SummaryUrinary excretions of creatinine (CR) and 3-methylhistidine (3-MeHis), and skeletal muscle mass (SKM) were measured in rats (male, Wistar, weighing about 290g) fed a 20% casein diet ad libitum, rats restricted to 70% of the food intake of the ad libitum group, and rats deprived of food for 20days. At the same time, catabolic rates of muscle protein derived from 3-MeHis/CR, 3-MeHis/BW, and 3-MeHis/SKM were compared. The CR/SKM ratio in the energy-restricted group was similar to the ad libitum group, while the ratio was higher in the food-deprived group than in the other two groups. This means that CR excretion was promoted per unit of skeletal muscle mass by food deprivation. These results indicate that CR excretion can be an index for SKM under the condition of energy restriction, but not under the condition of food deprivation. The present data also suggest that it is possible to use the 3-MeHis/CR ratio instead of 3-MeHis/SKM which is the most reliable index of the three indices, under the condition of mild energy restriction. In food deprivation, however, the 3-MeHis/CR data tended to underestimate the catabolic rate of muscle protein compared with the 3-MeHis/SKM data. The 3-MeHis/BW data was similar to the 3-MeHis/SKM with all dietary conditions. The catabolic rate of muscle protein was constant or decreased very slightly under ad libitum feeding, decreased slightly in energy restriction, and increased drastically with food deprivation.
It is well known that in protein-energy malnutrition of the kwashiorkor type and in experimental protein deficiency, essential amino acids (EAAs) in the plasma tend to be decreased, while some nonessential amino acids (NEAAs) are increased. Previously, we examined the effect of dietary energy on plasma free amino acids in young men on low egg or rice-protein diet (1). Longnecker and Hause (2) reported that free amino acids in plasma roughly reflect the amino acid pattern of ingested protein. But in the young men on diets containing graded levels of wheat gluten, plasma lysine was usually maintained at the normal level, while the plasma valine concentration decreased significantly, even in the group on 1.0 g/kg of gluten (3). These findings in young men differ from results in children (4, 5) and growing animals (6) fed on gluten diets, suggesting that the responses of free amino acids in the plasma or tissues to the dietary protein may differ in adult and growing animals and humans. These results also suggest that in protein deficiency, plasma free amino acids may reflect not the amount or pattern of the protein consumed, but rather changes in amino acid metabolism in various tissues in response to the diet. Accordingly, the metabolic and clinical significances of plasma free amino acids should be considered in relation to changes in tissue levels.However, most previous studies on the effect of dietary protein on free amino acids in the plasma and tissues have been performed on young, not adult animals. Therefore, in this study we used adult rats and examined 1) the effects of the quality and quantity of ingested protein on free amino acids in the plasma, 2) the effect of dietary protein on the tissue free amino acid level and the influence of the latter on free amino acid levels in the plasma, and 3) the significance of plasma free amino acids as an index of the protein nutritional status.
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