I. Water

Estimated as a force producer within the body, water may be said to have comparatively little value. Much of the water which is either drunk or ingested in combination with foods passes through the body unchanged, and is eliminated from one or more of the excreting surfaces; but some of it is undoubtedly altered or split up into elements which unite with other compounds. The nature of these processes is obscure, and as yet very little understood. It is - believed also that a certain quantity of water is produced in the body by thermion of oxygen and hydrogen which occurs incident to other chemical change, or by the liberation of water from more complex molecules. Water is entitled to rank as a food because it enters into the structural composition of all the tissues of the body, and, in fact, constitutes rather more than two thirds (70 per cent) of the entire body weight. Its importance is readily appreciated after it has been withheld from the diet for a short time, when striking physical and physiological alterations in the functions of the body occur.

Yeo says that, " assuming the water-free food to be 23 ounces, and a man's weight to be 150 pounds, each pound weight of the body receives in twenty-four hours 0.15 ounce, or the whole body receives nearly a hundredth part of its own weight. But ordinary solid food contains usually between 50 and 60 per cent of water; and if we add this to the water-free solids, the total daily amount of so-called dry food (exclusive of liquids) is about 48 to 60 ounces. But from 50 to 80 ounces of water in the liquid form is usually taken in addition, and this would make the total supply of water equal 70 to 90 ounces, or half an ounce for each pound of body weight".

II. Salts

The salts have practically no force-producing power, but they are concerned in tissue formation to some extent, especially in the bones and teeth, where the lime and magnesium compounds are the most stable of any in the body. Some of the compounds of sulphur and phosphorus which exist in meat and bran are undoubtedly associated with the development of energy in the body, but the salts are already saturated and stable, and, although their functions as laboratory reagents within the body are numerous, they may be left out of calculations of force-producing foods.

III. Proteids, Carbohydrates, And Fats

It is easy to determine with accuracy the force value of a pound of albumin by chemical analysis, but it does not follow from this that the energy contained in it will all be developed in the body at exactly the right time or even within a sufficiently short period to enable the system to utilise it completely. The majority of physiologists are agreed that muscular energy is chiefly derived from non-nitrogenous foods. This statement receives further confirmation in the results of an interesting series of experiments made upon sixteen persons by Prof. Charles E. Waite, of the University of Tennessee (U. S. Department of Agriculture Bulletin No. 89, 1901). The experiments included rest intervals followed by work periods, usually of about four hours per diem. Waite found that the digestibility of the diet was uninfluenced by moderate work, animal protein being more completely digested than vegetable protein. He concludes as follows:

" A study of the nitrogen balance shows that in the majority of cases if there was a gain during the rest period it was increased during the work period, and if there was a loss it was diminished. In other words, comparing the elimination of nitrogen in the urine during the periods of little muscular activity and normal diet with that during periods of increased activity and a diet furnishing energy largely in excess of the heat equivalent of the measured work performed, there seems to be a slight decrease under the latter condition. This is true even when we consider the possibilities of a small loss of nitrogen in the perspiration and a lag of considerable duration between the breaking down of nitrogenous material within the body and the excretion of nitrogen in the urine".

W. O. Atwater and H. C. Sherman state (U. S. Department of Agriculture, Bulletin No. 98, 1901) that:

"Practically all of the recent experimenting with men sustains the view that muscular work normally results in an increased excretion of nitrogen when the work is at all severe and there is not a corresponding increase in the fuel ingredients (fats or carbohydrates) of the diet. It also implies that the increased output of nitrogen continues after the work stops, so that if the experiment continues but one day the larger part of the increase may be found on the succeeding day.

"Well-trained professional athletes when engaged in severe muscular exertion metabolise relatively large amounts of protein, the body tissue being drawn upon unless the protein of the food is very abundant".

The daily quantity of solid food consumed by an adult male at ordinary work will range between 50 and 60 ounces, and that of water drunk is about the same. The requirements of severe labour, if continued, exceed this range, so that as much as 75 ounces of solid food may become necessary, and this should be mainly in the form of albuminates and fats. Fats, compared with carbohydrates, yield two and one fourth parts as much energy. The standard diet for twenty-four hours for a healthy adult male at ordinary work should contain, in round numbers, 20 grammes of nitrogen (contained in 120 grammes of proteid) and 320 grammes of carbon, a proportion of 1 to 16, but, because the food is best taken in the form of a mixed diet, the proportion may be stated as one part of nitrogenous or animal food to three or three and one half of non-nitrogenous or vegetable food. Ordinary albuminous food yields, on the average, about 16 per cent of nitrogen.