202 THE INDIAN JOURNAL OF VETERINARY SCIENCE AND ANIMAL HUSBANDRY [VI, II

Protein ingested alone by animals causes a marked specific dynamic effect,
much greater than any other nutrient, but when incorporated into a protein-free
diet, otherwise complete, it must decrease the specific dynamic effect of such a
diet rather than increase it, because the combination will be more efficient in
maintaining the energy balance of an adult animal or in increasing the energy
balance of a growing animal. Weiss and Rapport ⁴ were greatly mystified when they
found that calorigenic amino-acids, administered to dogs along with proteins,
failed to increase the calorigenic action of the latter. But equally mystifying
from the standpoint of the current theories of the specific dynamic effects of food
is the action of amino-acids in improving greatly the efficiency for growth of rations
containing protein complexes deficient in those amino-acids ⁵. In all probability
this increase in efficiency means a decrease in the specific dynamic effect, assuming
again that the basal metabolic rate and the activity of the experimental animals
was not depressed by the amino-acid supplements.

Apparently the specific dynamic effects of isolated nutrients fed as such have
very little if anything to do with the specific dynamic effects of mixtures of
nutrients, particularly balanced mixtures. Without being able to specify the
exact causes of the metabolic stimulation induced by the consumption of food,
we may nevertheless conclude reasonably that its intensity is dependent
primarily upon the degree of accumulation of the end-products of digestion within
the tissues, which is in turn dependent for any given intake of food upon the rate
of utilization of these products by the tissues. Their rate of utilization will be
determined by the proportions existing among them, such that the better the
balance with reference to the requirements of the animal the more rapid the rate of
utilization and withdrawal from the cellular fluids. The metabolic stimulation
thus occurs only when there is an excess of nutritive material in the tissues, and is
to a considerable extent proportional to this excess. It is possibly a mechanism
operating merely for the removal of excess food material from the body cells in the
interests of physiological efficiency.

These speculations are now being investigated experimentally in the Division
of Animal Nutrition of the University of Illinois.

⁴ Jour. Biol. Chem., 60 : 513, 1924.
⁵ H. H. Mitchell and D. B. Smuts, Jour. Biol. Chem., 95 : 263, 1932.