Carbohydrate and Fat as Factors in Protein Utilization and Metabolism

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Abstract
Author''s general conclusions abridged: In the field of nutrition 3 aspects of protein-sparing action can be distinguished: (1) A change in energy intake, whether in the form of carbohydrate, fat, or alcohol affects N balance. (2) Carbohydrate also plays another role in promoting the utilization of dietary protein. N balance is temporarily impaired when fat is substituted isocalorically for carbohydrate; this special sparing action is probably largely dependent on the carbohydrate being fed along with the protein of the diet. A curious feature of this relationship is the lack of conclusive evidence that carbohydrate in the diet favors protein utilization by the growing animal. (3) Carbohydrate plays a special part in conserving protein of endogenous, as well as of dietary origin. Feeding carbohydrate to fasting animals reduces the N output, but feeding fat does not generally do so until the fat stores are exhausted. The protein-sparing action of carbohydrate in fasting animals is probably based on the same mechanism as its interaction with dietary protein. Metabolically, the energy needed for protein synthesis comes from a pool of energy-yielding metabolites contributed mainly by carbohydrate and fat. The sensitivity of N balance to a change in carbohydrate or fat intake suggests that protein metabolism is in equilibrium with the level of metabolites in this pool. The evidence is as yet insufficient to show whether the rate of protein synthesis is directly affected by changes in carbohydrate and fat supply or indirectly through other reactions in protein metabolism. The special protein-sparing action of carbohydrate apparently does not depend primarily on improved synthesis of non-essential amino acids from the carbohydrate, nor on stimulation of some endocrine gland. It is possibly due to a temporary increase in the level of available energy, with consequent stimulation of protein synthesis. It is conceivable that further investigation of the protein-sparing actions of carbohydrate and fat may throw some light on the problem of protein synthesis.