While the requirements of the body are necessarily determined by its constitution, there is no correspondence of quantitative relations, and the qualitative relations are disturbed by various factors. Thus the elemental analysis of the body throws practically no light on its requirements in foods and the analysis by proximate principles, very little. This paradox is due to two main reasons. First, there is a great difference in the rapidity with which different ingredients are wasted. Water and soluble salts pass through the body - not in the sense of a direct passage through the alimentary canal - quite rapidly. Insoluble salts, as calcium phosphate, etc., in bone, are wasted very slowly, and the silicates, mainly represented in dental enamel, are scarcely at all replaceable. Secondly, foods serve two quite distinct purposes, the replacement of tissue waste and the furnishing of heat and energy. The waste of protein tissues is quite slow and uniform, depending very slightly on differences of activity. Conversely, carbohydrates never strictly form any part of the body, though present to the amount of two or three hundred grams as a reserve supply of energy in the form of glycogen and as a supply in process of use, in the form of dextrose in the blood. Yet nearly four hundred grams a day are required under normal conditions. Fats, although forming 16% of the average, well nourished body and practically never falling below 3%, even in starvation, are not necessary at all in the food, as they may be produced both from proteins and carbohydrates. It should be remembered also that however useful as a reserve supply of nutriment and as packing, and to prevent rapid loss of heat by radiation, they do not, any more truly than carbohydrates, form an essential part of the body.

Thus, for practical purposes, the nutritive requirements must be determined by actual empiric observation. Subject to considerable variation, they are as follows:

Proteins, 50. - 125 grams.

At least 2/3m must be given for every kilo of body weight to prevent consumption of tissues. On the other hand, unless necessitated by the failure to assimilate fuel-foods, more than 125 grams is objectionable on account of producing toxic waste products.

Fats, 0. - 150. grams.

Ordinarily about 50 - 80 grams may be used.

Carbohydrates, 80. - 400 grams.

Less than 80 grams requires so much destruction of fats and proteins that fatty acid intoxication is imminent.

Albuminoids.

These are produced by the partial decomposition of proteins, though some hold that some nuclein compounds are required. Gelatin may be used as a fuel food, to the amount of 100 grams a day, but not indefinitely.

Lecithin, iodothyrin, etc.

The exact amount needed or whether any is needed, has not been determined. Enough is included in any average diet.

Haemoglobin.

The small quantity contained in an allowance of 50 grams of meat daily is sufficient and probably, so long as 10 centigrams of iron is furnished in any form, the normal body can anabolize it into haemoglobin.

Water, 2000 - 3000 c.c.

About 1000 c.c. is contained in an ordinary so-called dry diet. On the other hand, about 1000 c.c. should be given as water.

Sodium chlorid, 10 - 20 grams.

Ordinarily, 20 or even 30 grams produce no appreciable harm. While some claim that the amount contained in unsalted meat, vegetables, etc., is sufficient, about 10 grams additional is craved by most human beings and a proportionate amount by the higher quadrupeds.

Other mineral salts.

Ordinarily supplied in sufficient amount in the diet.

Antiscorbutic principle.

Whether this consists essentially in potassium or in vegetable acids, is disputed. At any rate, a small quantity of raw milk or of fresh or cooked fruit or vegetables, is required.

Cellulose and other indigestible material is not theoretically required, but must be taken to the amount of 50 grams or thereabouts, a day, to insure proper peristalsis and, indirectly, to maintain health.