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Plasma Insulin and Amino Acid Concentrations in Rats Given an Adequate or Low Protein Diet^1,2,

Department of Nutrition and Food Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

Plasma insulin and free amino acid levels were measured in young male rats given inadequate amounts of dietary protein. In experiment 1, rats were fed an adequate (18% lactalbumin) or low protein (0.5% lactalbumin) diet for 5 weeks; some protein-depleted rats were then refed with the adequate diet for 1 week. After 1, 3 and 5 weeks of protein deprivation and after refeeding, groups of rats from each diet treatment were fasted overnight and force-fed a test dose of one of the experimental diets; plasma insulin, glucose and amino acid levels were measured after 30 and/or 60 minutes. Basal insulin levels rose after the oral load; after the 3- and 5-week periods, however, the increment was significantly less in protein-deprived rats than in well-nourished controls. The response of plasma amino acid levels to force-feeding suggested an impaired uptake of amino acids by muscle in rats deprived of protein for 3 or 5 weeks. In experiment 2, rats were given the experimental diets for 1 week and then groups of 10 were killed at 3-hour intervals throughout the day, beginning at 1200 hours, while they were fed their usual low protein (group A) or adequate protein (group B) diet. Another group of protein-depleted rats (group C) was fed the adequate protein diet, from 1200 hours. Plasma insulin and essential amino acid levels were lower at all times in the low protein group (group A) than in the controls (group B); they decreased further still at the beginning of the dark period and then rose gradually during the next 12 hours. In contrast, plasma amino acid concentrations (from group B) rose at the beginning of the dark period and peaked at 2400 to 0600 hours. In group C, insulin levels rose immediately after refeeding and remained high throughout the 24-hour day; amino acid levels also rose immediately after refeeding. The pattern of daily fluctuation in amino acid levels for group C was comparable to that of well-nourished rats. The results are discussed in relationship to the importance of both insulin and amino acids as factors associated with the altered rates of muscle protein synthesis during dietary protein restriction and refeeding.

KEY WORDS: • insulin • plasma amino acids • protein depletion

¹ Supported by Grant No. AM-13432 from the National Institutes of Health and a grant from the William S. Merrill Company.

² Contribution no. 1940 from the Department of Nutrition and Food Science, M.I.T.

Manuscript received 26 October 1972.