have a supply of energy to keep your organs functioning. Once you use up all of your available glucose during the digestive phase of metabolism (your body only stores about 300 calories in the short-term glycogen reservoir), it taps a long-term reservoir— fatty tissue in the form of triglycerides (molecules that include a carbohydrate containing glycerol). This keeps you going until you break the fast with breakfast. This is where weight loss starts to take place.