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What is Food Energy?

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Food energy is measured in units called large, or kilogram, calories. One calorie is equal to the amount of heat needed to raise the temperature of 1 kilogram (2.2 pounds) of water from 14° C to 15° C (59° F to 60.8° F).

To determine the calories in a food a dry sample of the food is burned in a bomb calorimeter ; the heat given off by the food in burning is measured by noting the change in the temperature of a known weight of water surrounding the bomb. Calories furnished by foods are also determined indirectly by measuring the volume of oxygen needed to burn a dry sample of the food in an oxycalorimeter.

For example, one average egg, two large heads of lettuce, one and one-third cups of tomato juice, three lumps of sugar, and one square of butter (3/8 inch thick) each furnishes about 75 calories. Lettuce is made up largely of cellulose, which does not furnish calories and therefore has few calories per unit of weight. Tomato juice is low in calories because it contains a high percentage of water. Water does not furnish calories. Butter, however, is high in calories because it is mostly fat.

The energy or food requirement of an individual is determined in one of the following ways: (1) careful observation of the diet to determine the amount of food consumed, (2) measurement of the energy expenditure directly in a respiration calorimeter or indirectly by means of other types of respiration apparatus, or (3) determining the balance of intake and output through carbon and nitrogen balance experiments.

In determining the energy expenditure directly, the amount of heat given off by the body in performing a given activity is measured by means of a respiration calorimeter. This instrument consists of a chamber that is insulated and can be made airtight. The heat is measured by noting the change in the temperature of the water that circulates through coils in the chamber. The respiration calorimeter is also equipped for the measurement of oxygen used or carbon dioxide produced.

In determining the energy expenditure indirectly, the amount of oxygen consumed or carbon dioxide produced in performing a given activity is measured by means of any one of a number of types of respiration apparatus.

Whether the heat produced by the body is measured or the oxygen used in respiration or the carbon dioxide given off by the lungs makes little difference because each is proportional to the amount of combustion taking place within the body.

The basal metabolism determination is made by measuring the oxygen consumption in the morning when the subject is in a resting position, relaxed, comfortable, awake, and before any food is taken. Under these conditions the energy needed just to support the internal work of the body is determined.

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