Influence of low- and high-protein diets on glucose homeostasis in the rat

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Abstract
1. The influence of the protein content of the diet on glucose homeostasis was studied in the rat. Rats of 28 d of age received ad lib. a control diet containing (g/kg) 150 protein (P15), or a diet containing 50 protein (P5) or 450 protein (P45). Since P5 rats spontaneously reduced their food intake, a fourth group of rats (P25) received the same amount of energy as P5 rats and the same amount of protein as P15 rats.2. After 12–13 weeks on these diets, plasma glucose and insulin levels were similar in fed P45, P25 and control P15 rats, but were lower in P5 rats. In fasted animals, plasma glucose and insulin levels were also decreased in P5 rats, whereas plasma glucose levels were increased in both P45 and P25 animals.3. During an oral glucose tolerance test, the glucose rise was only slightly larger in P5 than in P15 rats in spite of a considerably smaller increase in insulin levels. P45 rats displayed a normal tolerance to glucose with a normal insulin response, whereas tolerance to glucose was slightly poorer in P25 rats in spite of a normal insulin response.4. Pancreatic insulin stores were lower in P5 than in control P15 rats, not only because of the smaller size of their pancreas, but also because of a decrease in the insulin concentration in the gland. A much smaller decrease was also observed in P25 rats, whereas insulin reserves were not altered in P45 rats.5. It is concluded that the changes in glucose homeostasis observed in protein-energy malnutrition (P5 rats) are due to protein deprivation rather than to energy deprivation. A high-protein diet has little influence on glucose homeostasis in the rat.