It is a well-known fact that if more fat than the animal organism can advantageously oxidize is supplied, such fat, in so far as it is absorbed, is stored as fat in the body. If an excess of carbohydrates is taken, such excess is also converted into fat and is likewise stored as fat for future use. Further Pfli'iger has recently advanced the not improbable hypothesis that fats are not completely oxidized as such within the animal organism, but are first converted into carbohydrates. The animal organism is then able to convert carbohydrates into fat and fat into carbohydrates according to its needs, and the logical conclusion therefore is quite in harmony with the accepted view that it is theoretically a matter of relatively small importance what ratio is selected for the fats and carbohydrates. The two taken together must of necessity furnish the greater part of the fuel value of the food, but upon individual peculiarities, relative cost, and a number of other accidental factors depends what ratio between the two may be most suitable in any given case.
With regard to the protein prescribed by the dietary standards the case is different. The animal or human organism, while able to convert carbohydrates into fat and probably also fat into carbohydrates, can effect no such transformation of non-nitrogenous food into the highly nitrogenous proteins. It may be able to produce carbohydrates, and therefore also fat, out of protein, but it certainly can not produce protein out of fat or carbohydrates. The protein of the food not only furnishes energy and heat, as do the fats and carbohydrates, but it, and it alone, furnishes the material which replaces the constant loss of living protoplasm. It is therefore clearly-necessary that the daily food should contain enough protein to protect the organism against loss of body tissue. On the other hand, it is generally believed that instead of being advantageous it is probably detrimental to the full-grown organism to have to take care of more protein than is needed for the replacement of lost tissue material. An excess of fat or carbohydrates, the human organism can to a very great extent take care of by adding it to its store of body fat, but it has not the power similarly to increase its supply of reserve protein. Any excess of nitrogenous material supplied with the food leads at once to a correspondingly increased destruction of protein. And the formation of excessive quantities of nitrogenous katabolism products within the body is supposed to be more or less a source of danger. I think all are agreed that gout at least is largely the result of "high living".
The important question then is, How much protein must the diet of normal persons contain? Voit came to the difinite conclusion that 118 grams are needed for a man weighing 70 kilos (150 lbs.), and for more than a generation this figure has been generally accepted as substantially correct. But is it? Since the publication in 1881 of Voit's great monograph on metabolism it has been shown by Hirsch-feld, Klemperer, Pechsel, and Siven that the daily-protein destruction in men of average size can be reduced to 40 grams or less, and that nitrogen equilibrium can be maintained by furnishing such small amounts of protein with the food. The experiments by means of which Voit secured the almost universal acceptance of his standard minimum protein requirement are essentially similar and in no way superior to these more modern experiments which seem to prove that 40 grams of protein, or less, are enough to maintain nitrogen equilibrium. One might therefore suppose that the later experiments would have been accepted as proving the erroneous-ness of Voit's figures, or that they would at least have been deemed sufficiently important to lead to a general reopening of the whole question. But the earlier conclusions and generalizations of Voit had in the meantime, so to speak, survived the probation period, and had become the accepted doctrine, not to say tradition, of the scientific public. In addition, it must be remembered that Voit's standard comes much nearer the average common usage. The widespread and earnest acceptance of Voit's figure is undoubtedly in a great measure due to the fact that it agrees tolerably well with the protein consumption actually prevailing among the people, specially among those not too poor to procure the more expensive articles of food.