From what has been said above it will be apparent that, within rather wide limits, the greater the amounts of carbohydrates and fats eaten, the smaller will be the amount of protein required to maintain nitrogen equilibrium.

For practical purposes, however, we may eliminate the question of the extent to which protein metabolism can be restricted by the use of excessive amounts of other food and reduce the problem to this: When the total food is properly adjusted to the size and activity of the subject so that there is sufficient but not excessive fuel to meet all the energy requirements, how much protein must the daily food contain in order to keep the body in nitrogen equilibrium?

The most extended investigation on the protein requirement of man is that of Chittenden.* The general plan followed in this investigation was to have each man reduce his protein food gradually without any great change in his other habits. This gradual reduction of the protein intake was continued usually for some weeks, sometimes for several months, before any comparison of intake and output was attempted. During this preliminary period upon a restricted diet there was in almost every case a loss of weight, and from previous observations† under similar conditions we may safely assume that there was a considerable loss of body protein. After a sufficient period of adjustment there was usually a tendency for the body weight and the rate of protein metabolism (measured by the amount of nitrogen eliminated through the kidneys) to become fairly constant, indicating that the body had adapted itself to the new conditions. When this point had been reached, a nitrogen balance experiment was made, the intake and output being determined by weighing and analyzing for nitrogen all food consumed and all nitrogenous material given off from the body except that in the perspiration. The fuel value of the food consumed during the same period was calculated by means of figures taken from standard tables. From these calculated fuel values it would appear that the energy of food consumed by Chittenden's subjects was in general about equal to the usual estimates of the energy requirements for similar occupations, though in several specific instances the subject may have unduly restricted his total food intake and thus created an energy deficit and a tendency toward negative nitrogen balance.

* See Chittenden's Physiological Economy in Nutrition and Nutrition of Man.

† Neumann, for example, in 35 days on insufficient diet lost 06 grams of nitrogen corresponding to 600 grams of protein, equivalent to about 2.5 kilograms (5.5 pounds) of muscle tissue.

Chittenden bases his estimate of the protein requirement, not only upon the nitrogen balances, but also upon the amounts of nitrogen observed to be eliminated daily through the kidneys over long periods in which the body may or may not have been in complete equilibrium, but in which health and efficiency were certainly maintained. The first men to serve as subjects in this investigation were Chittenden himself and his associates, who all continued their professional work and either reported no effect or felt benefited by the change to the low protein diet. Similar experiments were then made upon a squad of soldiers, who during the test were quartered near the laboratory and were given regular exercise in the gymnasium in addition to light duties about their quarters. These men showed marked improvement in physical condition during the test, probably due in part to their more regular habits of life and their gymnastic exercises. In order to eliminate this latter factor while still applying the low protein diet to young and physically active men, the investigation was extended to cover a group of university athletes who were already well-trained and in prime physical condition at the beginning of their dietary experiment. These athletes not only maintained, but in many cases improved, their gymnastic records while on the low protein diet, one of them winning an all-round gymnastic championship during the time. Chittenden statesl that his data "are seemingly harmonious in indicating that the physiological needs of the body are fully met by a metabolism of protein matter equal to an exchange of 0.10 to 0.12 gram of nitrogen per kilogram of body weight per day, provided a sufficient amount of non-nitrogenous foods is taken to meet the energy requirements of the body." This would correspond to 44 to 53 grams of protein per day for a man of average weight (70 kilograms, 154 pounds, without clothing), and Chittenden considers that for such a man an allowance of 60 grams of protein per day should certainly be entirely adequate.

1 Nutrition of Man, pages 226, 272.

In a recent examination of the available literature upon this subject there were found 86 experiments upon adults showing no abnormality of digestion or health, in which the diet was sufficiently well adjusted to the probable requirement and the nitrogen balance showed sufficient approach to equilibrium to make it appear that the total output of nitrogen might be taken as an indication of the protein requirement. These experiments are taken from 20 independent investigations in which 41 different individuals (37 men and 4 women) served as subjects. For purposes of comparison the daily output of total nitrogen in each experiment was calculated to protein and this to a basis of 70 kilograms of body weight. Reckoned in this way, the apparent protein requirement as indicated by the data of individual experiments ranged between the extremes of 20.0 and 79.2 grams, averaging 49.2 grams of protein per man of 70 kilograms per day.. Thus the average falls well within the range of Chittenden's estimate of the amount of protein required for normal nutrition when the energy value of the diet is adequate.

Examination of the data recorded in the original papers indicates that the wide differences in amounts of protein catabolized in the different experiments cannot be attributed primarily to the kind of protein consumed nor to the use of diets of fuel values widely different from the energy requirements. Apparently the most influential factor was the extent to which the subject had become accustomed to a low protein diet.