150. Food Balances

The fundamental facts upon which a food balance is based are the following: The general balance of gain or loss of tissue is obtained by comparing the income and outgo of carbon. All organic compounds, whether in the plant or animal, contain carbon, and cannot be formed without it. If, therefore, the carbon taken into the body is more than that given off through the various channels, it is proof that the body substance has increased. If the balance is the other way, there has been a loss of body tissue.

It is possible to know the kind of tissue that is lost or gained. All the nitrogen excreted from the body passes out in the urine and feces, that in the urine coming from the digested protein. No protein tissues can be formed without the use of nitrogen, and no other tissues require its retention. If, therefore, the body retains nitrogen, it is evidence that muscular tissue or some other form of nitrogenous substance has been deposited. If at the same time the body has retained more carbon than would be required for the increase of protein tissue, then it is necessary to conclude there has been also a deposition of fat or other non-nitrogenous material. By such means it is possible, for instance, to discover the effect of a given dietary upon protein storage, or to learn if a change in activity, such as passing from rest to hard work, causes a greater utilization of protein, or whether the increased need for food to sustain increased labor may be met by eating more carbohydrates. The ratio in which one nutrient may replace another is also an important consideration that the respiration calorimeter has made it possible to study.

161. Energy Balance And Use

Very delicate modern apparatus now accurately measures the potential energy of various food compounds; that is, the energy that they give up in the form of heat or motion when burned. The respiration calorimeter measures the heat given off from the human body, heat being the end product of all vital activity. This may be done with the subject both at rest and at work. If, then, by the metabolic balance it is shown how much food has been oxidized and how much has been retained, conclusions may be drawn as to the food energy utilized for the vital processes of the subject, and, by taking such measurements with the subject at rest and doing different amounts of work, it is possible to learn how much food is needed to accomplish a given amount of work. By such means the law of the correlation and conservation of energy has been shown to hold with human machines as well as with those of wood and iron.