Determination of Optimal Dietary Protein Requirements of Young and Old Dogs

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Abstract
The purpose of these investigations was to determine the optimal nitrogen intake that would produce maximal filling of the protein reserve of the animal. Data were obtained which indicate that the present method of calculating protein maintenance requirements as 2 mg of nitrogen/basal kcal is considered to be endogenous nitrogen and may not adequately fill the “protein reserves” of the animal. Both young dogs (one-year-old) and old dogs (12 to 13 years of age) can be placed in nitrogen equilibrium and maintained with 0.2 to 0.6 g casein nitrogen/day/kg of body weight. Liver and muscle protein to DNA ratios reached maximal values in young dogs that were fed 0.4 g of casein nitrogen/day/kg of body weight, whereas the older animals required 0.6 g of nitrogen to obtain optimal filling of the protein reserves. Under these conditions, the dogs cannot be placed in positive nitrogen balance by feeding more dietary protein. Liver and muscle RNA to DNA values are closely correlated with changes in protein to DNA levels. When all dogs reached nitrogen equilibrium when fed 0.5 g of casein nitrogen/day/kg, higher cellular RNA concentrations and rate of incorporation of radioactive leucine into tissue protein were obtained in the liver and muscle of young adults as compared with the older animals. A kinetic model was developed which can be used to explain the change in tissue protein metabolism that is associated with variations in the intake of dietary nitrogen.