Sometimes, as you polish off that last bite of ultra-cheesy, deep dish pizza, you get the feeling that calories are rushing headlong toward your waistline, where they plan to retire. And that sentiment might not be entirely inaccurate. After you eat, it takes some time for calories to enter your bloodstream, in the form of glucose. If your other energy stores are full, glucose is immediately transformed into fat and deposited into fat cells throughout your body for longer-term storage. If you manage your diet properly, the fat won't stay for long. If you make a habit of over-indulging in calories, your fat deposits grow.
Energy Management
Your brain, organs, cells and tissues use energy to operate, maintain and repair themselves during every moment of the day. You obtain that energy by eating food, which has energy and other nutrients your body needs. As appealing as it may sound, you can't spend the entire day eating to ensure a steady supply of energy. Because you need energy constantly, but it only comes into the body in bursts when you eat, your body has several ways of storing it for use between meals.
ATP
The basic fuel your cells use for energy is stored in the high-energy bonds of a molecule, adenosine triphosphate, or ATP. Your body synthesizes ATP in mitochondria, microscopic power plants contained in every cell of your body. The raw material used to create ATP comes from carbohydrates, fats or proteins stored in your body. You have some ATP reserves, perhaps 30 seconds worth -- but your cells constantly demand raw fuel from your energy stores to produce ATP.
Energy Stores
One source of stored energy is glucose, a type of carbohydrate that circulates in your blood, providing fuel to cells and tissues throughout your body. A second source of stored energy is another carbohydrate, glycogen. Glycogen is stored in your muscles for local use by those muscles, and in your liver, for general use by other cells and tissues. The third type of stored energy is fat. You have 50 to 200 billion fat cells throughout your body, distributed in adipose tissue layered under your skin and above your muscles. You also store fat in the omentum, a sheath that hangs off your stomach and wraps between organs in your abdomen. These three sources of energy -- glucose, glycogen and fat -- provide for most of your energy needs, according to exercise researcher Carole Conn, University of New Mexico. Protein is generally used for repair and maintenance of body tissue, but can be used to power muscles when other energy supplies run low, according to WeightLossForAll.
Fasting Energy Management
Between meals, some of your energy stores become depleted. In healthy people, the body maintains a relatively stable amount of blood glucose, orchestrated by your pancreas. Your pancreas secretes two hormones, insulin and glucagon. When blood sugar gets low, the pancreas releases glucagon into the bloodstream. Protein molecules ferry glucagon through the bloodstream to glycogen stores in the liver and muscles, as well as to fat stores throughout your body. The glucagon transforms glycogen into glucose, helping to maintain blood glucose levels. Glucagon also releases fat, stored as triglycerides, from fat cells, transforming it into free fatty acids. The FFAs are taxied by protein molecules in the blood to muscles, where they're used by mitochondria to produce ATP.
Feeding Time
A few minutes to two hours after you've eaten, depending on the amount, type and mix of food you've eaten, glucose starts hitting your bloodstream. As your blood glucose levels rise, your pancreas releases insulin into the bloodstream. Insulin immediately transports blood glucose into cells for use in energy production. It transforms remaining glucose into glycogen and transports it into stores in the muscles and liver. As glycogen stores fill, glucose is transformed into triglycerides and packed away into fat stores. When you eat a lot of food at once -- especially if you eat fat or carbs -- which are released quickly into the bloodstream, you fill your glycogen stores, and excess calories are immediately transformed into fat, for later use. So, once your glycogen stores are filled, calories turn to fat anywhere from a few minutes to a couple hours after you eat.
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