There are people who can rattle off the caloric content of practically any food in the U.S. Dept. of Agriculture Guidebook. Ask 'em about a bagel, a carrot, a PowerBar, two stalks of broccoli, a slab of butter or a pound of meat, and they'll give you the Book of Food Counts reading without so much as a furrow of the brow.

One of the most common topics I'm asked about is calories, usually in the form of, "How many should I eat?" Like most seemingly simple questions, this one is more difficult to answer accurately than you might think.

First of all, let's get to know the cast of characters. A calorie is a simple measure of energy. In chemistry terms, it's basically the amount of heat it takes to raise the temperature of a kilogram of water 1 degree centigrade. Scientists figure out the amount of calories in food by burning it. When the food burns, the chemical bonds break, releasing energy in the form of heat. The amount of heat given off is a direct measure of the food's energy value (i.e., how many "calories" it has).

The body uses calories for several classes of activity. By far the biggest portion of our caloric expenditure goes to keeping us alive. These are the calories that keep us breathing, that grow hair and nails, make new red blood cells, maintain body temperature.... This number of calories is called the BMR, or Basal Metabolic Rate, and it accounts for about 2/3 of all the calories we "spend" during the day. When you calculate how many calories you need a day, you usually start with a formula that estimates your BMR and then add more calories depending on how much you exercise and other factors we'll talk about later.