The whole of the nitrogen taken in leaves the body in urea, unless, as we have said, growth of tissue is taking place. The body has not the same temptation to store nitrogen as it has to store carbon. Consequently, it is very sensitive to any deficiency of nitrogen in the diet. If food does not contain as much protein as is needed, the deficit is made up at the expense of the tissues. It does not necessarily follow that under these circumstances a man loses in weight. He may be putting on fat, although losing in strength owing to waste of muscle. For observations upon the income and expenditure of the body to be of any value, a condition of “nitrogenous equilibrium” must be established. The nitrogen taken in must equal in amount the nitrogen given out.
Very exact determinations of income and expenditure may be made by placing an animal, or even a man, in a box through which air is drawn. A record is made of the volume of air drawn through the box. The percentages of water vapour and carbonic acid which the air contains are estimated before it enters and after it leaves. The solid food consumed and the urea excreted are also measured.
If it is desired to measure the amount of heat given off, an animal may be placed in a calorimeter.
Even when most passive, the subject under examination, whether an animal or a man, is expending energy in keeping the body warm, in movements of respiration, and in shifting position. If it is desired to ascertain the relation of oxidation to external work, it is easy to devise a form of resistance, such as the turning of a wheel, or the lifting of a weight which can be measured.
In testing diets, it suffices to make sure that nitrogenous equilibrium is maintained, and then to estimate the gain or loss in weight and the output of energy in external work.
The Relative Value of Foods.—Dried proteins contain about 15 per cent. nitrogen, 54 per cent. carbon, 7 per cent. hydrogen, 22 per cent. oxygen, a little sulphur, and frequently some phosphorus. A large proportion of their carbon and hydrogen is available for combustion. Fats contain 75 per cent. of carbon, and a considerable quantity of hydrogen available for combustion; carbohydrates, 40 per cent. of carbon, with hydrogen and oxygen in the proportions in which they occur in water. If 1 gramme of protein is oxidized to the condition of urea, carbonic acid, and water, sufficient heat is liberated to raise the temperature of 4,100 grammes of water 1 degree centigrade. Its calorific value is therefore expressed as 4,100 calories, the unit of measurement—a calorie—being the amount of heat needed to raise 1 gramme of water 1°. The calorific value of 1 gramme of fat is 9,300 calories; of 1 gramme of starch, 4,100 calories. Thus, the energy potential in protein and in starch is the same; that in fat more than twice as great as that in either of the other foods.
A Normal Diet.—Nitrogenous equilibrium and body-weight can be maintained and work done on diets which vary widely in percentage composition. This is a question which we shall consider at greater length later on. In the meantime, for the sake of illustration, it is necessary to formulate a diet which is fairly representative of the selection of foods made by a man of average weight—say 70 kilogrammes (145 pounds)—who desires to do a moderate day’s work in comfort. It has been found to amount to about 100 grammes of protein, 100 grammes of fat, 240 grammes of carbohydrate, all measured dry and as pure foods. If the several elements of such a diet be multiplied by the figures which represent their calorific value, it will be found that the man is supplied with 2,324,000 calories. The illustration that we have chosen is the diet of a professional man who is not engaged in hard physical work. The pure foods would be found to the amounts stated in 17 ounces lean meat, 4 ounces butter, and 17 ounces bread. The day’s diet would, of course, be much more varied than this, but it is simpler to express it in these terms.
Such a diet would hardly answer the requirements of a man doing hard muscular work. Experience shows that he would expect to receive a more liberal supply of energy, and that to obtain it he would increase slightly his allowance of proteins, and very considerably increase the quantity of carbohydrates that he consumed. The diet of European workmen is remarkably constant in the relative amounts of its several constituents, no matter what their nationality or the exact form of their work may be: Proteins, about 135 grammes; fats, 80 grammes; carbohydrates, 500 to 700 grammes—giving a supply of energy equal to 3,500 to 4,000 kilo-calories.
Speaking generally, carbohydrates are the source of muscular force, and fats of heat. In warm climates men work on carbohydrates. The ’rickshaw men of Japan are said to eat only rice on working days, and to reserve fish for days of leisure. The Japanese, as is well known, consume extremely little fat. The Esquimaux and other inhabitants of high latitudes eat immense quantities of fat. Proteins constitute the luxurious element of a diet. Not only are they more attractive to most palates, and therefore preferred by persons whose dietary is not severely regulated by price, but the body prefers them. It works with greater alacrity when supplied with more protein than, in a strictly physiological sense, it needs.
The supply of food must exceed the apparent demand. The most efficient of motors cannot convert more than 15 per cent. of the energy potential in its fuel into work. If a man endeavours to obtain a better result than this from his muscular system, if he tries to make his machine do more than 15 units of work for every 100 units of energy with which he supplies it, he does it at the expense of his own tissues. First he loses in weight, owing to the consumption of fat; then the excess of nitrogen discharged over nitrogen consumed shows that he is burning up the proteins of his own tissues. It is needless to add that the weakness which results puts a stop to excessive work. Muscles, as we shall find when we consider the relation of their output of work to the energy supplied to them, can produce a much better result than the best of engines; but we are speaking of the body as a whole, which wastes energy in the movements of respiration, masticating food, shifting position, maintaining the body temperature, etc.