Food as Fuel

Q.How is the caloric value of foods determined? Is it possible for food to cause us to gain more weight than it weighs itself?

A.Food is, simply put, fuel, and some fuels burn hotter than others. The amount of energy produced by burning it is measured in calories. A calorie is the amount of energy needed to raise the temperature of a gram of water by 1 degree centigrade, but for convenience, dietitians use a unit called a kilocalorie, equal to 1,000 calories.

Caloric values of foodstuffs are determined by burning them in a container. The usual method is to weigh a small amount of the food, place it in a sealed container called a bomb calorimeter, and ignite it. The calorimeter is immersed in a known volume of water. The rise in temperature when the food is completely burned gives the calorie value.

In general, fatty foods have the most calories per unit of weight, while carbohydrates and proteins have many fewer calories in the same weight. Roughly speaking, the American Medical Association says, a gram of fat has 9 calories, while a gram of carboydrates produces 4 to 5 calories.

After food is digested, or broken down into its components, some, like vitamins, may pass into the blood unchanged, some turn into products that are stored in the body or that become part of the body tissues themselves, and some, like the cellulose in lettuce, may pass through the body without burning.