This question has been studied by many investigators, with very interesting and suggestive results. Thus, in 1887, Hirschfeld[55] reported some experiments on himself, twenty-four years of age and weighing 73 kilos. His ordinary diet contained daily 100 to 130 grams of proteid, and the amount of nitrogen excreted varied from 16 to 20 grams per day, corresponding to a metabolism of proteid equal to the amount ingested. In other words, the body was essentially in nitrogen equilibrium. Then, for a period of fifteen days, during which he was unusually active, he lived on a diet in which the content of proteid corresponded to only 6 grams of nitrogen per day, and yet he remained in nitrogen equilibrium. The diet made use of was composed essentially of milk, eggs, rice, potatoes, bread, butter, sugar, and coffee, with some wine and beer, and on two days a little meat. It is to be observed that the nitrogen or proteid intake per day was only one-third of what he was accustomed to consume. In a second experiment, covering ten days, similar results were obtained. So that evidence was afforded that a young and vigorous man can maintain his body in nitrogen equilibrium, for fifteen consecutive days at least, on an amount of proteid food equal to only one-third of the minimal requirement called for by common usage. Plainly, the difference between a daily consumption of 118 grams of proteid food and 40 grams represents a large percentage saving, both of proteid and in the metabolism of proteid matter with all the attendant transformations. In these experiments, however, the subject consumed relatively large amounts of non-nitrogenous food, notably butter, of which on some days he took as much as 100 grams. The average fuel value of his food ranged from 3750 to 3916 calories per day; a fact of some importance, since it is to be remembered that both fat and carbohydrate tend to protect proteid metabolism. [55] Felix Hirschfeld: Untersuchungen über den Eiweissbedarf des Menschen. Pflüger’s Archiv f. d. gesammte Physiologie, Band 41, p. 533.