It is well known that the non-nitrogenous foodstuffs may spare body protein. The carbohydrates are unquestionably more efficient as protein sparers than are fats. The starvation output of nitrogenous waste products, or at least of urea, can be notably diminished by the ingestion of either of the non-nitrogenous nutrients mentioned, though the superiority of the carbohydrates in depressing the loss of nitrogen is always manifestly greater. If the carbohydrates are omitted from the diet or even replaced by fat, the effect on the nitrogen output is promptly seen.
Various theories have been proposed to explain this unique function of the carbohydrates in nutrition. One view has maintained that a certain concentration of blood sugar is always necessary for proper maintenance of physiologic activities. This sugar can be produced from proteins if carbohydrates are not available directly; and since, in the absence of carbohydrates, under physiologic conditions fats cannot well supply
THE PROTEIN-SPARING ACTION OF CARBOHYDRATES. JAMA. 1916;LXVII(17):1233. doi:10.1001/jama.1916.02590170041017