170. Factors Used In Computing Food Values

It is shown on pp. 161-162 that the different food compounds, even of the same class, have somewhat different heat values. These are total values, also, and make no allowance for losses in the urine solids and in digestion. Riibner, basing his figures on experiments with dogs, adopted certain factors for the calorific values of the several classes of nutrients. Sherman,1 in a very recent publication, suggests somewhat smaller factors as more nearly representing the net value of the constituents of foods. Both sets of factors are riven below.

1 " Chemistry of Food Nutrition," p. 126.

171. Energy Relations Of The Several Nutrients

As has been pointed out, the animal body is the field of numerous mechanical activities which are sustained by the energy derived from the food. What is the relation of the several nutrients to these manifestations of vital energy is an interesting and, in some ways, an intensely practical matter. For instance, has protein a peculiar function in the maintenance of muscular activity which no other nutrients have? The belief prevailed at one time that muscular contraction caused a wasting of the muscle substance which must be replaced by the protein compounds of the food; in other words, protein alone was believed to sustain the work of the animal body, both internal and external. It would follow from this that the more work is done, the more protein is needed. This view is no longer held. The more exact methods of modern research have revealed the fact that an increase of muscular effort, even up to a severe point, increases but little, if any, the nitrogen compounds of the urine, these being the measure of the protein that is destroyed. There has come to light a corresponding fact that the consumption of fuel in the body other than proteins increases proportionately with the increase of work. This means that mechanical work is largely sustained through the combustion of carbohydrates and fats, and that while, for reasons we do not yet wholly understand, a fairly generous amount of protein seems to promote the well-being of the laborer, the non-nitrogenous nutrients mostly supply the extra energy demanded for the labor.

172. Heat Relations

The question is very naturally asked, as no energy is lost, into what is the energy of muscular contraction converted, as, for instance, that required for walking, the beating of the heart and the work of the intestines? It is concluded by physiologists that muscular energy used by the living organism is partly transformed into external motion and partly into heat, and this certainly is consistent with facts as observed. Violent exercise by the animal greatly increases the production of heat. We know this is so, because, under these conditions, an increased amount of blood is thrown to the surface of the body, thereby greatly increasing the loss of heat by radiation; perspiration sets in, and with it the consequent evaporation of much more moisture, thus disposing of much heat. Accurate determinations1 reveal an increase of insensible perspiration (from lungs and skin) from an average of 960 grams of water per day for men at rest, to an average of 4272 grams for men at work. This shows what an important factor is the evaporation of water from the body in heat regulation. The dog, and sometimes other animals, pants and thereby causes a large loss of heat from the expanded surface of the moist tongue. All this occurs without reducing the body temperature below the normal. In fact, nature adopts these various devices, such as increased circulation of the blood and perspiration, in order to regulate the body temperature and prevent its rising above the proper point. The explanation of this greater heat during labor is that the mechanical energy manifested by the muscles is converted to heat, which, under circumstances of severe exercise, is more than enough to keep the body at its usual temperature and maintain the usual radiation. When it is severely cold, on the other hand, vigorous exercise is sometimes necessary in order to keep sufficiently warm.

1 " Metabolism and Energy Transformations of Healthy Man during Rest," Benedict and Carpenter, p. 114.

The view is now held that all body heat is a secondary product, that combustion first supports muscular activity with heat as the waste product; in fact, that, under the majority of conditions, no food is burned primarily to keep the animal warm. There is much evidence to support this position.

173. The Critical Temperature

The possible combustion of food for the purpose of warming the animal body should not be denied, however. Recent investigations indicate that under given conditions there is an air temperature called the critical temperature, at which metabolism (oxidation) reaches a minimum. If the air temperature falls below this point, thus causing a greater radiation of heat from the body surface, increased oxidation occurs. If the temperature rises above this point, there is no diminution of oxidation, but rather a slight increase; hence the conclusion that there is a minimum oxidation necessary to the maintenance of the vital functions which must go on, however much the demands for the radiation of heat may be lessened by a rise of the air temperature. Down to a certain temperature point, the oxidation necessary for maintaining the work of the body gives off enough heat as a waste product to keep the body temperature up to 98.6° C. Above the critical temperature an excess of heat must be disposed of. What this critical temperature is for man does not appear to have been determined. Whichever way the air temperature moves from the critical point, there is heat regulation, this being chemical for the lower temperature, and physical for the higher.