Br. J. Nutr. zyxwvutsrq (rg80), zyxwvuts 4.(. 381 Dietary protein intake and 3-methylhistidine excretion in the rat BY L. C. WARD* AND P. J. BUTTERY Department zyxwvut of Applied Biochemistry and Nutrition, University of Nottingham, School of Agriculture, Sutton Bonington, Loughborough, LE12 5RD (Received 24 November zyxwvu 1978 zyxw - Accepted 27 June 1980) I. Rats were fed for 14 d on diets containing zyxwv 50, zyxwvut 150 or 250 g/kg casein as the protein source. The daily excretion of “methylhistidine (His(TMe)), a non-re-utilized amino acid, was determined. 2. His(rMe) excretion/Ioo 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 rnyofibrillar protein catabolic rate. The excretion rate/roO 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 @Me) excretion rate/IoO g body-weight to increase with age. 4. Nitrogen balance and creatinine excretion results are also presented. The measurement of the rate of excretion of Nr-methylhistidine (His(TMe)) provides a method for the continuous monitoring of the breakdown of myofibrillar proteins in muscle. Whilst the caution with which results obtained using this technique should be interpreted cannot be over emphasized, the technique would appear to be valid (for review see Ward & Buttery, 1978). Using this technique Young et al. (1973) demonstrated a decrease in myo- fibrillar protein breakdown in man during starvation for 20 d while Rao & Nagabhushan (1973) have demonstrated a similar reduction in catabolic rate in man during protein-energy malnutrition. Haverberg et al. (1975) have also reported similar results in protein-energy malnourished rats. Nishizawa et al. (1977) observed an increase in the breakdown of myofibrillarproteins followingfeeding high-protein diets to rats. Since in these previously published studies only relative extremes of protein depletion and supplementation were investigated it seemed desirable to study the changes in urinary His (TMe) excretion that occur when animals are fed on diets containing a comparatively moderate excess or deficiency of protein and to relate these changes to the over-all nitrogen status of the animal. Such a study has particular relevance to comparable studies in large farm animals, particularly cattle where the His(TMe) technique would appear to be valid (Harris & Milne, 1978), as such animals are unlikely to experience such extreme variation in the composition of the diet as used in the previously reported studies with the rat referred to previously. In the present report the results of the effects of diets containing 50, 150 or 250 g casein/kg on the His(7Me) excretion and N status of rats are reported. Australia 4067. * Present address: Department of Biochemistry, University of Queensland, Brisbane, Queensland, 0007-1 145/80/3255-2706 Sol .oO @ 1980 The Nutrition Society Downloaded from https://www.cambridge.org/core. IP address: 54.234.194.95, on 29 Nov 2021 at 10:49:10, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1079/BJN19800053