Editor Popular Science Monthly.
THE article under the above title in your July number appears to be based on chemical theories of nutrition that find no place in the modern science of physiology. The classification of food constituents into proteids or albuminoids, fats, and carbohydrates is a convenient one for certain purposes, but it is now known that these groups of nutrients have not the specific functions that were formerly attributed to them.
The assumption of Dr. Davies (page 366) that "the foods that are converted into heat—that is, keep up the heat of the body—are starches, sugar, and fat, and those that more particularly nourish the nervous and muscular system are the albumen and salts," is a survival of an obsolete theory of respiration; and tables giving the proximate composition of different articles of food are of little value in formulating diets for special purposes.
The law of the conservation of energy is now recognized as a significant factor in all physiological processes, and it furnishes the only consistent explanation of the phenomena of animal heat. The energy used in the constructive processes is stored up as potential energy, as an essential condition or constituent of all organic substances, and on their disintegration in the processes of destructive metabolism it is liberated in the form of heat—as in the digestion of foods, and in the wear and tear of the tissues that are constantly taking place in all vital activities.
Animal heat is not produced by a combustion of certain food constituents that serve as fuel, but it is the result of the liberation of the stored energy of foods and tissues, in the disintegrating processes they undergo in the system. Dr. M. Foster, who is recognized as one of the best authorities in physiology, estimates the potential energy of food constituents as follows:
From this it appears that the "starches and sugars," which are included in the group of carbohydrates, contain less potential energy than the proteids, and the heat obtained from them is accordingly less. The inference that starch and sugar should be avoided in hot weather, and largely replaced by lean meat, which consists in the main of proteids or albuminoids, is, therefore, not warranted by the evidence in regard to their constitution.
Without noticing the numerous fallacies in the article in question, attention is called to the widely different conclusions reached by Dr. Foster, in the last edition of his Physiology, in discussing the adaptations of diets to climatic conditions, and to the requirements of the system in brain-work and severe muscular labor.
He says: "With regard to climate the chief considerations attach to temperature. When the body is exposed to a low temperature, the general metabolism of the body is increased, owing to a regulative action of the nervous system. We might infer from this that more food is necessary in cold climates; and, since the increase in the metabolism appears to manifest itself chiefly in a greater discharge of carbonic acid, and therefore to be especially a carbon metabolism, we might infer that the carbon elements of food should be especially increased. When the body is exposed to high temperatures the same reflex mechanism tends to lower the metabolism; but the effects in this direction are much less clear than those of cold, and soon reach their limits; the bodily temperature is maintained constant under the influence of surrounding warmth not so much by diminished production as by increased loss. We may infer from this that in warm climates not less, but if anything, rather more food than in temperate climates is necessary in order to supply the perspiration needed for the greater evaporation and discharge of heat by the skin. . . . Indeed, the evidence that the increase of metabolism provoked by cold bears exclusively on carbon constituents, is so uncertain that it may be doubted whether any change in the normal diet, beyond some increase in the whole, should be made to meet a cold climate. Similar reasons would lead one to infer that man in the warmer climate would maintain on the whole the same normal diet, the only change perhaps being to increase it slightly, possibly throwing the increase chiefly on the carbohydrates with the special view of furthering perspiration. . . .
"In choosing a diet for muscular labor we must have in view not the muscle itself but the whole organism. And although it is possible that future research may suggest minor changes in the various components of a normal diet, such as would lessen the strain during labor on this or that part of the body, on the muscles as well as on other organs, our present knowledge would rather lead us to conclude that what is good for the organism in comparative rest is good also for the organism in arduous work; that the diet, normal for the former condition, would need for the latter a limited total increase, but no striking change in its composition. . . . The principles of such a conclusion with regard to mus-