148. Necessary Measurements

It is clear that first of all it is necessary to measure both the income of the body, that is, the food plus water and oxygen, and also the outgo, that is, the excretions which, as we have learned, are practically all included in what passes out through the lungs, the skin, the kidneys, and in the feces. We must do this not only that we may determine whether with a given diet the individual is losing or gaining body substance, but we must also in this way ascertain in what the body gain or loss consists, whether of protein or of fat. In being able to measure the balance in this way, it is possible to determine what is the minimum amount of food that will maintain an individual under given conditions of activity without loss of body substance, and also whether a given ratio of nutrients is efficient. Indeed, it is by this method that we have learned how and to what extent the body substance is utilized to supplement food deficiencies, whether of kind or quantity. It is also possible to determine the extent to which nutrients of one class may be substituted for, or conserve, those of another class, or what is the effect of omitting from the diet all of one class of compounds.

1 "Metabolism and Energy Transformations of Healthy Man during Rest," Benedict and Thorne, p. 110.

149. How Measurements Are Made. Respiration Calorimeter

Let us consider more in detail how it is possible to arrive at a balance between the income and outgo of the body. It is a mere matter of weighing and chemical analysis to ascertain what enters the body in the food and drink, - the quantities of carbon, nitrogen, phosphorus, sulfur, and all the other elements. Recently it has become possible to measure the oxygen consumed in breathing. The food, drink, and oxygen constitute the income. Of the outgo, viz., the feces, urine, carbon dioxid from the lungs, and water from the lungs and skin, the feces and urine can be weighed and analyzed, as is done with the food. For measuring the carbon dioxid and water excreted, a special apparatus has been devised, which also now measures the oxygen consumed and heat given off. This apparatus is known as a respiration calorimeter. By means of the respiration apparatus it is possible to measure, not only the gaseous excreta delivered to the air or the respiration chamber, but also to discover what particular nutrients are being oxidized at any given time. This is done by determining what is known as the respiratory quotient, which is obtained by dividing the volume of carbon dioxid evolved by the volume of oxygen used. If a carbohydrate alone is oxidized, the two volumes are equal and the quotient is 1.00. If fat alone is being burned, the carbon dioxid is less in volume than the oxygen, and the quotient is approximately. 70. When a mixture of fat and carbohydrates is being utilized, the quotient will range somewhere between 1.00 and .70 and the relative proportion of these two classes of nutrients that are undergoing oxidation can be determined by calculation.