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James A. Pierce, age 12, of Shreveport, Louisiana, for his question:

How do they determine the calorie value of foods?

Scientists perform these wonders by testing the amount of energy each bite of food can give to the body. The calorie, with a small c, is a unit of heat energy. The large Calorie, spelled with a capital C, is worth 1,000 small calories. This is the unit used to determine the value of foods. So if we wish to be strictly scientific, the values on a diet chart should be spelled Calories.

The body breaks down the food we eat to produce energy that can be measured as heat. It uses a very slow burning process called combustion. In mechanical terms, the heat produced by this process can be called energy because it can be converted into energy to run the multitude of chemical activities in the cells. Hence it is easier to determine the value of foods according to how much heat the body can extract from them. And the basic unit for measuring small amounts of this sort of heat is the small calorie.

The calorie is defined as the amount of heat required to raise the temperature of a gram of water by one degree centigrade. The weight unit of one gram equals only 0.0353 ounces    and one degree centigrade equals 1.80 of the Fahrenheit degrees we use to take the temperature of the weather. It is necessary to base the values on the weight of the food, so the test requires small samples. However, the small calorie is too small for this job.

If we multiply it by 1,000 we have the Great Calorie, the one used to determine the heat energy value of foods. In the laboratory, the chemist weighs a food sample in grams, then puts it into the chamber of a calorimeter. This instrument consumes the food sample by a combustion process similar to the body's slow burning process. As it burns, the food gives off heat energy. The calorimeter is fitted with a thermometer that registers the rising temperature inside the chamber, plus dials to report the results to those outside.

The combustion that goes on inside the calorimeter breaks the molecules of food down into small particles. When this is done, the food has released all the heat energy it can. The temperature inside the chamber is as high as it can go. The weight of the food sample compared with the degrees gives its potential heat energy in terms of Calories.

As we know, some items on the menu yield more Calories than others. The basic building block in the starches and sugars in carbohydrate foods is glucose. When 6.4 ounces of glucose is burned in a calorimeter, it yields heat energy equal to 673 Calories. The same amount of buttery fat yields 1670 Calories and the same amount of lean meat yields 670. Many dietary charts spell out calories when they mean Calories. This is an error and we have to multiply them by 1,000 to get the true values.

Calories tell only part of the food value story. Vitamins and other essential elements may be destroyed in cooking and food processing. The body uses or loses about 60 per cent of its Calorie intake as heat. The other 40 per cent is converted into chemical energy to carry on its cellular activities.

 

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