§ 5 The Plastic Rôle of Food
Opinions of the Early Physiologists.—It is from this point of view that the ancients regarded the rôle of alimentation. Hippocrates, Aristotle, and Galen believed in the existence of a unique nutritive substance, existing in all the infinitely different bodies that man and the animals utilize for their nourishment. It was Lavoisier who first had the idea of a dynamogenic or thermal rôle of foods. Finally, the general view of these two species of attributes and their marked distinction is due to J. Liebig, who called them plastic and dynamogenic foods. In addition he thought that the same substance should accumulate the same attributes, and that this was the case with the albuminoid foods, which were at once plastic and dynamogenic.
Preponderance of Nitrogenous Foods.—Magendie, in 1836, was the pioneer who introduced in this interminable list of foods the first simple division. He divided them into proteid substances, still called albuminoids, nitrogenous, quaternary, and ternary substances. Proteid substances are capable of maintaining life. Hence the preponderant importance given by the eminent physiologist to this order of foods. These results have since been verified. Pflüger, of Bonn, gave a very convincing proof of this a few years ago. He fed a dog, made it work, and finally fattened it, by giving it nothing at all to eat but meat from which had been extracted, as thoroughly as possible, every other substance.12 The same experiment showed that the organism can manufacture fats and carbo-hydrates at the expense of the nitrogenous food, when it does not find them ready formed in the ration. The albumen will suffice for all the needs of energy and and matter. To sum up, there is no necessary fat, no carbohydrate is necessary; albuminoids alone are indispensable. Theoretically, the animal and man alike could maintain life by the exclusive use of proteid food; but, practically, this is not possible for man, because of the enormous amount of meat which would have to be used (3 kilogrammes a day).
Ordinary alimentation comprises a mixture of three orders of substances, and to this mixture albumen brings the plastic element materially necessary for the reparation of the organism; it also is the source of energy. The two other varieties only bring energy. In this mixed regimen the quantity of albumen must never descend below a certain minimum. The efforts of physiologists of late years have tended to fix with precision this minimum ration of albuminoids—or as we may briefly put it, of albumen—below which the organism would perish. Voit had found 118 grammes of albumen necessary for the average adult man weighing 70 kilos. This figure is certainly too high. The Japanese doctors, Mori, Tsuboï, and Murato, have shown that a considerable portion of the population of Japan is content with a diet much poorer in nitrogen, and suffers no inconvenience. The Abyssinians, according to Lapicque, ingest, on the average, only 67 grammes of albumen per day. A Scandinavian physiologist, Siven, experimenting on himself, found that he could reduce the ration of albumen necessary to the maintenance and equilibrium of the organism to the lowest figures which have been yet reached—namely, from 35 to 46 grammes a day. These experiments, however, must be confirmed and interpreted. Besides, it is important to point out that the most advantageous ration of albumen requires to be a good deal above the strictly sufficient quantity.
It only remains to refer to several other recent researches. The most important of many are those published by M. Chauveau, on the reciprocal transformation of the immediate principles in the organism according to the conditions of its functioning and the circumstances of its activity. To deal with these researches with as much detail as they deserve, we must study the physiology of muscular contraction and of movement—that is to say, of muscular energetics.