There is another way of attacking the problem of protein requirement - a more scientific way. But before we describe this, a few preliminary observations become necessary.

The reader will remember what we said above concerning the three classes of foodstuffs, - that only the proteins contain the element nitrogen. When we come to trace the course of this protein in the body we find that, in so far as the nitrogen part of the protein is concerned, it is mainly occupied in replacing decayed tissue. Whenever the cell takes up the nitrogen compound to build up tissue, it gives up a corresponding amount of a nitrogen compound which represents the waste material. This waste material finds its way chiefly into the urine. Very small quantities are found in the feces and sweat.

Now suppose we determine the amount of nitrogen in the food we eat, and then determine the amount of nitrogen excreted. If the quantities are approximately equal we say we have reached a "nitrogen equilibrium": the expenditure is equal to the income.

All normal, adult people show such a "nitrogen equilibrium." The case is different with growing children. The child grows; the number of his cells multiply; he must therefore keep some of his nitrogen for additions to his little house. Here the nitrogen intake will be greater than the output.

Sick men and sick children may serve as the reverse example of the healthy, growing child. Here the tissues may go to waste without any corresponding replacement. That would mean that the nitrogen eliminated from the body is more than the amount the body receives from its food.

Where man has reached the limit of growth and is in a healthy condition, the output and income of nitrogen equal one another. If you give him twice as much protein (and therefore twice as much nitrogen) one week as another, you will soon find that this healthy man will begin to eliminate twice as much nitrogen as he did formerly. The body, you see, does not store protein the way it stores fat. Eat more fat than you can handle, and the extra fat accumulates in your adipose tissue and you become a fat man. (Sometimes you become a fat man without eating too much; but such cases belong to pathology.) Eat more carbohydrate than the body can handle, and the surplus stock is converted into fat and stored as before. Eat more protein than the body can utilize and the surplus is thrown out. As has already been said, the last statement does not hold true for children.

On the face of it you would say that if your adult shows that his nitrogen output and income balance one another - that he is in "nitrogen equilibrium"; that then he gets all the nitrogen necessary to rebuild waste tissue. He gets what he needs and in sufficient quantity.

Now the nitrogen comes from the protein, and the protein alone. Experience has shown that this nitrogen constitutes about 16 per cent of the total protein. This means that we need merely multiply the amount of nitrogen found by the number 6.25 to give us the amount of protein eaten; for 16 times 6.25 equals 100.*

Suppose an extended series of trials on a man show us that when he eats food containing the equivalent of 16 grams of nitrogen per day he also eliminates the equivalent of 16 grams of nitrogen. His balance sheet is clear. He is probably a healthy, normal individual. He is supplied with food in sufficient quantity. These 16 grams of nitrogen in the food show that he must have eaten 16 times 6.25, or 100 grams of protein. So we may arrive at the conclusion that 100 grams of protein are probably necessary for our individual to retain his good health. This will give an idea of how we arrive at a minimum protein requirement.

Now let us take one or two examples to illustrate this method of investigation.

Professor Chittenden's Experiment

Yale men will remember Chittenden, the director of the "Shef." Scientific School. Some years ago Chittenden selected a number of instructors, including himself, students and army men attached to the hospital corps, and made them the subjects of an experiment in which the amount of protein in the diet was gradually reduced from a little over 100 grams to 50, and in some cases to 30 grams per day. The loss in energy due to reduction in the protein supply was counterbalanced by increasing the quantity of fat and carbohydrate, so that the total number of calories remained constant. Even with as low as 30 grams of protein (about one ounce) the "nitrogen equilibrium" was maintained, showing apparently that the health of the subjects was in no way impaired. Chittenden of course drew the obvious conclusion that our protein consumption could be cut to one-half without in any way lowering the vitality of the individual. He maintained - and here he agreed with Fletcher - that some of the ills of humanity were due to excessive protein intake; and that therefore the reduction of the protein in the food eaten also lessened the possibilities of disease.*

* The food may also include nitrogen compounds other than protein, but it is unnecessary at this stage to complicate the situation more.

Dr. Hindhede. Chittenden's views were supported by Dr. Hindhede, of the Nutrition Laboratory in Copenhagen; and during the strenuous days of the late war, when the food of people even in the neutral countries was limited, Hindhede's influence was such that the feeding of the Danish population was left very much in his charge.

Objections to a Too Low Protein Diet. From what has been said, I do not want the reader to get the impression that the problem of protein requirement has been definitely solved in favor of the rather low figures of Chittenden and Hindhede. As a matter of fact, the tendency among the most prominent food experts is to retain a figure nearer to 100 grams of protein than 50, as the rations of armies and civilian populations, guided by the advice of such experts, shows (see below). There are one or two important reasons for preferring the older figures. One is that scientific experiments are usually of short duration - a few weeks, sometimes a few months;* and a diet that may do little harm in the course of a month may do infinite harm if extended over a period of years. This objection can be made to Professor Chittenden's work, and is, in fact, so general that it can be raised against many of the experiments in nutrition, unless animals, such as rats, are employed whose duration of life is considerably shorter than man's; so that months in the life of a rat may correspond to years - and even more - in the life of a human being. But here again you may say that what is true of the rat need not necessarily be true of man. Your point is well taken.

* For further details the reader may consult Dr. Chittenden's very readable works: Physiological Economy in Nutrition and The Nutrition of Man. Both are published by Frederick A. Stokes Co.