Dietary protein intake and 3-methylhistidine excretion in the rat

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Abstract
1. Rats were fed for 14 d on diets containing 50, 150 or 250 g/kg casein as the protein source. The daily excretion of Nt-methylhistidine (His(τMe)), a non-re-utilized amino acid, was determined.2. His(τMe) excretion/100 g body-weight appeared to be unaffected by increasing the concentration of dietary protein from 150 to 250 g/kg. Assuming no change in the proportion of muscle in the animals these results are indicative of no change in myofibrillar protein catabolic rate. The excretion rate/100 g body-weight of the animal given 50 g/kg casein was lower than the other two treatments, especially towards the end of the 14 d treatment period. Thus at this time the myofibrillar-protein catabolic rate was lower than in the animals fed on the higher protein diet.3. In the animals fed on the high protein diet there was a tendency for this (τMe) excretion rate/100 g body-weight to increase with age.4. Nitrogen balance and creatinine excretion results are also presented.