The influence of energy intake on protein metabolism

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Abstract
Studies of N balance and the protein content of the liver were made on rats receiving protein rich or protein deficient diets which provided various levels of energy intake (from about 850 to 1700 kg. cal./sq. m. of body surface area). When the diet contained adequate amts. of protein, increments in energy intake, produced by adding either carbohydrate or fat to a sub-maintenance diet, caused a linear improvement in N balance. Protein metabolism must thus be in a state of dynamic equilibrium with energy intake over the range studied. The total amt. of protein in the liver also responded linearly to increasing energy intake. Changes in energy intake caused a smaller change in % of N in the carcass than in the N content of liver. When the diet contained no protein, addition of fat to bring energy intake up from 900 to 1700 kg. cal./sq. m. failed to influence N balance. Addition of carbohydrate produced some improvement up to 1200 kg. cal./sq. m., but not thereafter. In neither case was the amt. of protein in the liver significantly altered. Apparently when the supply of amino acids circulating to the tissues comes solely from endogenous sources, this becomes a limiting factor in the rate of protein synthesis at quite low levels of energy intake, but when the diet supplies adequate amts. of protein this limitation no longer obtains.