Maureen Grace, age 13, of Spokane, Wash., for her question:

HOW DO THEY FIGURE FOOD CALORIES?

We all know that too many~food calories add chubby bulges to the body. Dieters consult charts that give the number of calories in measured servings of various foods  and a general reduction of calories is supposed to lead to a general reduction of bulges. Figuring the calorie content of foods is a tricky job and, though fairly accurate, not quite as perfect as it might be.

The calorie is a unit of heat energy, and heat energy is involved in the slow burning process which the body uses to digest its food. This calorie unit is the amount of heat used to raise the temperature of one gram of water by one degree centigrade. The sample is too small for testing foods. So for this they use the Great Calorie, which equals 1,000 small calories. By rights, our dieting charts should be listed in Calories, rather than calories. But the small "c" is in common usage.    

A calorimeter is used to measure the calorie content of foods. It is a special chamber which uses a combustion process, similar to the slow burning process that goes on in the body. A sample of this or that food is weighed in precise grams and consumed in the calorimeter chamber. As the food burns, the heat energy it yields is measured on a thermometer. The thermometer registers the rising temperature inside the chamber. It is attached to dials that report the findings to observers on the outside.

The combustion breaks down the molecules of the food sample, which releases heat. When all is consumed, the food has released all the heat energy it can. The temperature reaches its highest point. This is compared with the weight of the sample and the results computed to give the calories that a given helping of a certain food can yield.

Actually the system works fairly well, and a reduction of calories in the diet leads to a reduction of surplus fat. However, the body's private food processing is an infinitely complex system involving numerous small chemical miracles. For example, calories are used to move muscles  which is why more calories are needed by busy folk who do a lot of physical exercise.    

The body uses about 60% of its calorie intake to keep warm, and 40$ is used to carry on its miraculous cellular activities, including exercise. Surplus calories in the diet may be converted to fat and stored for future emergencies. Portions of this are used up between meals  but surpluses tend to accumulate.