RAYMOND W. SWIFT
OUR need for energy rather than our need for the other nutrients in food undoubtedly is the basis of our instinct and desire to eat. A person could die for lack of a particular food nutrient other than energy with no feeling of hunger.
The energy content of fuels or foods signifies the potential chemical energy which may be released as heat upon combustion and which is measured in heat units Calories. This measure of the gross energy of foods is the basis of calorimetry.
We use the bomb calorimeter to determine the heat evolved during combustion. It is a steel container in which the food is burned. The container is immersed in a known amount of water. A weighed amount of food is ignited by an electric fuse in the bomb calorimeter, which is filled with oxygen under high pressure. The rise in the temperature of the water (and bomb) upon ignition provides the means of measuring the heat produced.
A calorie (spelled with a small c), a unit measure of heat, is the amount of heat necessary to raise the temperature of 1 gram of water 1 degree centigrade (Specifically, from 14.5 to 15.5 C.).
The Calorie (spelled with a capital C) is the amount of heat necessary to raise the temperature of 1 kilogram (1,000 grams) of water 1 degree centigrade. This is the unit commonly used in expressing energy values of foods. A kilogram is equal to 2.2 pounds. A teaspoonful of sugar (4 grams), for example, provides about 16 Calories (16,000 calories).
METABOLISM is a term used to denote the chemical changes constantly taking place in living matter. The oxidation of foodstuffs by the human body provides the energy that serves it in a manner somewhat like that of a fuel in a furnace or machine.
Our bodies may be thought of as machines, which run 24 hours a day throughout life. The machines must continue to run, even when no food is available. Then the energy comes from the oxidation of body tissue. The body itself, particularly its fat, is thus a readily available reserve supply of food and is used continually in metabolism.
But when we think of the body as a machine that gets its fuel in the form of food, we must be aware of some differences. We can make a clear distinction between an engine and its fuel. In the human body, the food is absorbed and becomes a part of the body before it is available for any purpose.
Dr. Henry P. Armsby (1853-1921), a nutrition expert of international renown, has likened the intake, storage, and disposal of food energy to the exchange of water in a millpond:
"The water in the pond may represent the materials of the body itself, while the water running in at the upper end represents the supply of matter and energy in the food, and that going down the flume to the millwheel the metabolism required for the production of physiological work. . . . The water flowing into the pond does not immediately turn the wheel, but becomes part of the pond and loses its identity. Part of it may be drawn into the main current and enter the flume comparatively soon, while another part may remain in the pond for a long time."
Our bodies are not heat engines. A steam engine receives its mechanical power from the heat evolved during the combustion of the fuel. Fuel is burned before any work is done.
Muscular activity is more like the operation of a storage battery. Muscular contraction takes place as a result of a discharge of energy stored in the muscle. The energy is replenished during the recovery period. The net result in a complex series of reactions is the oxidation of lactic acid, a product formed when sugar is metabolized by the muscle. The heat associated with muscular activity is a waste product.
Heat is not a food nutrient. The common statement that the primary purpose of carbohydrates is to furnish calories is not wholly true. A part of the potential energy of ingested carbohydrate may take the form of mechanical energy, or it may be converted into fat and stored. In any event, our bodies do not use heat as their source of energy.
Heat from outside our bodies cannot replace the heat produced by the oxidative reactions in normal metabolism. Heat and mechanical energy are end products of the potential chemical energy of the food nutrients that are oxidized within the body. All forms of energy can be converted into heat. It is convenient to express the energy exchanges of the body processes in terms of heat units Calories.
Another distinction between the human body and a mechanical engine is that the "human engine" can perform 10 times more work for a time than its current oxygen supply would permit. This process involves an "oxygen debt," which is made up during periods of rest the recovery periods.
Our bodies differ from mere machines also in that all repairs are made while they operate at full schedule.
The absorption of oxygen does not cause the oxidation of food. There is no lack of oxygen in diabetes, but sugar is not oxidized.
Absorption of oxygen does not cause metabolism, but the amount of the metabolism determines the amount of oxygen to be absorbed. The body heat produced is a manifestation rather than a cause or regulator of metabolism.
All organic nutrients can serve the body as sources of energy. Their energy values provide a common basis for expressing nutritive value.
The main sources of energy are the carbohydrates, fats, and proteins. These nutrients, particularly protein, have other unique and specific functions as well, but that does not detract from their usefulness as sources of energy.
The gross energy of food as determined by the bomb calorimeter is not the same as the heat derived from it by the human body: Some of the food may be indigestible and so does not enter the body proper; some of the energy of the food may be represented by the storage of body tissue or by mechanical work; protein is oxidized in the body less fully than in the bomb calorimeter.
The heat-of-combustion values of all fats (and also of carbohydrates and proteins) are not identical. Differences in any one class are not very great, however. The heat-of-combustion value of fat is much higher than that of carbohydrate, because a much larger portion of the carbon and hydrogen in the carbohydrate molecule is already oxidized.
Studies with the bomb calorimeter have given us these averages in Calories per gram: Carbohydrates, 4.1; fats, 9.5; proteins, 5.7.
Carbohydrates sugars and starches are about 98 percent digestible and therefore furnish energy equivalent to a little more than 4 Calories per gram (98 percent of 4.1).
Human beings can digest different types of fat more or less equally well. The digestibility is commonly taken as 95 percent. One gram of dietary fat (equivalent to 9.5 Calories) makes available to the body 9 Calories (9.5 Calories X 95 percent). Thus a given weight of dietary fat furnishes to the body 2.25 times as much energy as does the same weight of carbohydrate. (The apparent digestibility of the ether extract "fat" of certain foodstuffs, such as whole-wheat flour, may be as low as 60 percent).
The end products of carbohydrate and fat when oxidized by the normal healthy person are the same as are obtained from combustion in the bomb calorimeter carbon dioxide, water, and heat.
The body does not oxidize protein completely but eliminates in the urine a residue of the protein molecule in the form of urea, creatinine, uric acid, and so on. This incomplete oxidation and the 92-percent absorption of the food protein make about 4 Calories available to the body for every gram of protein we eat.
Thus if we know the total carbohydrate, fat, and protein in a food we can estimate the total available Calories contained in it.