Most people call it fat. If you take a more gracious perspective, you can think of it as potential energy. As far as your body is concerned, fat is just one potential source of fuel. Whether you're lying down and resting or running full tilt for an hour, your body uses a mix of fuels, including carbohydrates, fats and proteins. The exact mix of fuel depends on the types of energy supplies that are available at the time, the intensity of the activity and the corresponding magnitude of the energy demands.
Energy Demands
Every moment of the day, even when you are resting or in deep sleep, your body requires energy. Your cells, tissues and organs constantly undergo maintenance and repair. Your heart pumps, your lungs expand and contract, and all your organs, including your brain, constantly carry on their jobs. When you exert your muscles, you require even more energy. You get this energy by eating food, but since you can't eat constantly -- as great as that may sound -- your body has to store the energy you ingest for future energy needs.
ATP
Your cells use only one fuel, adenose triphosphate, often referred to as ATP. Microscopic mitochondria, the power plants of your cells, manufacture ATP using raw material that comes from carbohydrates, fats and proteins. Your body stores 30 seconds or so worth of ATP, but constantly takes in raw material to synthesize fresh supplies.
Ever Ready
The primary source of raw fuel for your cells and body is glucose, a type of carbohydrate that circulates in your blood. When blood glucose levels rise too high, your pancreas releases insulin, causing glucose to be converted into two other forms of stored energy. Insulin causes some glucose to transform into another type of carbohydrate, glycogen. You store glycogen in your muscles for use by those particular muscles and also in your liver, for more general use. Insulin also promotes the transformation of glucose into triglycerides, a form of fat that is stored in fat cells under your skin and in your abdomen.
Energy Release
Between meals and when you exercise, some of your energy stores run low. As your cells and muscles demand energy, your blood sugar drops. This triggers your pancreas to release a hormone, called glucagon, into your bloodstream. Glucagon triggers the release of glycogen from your muscles and liver. Glycogen from your liver is converted into glucose and released into the bloodstream, helping to maintain your blood sugar level.
Glucagon also triggers the release of triglycerides from your fat cells. The triglycerides are converted into free fatty acids and transported by proteins in your bloodstream to muscles and tissues throughout your body, which convert the free fatty acids to ATP. So, when your blood sugar runs low, whether it is because you haven't eaten in a while or because you are exercising, you burn both carbohydrates and fat.
Fuel Mixture
During rest and sedentary activities, your body burns a combination of fuels, but so long as glycogen supplies are available, it primarily burns carbohydrates. During muscular exertion, you burn a combination of carbohydrates and fat. During low-intensity cardio exercise, you burn a higher proportion of fat relative to carbohydrates, exercise researcher Len Kravitz from the University of New Mexico notes. During more intensive cardio exercise, the ratio changes. You burn more carbohydrates relative to fat. However, during intensive exercise you burn more fat, carbohydrates and calories overall, even though the fuel mix is slanted toward carbohydrates.
Glycogen Depletion
The fuel mixture changes, too, when glycogen stores are depleted. You deplete glycogen supplies when fasting a long time between meals and when you engage in strenuous exercise for extended periods of time. As glycogen stores get tapped, your body relies more extensively on fat for energy, though it also uses protein.
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