180. Food Balance

The matter of the digested food, including water and oxygen, is exactly equal to that stored in the body or in milk, or both, plus that in waste products, - feces, water, carbonic acid, and urine solids. Such a balance may not be maintained for any particular day, but will ultimately be found to exist.

181. Food Requirements Definite

Under given conditions of species, sex, age, climate, and use, a definite amount of digested organic matter is necessary to maintain a particular person without gain or loss of body substance. This means simply that tissue wastes must be replaced, and the fuel supply must be kept up.

If an individual receives no food, or less than the amount needed for maintenance purposes, tissue waste and the production of energy do not cease, but go on wholly or in part at the expense of the body substance, and, as it is commonly expressed, the person "grows thin."

182. Production

Food supplied above a needed maintenance quantity may be utilized for the production of new substances or work. In the proper sense of the term, no production ever occurs without an excess of food above the maintenance requirement. Milk formation may sometimes go on at the expense of the body substance, but with proper feeding, milk, flesh or muscular work are produced at the expense of food supplied in excess of that needed for maintenance.

183. Specific Requirements

Regard must be had to the supply of particular nutrients as well as of total food. Even with a person doing no work a certain amount of protein will be broken up constantly into urea and similar compounds, an amount which will be withdrawn from the body tissues to the extent that it is not supplied by the food. In addition to this, nursing mothers, for instance, must have protein for the formation of the nitrogen compounds of milk, or a growing child, for the growth of bone and flesh in a quantity proportional to the production, and food must supply it. There is, therefore, a minimum supply of protein, which, in a particular case, is necessary for maintenance and for constructive purposes, less than which ultimately diminishes production to the extent of the deficiency, or else requires the use of body tissue.

184. Nutrients Interchangeable In Part

The different classes of nutrients are to some extent interchangeable in their functions. That is to say, all the organic nutrients may be burned to supply energy. Protein may be so used, even to withdrawing it from the purposes to which it is necessary, unless the carbohydrates or fats are sufficient to protect it from being consumed as fuel. A proper supply of the non-nitrogenous nutrients is required, therefore, to insure the application of the necessary minimum of food protein to its peculiar uses. Carbohydrates seem to have a special physiological adaptation to energy production.