Food is digested by enzymes, and its energy-containing molecules are absorbed by cells. In turn, cells generate ATP by the process of cellular respiration.

Energy, given off as heat, is a by-product of the respiration process. This energy is measured in calories. In fact, 1 calorie is the amount of heat needed to raise the temperature of 1 gram of water by 1 degree Celsius. Because this is a tiny amount of heat, we usually express food energy in kilocalories. A kilocalorie is equal to 1000 calories.

When nutritionists and other people talk about calories, they are usually referring to kilocalories, or Calories with a capital C. Otherwise people would be counting millions of calories each day!

A calorimeter measures the calories available in food. You burn a known amount of food and see how much it raises the temperature of a known volume of water.

By using the calorie as a measure of heat, we can get an idea of the amount of energy we get from food. 1 gram of fat provides 9.5 kilocalories of heat, 1 gram of carbohydrate generates 4.2 kilocalories, and 1 gram of protein generates 4.1 kilocalories.

The total amount of energy an animal uses in a given time period is called its metabolic rate. The basal metabolic rate, or BMR, is the amount of energy an animal uses at rest. For humans, the BMR is between 1300 to 1500 kilocalories per day for an adult female and between 1600 and 1800 kilocalories per day for an adult male. Metabolic rates are higher during exercise.

When an animal takes in more calories than it needs to maintain its metabolic rate, the excess energy is stored by the body. For short-term storage, the liver and muscles store excess energy as glycogen, a carbohydrate. But carbohydrate storage is inefficient, because the amount of glycogen needed to release a given amount of energy is rather high.

Fats are more suited to long-term energy storage. They provide more calories per gram than carbohydrates, partly because there is less water associated with fat storage. Carbohydrates are converted into fats called triglycerides by the procdess of glycolysis. Triglycerides are the form in which most fat exists in food, as well as in the body.

If fat storage is more efficient than carbohydrate storage, why use glycogen at all? The answer is availability. Glycogen is available to provide energy in the body very quickly, whereas fat becomes available only after about 20 minutes of continuous exercise. Most people don’t usually exercise for this long. However, regular exercise can create a calorie deficit that results in the loss of fat over a period of time. Also, aerobic exercise raises the level of several hormones that promote a greater use of fat by the body. The best way to crank up your body's fat-burning ability is to keep active.

Food isn’t used only as a source of energy for body processes. Food also provides the raw organic materials from which the body synthesizes its own components. Next, we’ll look at how food provides the building blocks for the body itself.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education