The nitrogen in the urine and feces may be regarded as an index of the proteid food assimilated, and when these two factors correspond, the body is said to be in "nitrogen equilibrium," i. e., all the nitrogen actually consumed is eliminated without storage in the body. When the body has attained its full adult size, it maintains this equilibrium with but very slight variation. A new diet containing either an excess or diminution of protein temporarily disturbs this equilibrium, but it is soon readjusted.

For an adult male the average consumption of nitrogen being 20 grammes per diem, the ordinary allowance for daily variation does not exceed 3 grammes on either side of this standard. For a man of average weight - that is, 67 kilogrammes, or 160 pounds - the daily allowance of food ranges from 6 to 9 grains of carbon, and 0.25 to 0.36 grain of nitrogen per kilogramme of body weight. Those elementary foods which approach most nearly in composition the economical proportion of nitrogenous to non-nitrogenous material are cow's milk and wheat flour. In the former this proportion is one to three, in the latter one to four and a half. The standard ratio for health of protein to the fuel ingredients - starch, sugar, and fat - is 1 to 5.8. This is the ratio adopted by the Experiment Stations of the United States Department of Agriculture. In metabolism experiments a time allowance must be made for the interval between the ingestion of proteid food and the corresponding nitrogen increase in urine and feces.

This interval is known as the "nitrogen lag," and it occupies a number of hours, varying under changing conditions.

The question of the nutritive value of any fixed diet cannot be solely estimated from variations in the body weight, for the loss of water or of albumin may increase while fat is stored up. Neither can the quantity of food eaten be relied upon for this purpose, for from habit one man may eat more than another while doing the same work, and may eat more than he needs. Nor is it possible to determine from the various excreta alone whether the body has had a suitable quantity of nourishment, for they are too constantly changing, and " life will soon become extinct if the nutriment given be measured by the metabolism of starvation " (Bauer).

A man in perfect health might easily lose weight upon the identical diet which would enable an emaciated and feeble invalid to gain flesh. In convalescence from typhoid fever, for example, one often sees evidence of a daily gain in weight while the quantity of food is still far below that necessary to support a healthy man in vigour. The needs of the organism at any particular time must, therefore, be taken into account as well as the strength of the digestive organs in prescribing the quantity of food consumed. In making calculations for the quantity of food required by large numbers of persons under any conditions, somewhat more than the averages above stated should be always included in order to make allowance for variations in absorption and assimilation of the food by different individuals. (See Quantity of Food Required, page 287).

The carbohydrates are not as available for the repair and growth of the tissues as the fats, but by their oxidation they save tissue waste, and furnish both heat and muscle force. They limit the formation of acetone from fat when the latter is eaten in excess (Schuman-Leclercq).

Bauer says: "The easy metabolism of the carbohydrates in the body must not be regarded as depending on their great affinity for oxygen; its cause is to be sought far more in the properties of the animal tissues.... The action of the carbohydrates agrees in many respects with that of fat, since they are in like manner capable of protecting from metabolism a certain amount of the circulating albumin and of assisting its transformation into organic albumin".

But the destruction of carbohydrates in the body is very complete, even when eaten in excess, and herein they differ from the fats, "an excess of which in the food invariably produces an accession of fat." From these and other statements made by Bauer he argues that probably the carbohydrates do not directly form fat in the body, but their well-known fattening action is, when eaten with albumin and fat, to spare the consumption of these latter substances, which are then converted into tissue fat.

Sugar furnishes, in addition to heat, considerable muscle energy, and it has been lately proved by Mosso, Vaughan Harley, and others to have distinct power in relieving muscular fatigue.

Vaughan Harley found that with an exclusive diet of seventeen and one half ounces of sugar dissolved in water he could perform almost as much muscular work as upon a full mixed diet. The effect in lessening muscle fatigue was noticeable in half an hour and reached a maximum in two hours. Three or four ounces of sugar taken before the expected onset of fatigue postponed or entirely inhibited the sensation. Schumberg recommends that the sugar be taken with chocolate or in lemonade. Experiments were made by him upon German soldiers which covered a period of thirty-eight days. The men were given daily ten lumps of sugar (about one sixth pound), and were able to withstand hunger, thirst, and fatigue much better than others whose diet contained no sugar. When a muscle contracts it is not the muscle tissue itself which is consumed, but its supply of glycogen. The candle wick is but very slowly burned so long as its supply of tallow does not fail, and so the muscle is spared while the carbohydrate glycogen furnishes force.

When a muscle is much used it increases in size, and needs more nitrogen to build new substance.

The food value of sugar is thus summarised by Mary Hinman Abel (U. S. Department of Agriculture, Farmers' Bulletin No. 93, 1899):

"(1) When the organism is adapted to the digestion of starch, and there is sufficient time for its utilisation, sugar has no advantage over starch as a food for muscular work except as a preventive of fatigue.

"(2) In small quantities and in not too concentrated form sugar will take the place, practically speaking, weight for weight, of starcrf as a food for muscular work, barring the difference in energy and in time required to digest them, sugar having here the advantage.

"(3) It furnishes the needed carbohydrate material to organisms that have as yet little or no power to digest starch. Thus, milk sugar is part of the natural food of the infant.

"(4) In times of great exertion or exhausting labour, the rapidity with which it is assimilated gives it certain advantages over starch".

The hard-working lumbermen of Canada and Maine eat a very large quantity of sugar in the form of molasses. I have seen them add it to tea and to almost everything they cook. Sugar has also been found of much service upon polar expeditions.

Albumin burned in oxygen outside of the body is almost completely oxidised, but after oxidation in the body, about one third of its substance is excreted, imperfectly consumed, as urea. Pfluger says: "All work of life can be performed by albumin alone, while no other material in the universe can do it. The integral ingredient of the living and working cell is the albumin, indeed often the only organic part of it." On the other hand, the consumption of some carbohydrates, such as alcohol and sugar, appears as complete within the body as it may be outside of it, and the amount of energy actually developed is identical with that which may be theoretically calculated.

The statement is sometimes made that the compounds of carbon and hydrogen are slowly oxidised, and give a more continuous yield of energy than nitrogenous compounds, but with less powerful individual discharges. In other words, they are said to be good "main-tamers of energy." There are, however, exceptions to this, such as alcohol.

The proportion of nitrogenous or proteid compounds required is greatest where growth is active. Many nitrogenous substances possess high potential and explosive energy, as illustrated by nitroglycerin and fulminating powder. Proteids are always present where the phenomena of life are exhibited, and bear a direct relation to their activity.