Liebig believed that fats and carbohydrates were burned in the body primarily to supply it with warmth, and that protein alone served as the source of muscular work and other forms of tissue activity. He therefore classed the non-nitrogenous as "respiratory" and the nitrogenous as "plastic" foodstuffs, and treated the proteins as playing a "nobler" part in nutrition than can be taken by fat or carbohydrate. Although it was soon demonstrated that carbohydrates and fats as well as protein serve the body in the production of muscular energy, yet the influence of Liebig's teaching, and of the great attention given to protein in Voit's classical researches on nutrition, together with the fact that protein is the most prominent constituent of protoplasm, has resulted in a strong tendency to associate high protein feeding with increased stamina and muscular power.

The reasoning of those who appreciated the results of more recent experimental work, and yet believed the general attitude of Liebig and Voit to have been largely sustained by experience, is well expressed by Von Noorden, who wrote in 1893: *

"When one considers that the dietary habits of peoples are the results of biological laws, it would seem that the action of these laws, extending through the thousands of years of existence of the species, would have resulted in the establishment of suitable habits regarding the amounts of protein consumed. The data gathered by Voit may be taken as showing that this normal habit involves the consumption of about 105 grams of digestible protein † per day, a smaller protein consumption being usually associated with weak individuals or inactive peoples. While men can maintain equilibrium on less, still it can rightly be said that a liberal protein consumption makes for a full development of the man. A single individual may for years, or even decades, offend against this biological law unpunished. When, however, the small consumption of protein continues for generations, there results a weak race."

Von Noorden, however, is careful to add:

* Freely translated from the first edition of Von Noorden's Pathologie der Stoff-wechsel.

† Corresponding to Voit's allowance of 118 grams of total protein when the food for the sake of economy, as contemplated by Voit, is taken somewhat largely from vegetable sources.

"On the other hand, the importance of protein must not be overestimated. A diet is not necessarily good because the amount of protein is right; it must have the proper proportions of the non-nitrogenous nutrients as well, since the protein is not to be depended upon for the necessary fuel value. Better somewhat less protein with a liberal amount of total food than more protein with insufficient fuel value; the latter brings a rapid loss of strength, the former can be endured very well, at least for a long time, and very likely throughout the life of the individual".

Chittenden, in 1905, had reached exactly the opposite conclusion, - that the products of protein metabolism are a constant menace to the well-being of the body, and that any excess of protein over what the body actually needs is likely to be directly injurious, and at best puts an unnecessary and useless strain upon the liver and kidneys. Chittenden had satisfied himself by his numerous and long-continued experiments that both physical and mental stamina are promoted by decreasing the amount of protein in the food: "Greater freedom from fatigue, greater aptitude for work, greater freedom from minor ailments, have gradually become associated in the writer's mind with this lowered protein metabolism and general condition of physiological economy" . . . (Physiological Economy in Nutrition, pages 51,127).

Hutchison, in 1906, concluded that the normal amount of protein in a diet furnishing 3000 Calories should be placed at about 75 grams. This allows some margin above the results of Chittenden's experiments and agrees with the relation of protein to calories in mother's milk, which Hutchison regards as nature's hint as to the proper balance of nitrogenous and non-nitrogenous food for the human species (Chemical News, Vol. 94, page 104).

Folin held that the argument for a high protein diet based on the fact that large amounts of protein are commonly eaten by those who can afford it can be equally well applied to the dietetic use of alcoholic beverages and is no more convincing in one case than in the other; while on the other hand, study of protein metabolism has given rather strong evidence that the body has no need of such amounts as are commonly eaten. The loss of body nitrogen which occurs in the early periods of restricted protein feeding, and which was not determined nor specifically discussed by Chittenden, is treated by Folin as follows: "All the living protoplasm in the animal organism is suspended in a fluid very rich in protein, and on account of the habitual use of more nitrogenous food than the tissues can use as protein, the organism is ordinarily in possession of approximately the maximum amount of reserve protein in solution that it can advantageously retain. When the supply of food protein is stopped, the excess of reserved protein inside the organism is still sufficient to cause a rather large destruction of protein during the first day or two of protein starvation, and after that the protein catabolism is very small, provided sufficient non-nitrogenous food is available. But even then, and for many days thereafter, the protoplasm of the tissues has still an abundant supply of dissolved protein, and the normal activity of such tissues as the muscles is not at all impaired or diminished. When 30 grams or 40 grams of nitrogen have been lost by an average-sized man during a week or more of abstinence from nitrogenous food (but with an abundance of carbohydrate and fat) the living muscle tissues are still well supplied with all the protein that they can use. . . . The continuous excessive use of protein may lead, however, to an accumulation of a larger amount of reserve protein than the organism can with advantage retain in its fluid media. It is entirely possible that the continuous maintenance of such an unnecessarily large supply of unorganized reserve material may sooner or later weaken one, or another, or all, of the living tissues. At any rate, it seems scarcely conceivable that the human organism, having all the time access to food, can gain in efficiency on account of such an excess of stored protein. The carrying of excessive quantities of fat is considered as an impediment, the carrying of excessive quantities of unorganized protein may be none the less so because more common and less strikingly apparent" (American Journal of Physiology, Vol. 13, pages 131-132, 136-137).