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The metabolic response to ingested glycine^1,2,3

¹ From the Metabolic Research Laboratory and the Section of Endocrinology, Metabolism, and Nutrition, Minneapolis VA Medical Center (MCG, JAN, and FQN), and the Departments of Medicine (MCG and FQN) and Food Science and Nutrition (MCG), University of Minnesota, Minneapolis.

Background: The metabolic effects of dietary protein are complex. In persons with type 2 diabetes, protein ingestion results in little or no increase in plasma glucose concentrations but a stimulation of insulin and glucagon secretion. Furthermore, when protein is ingested with glucose, a synergistic effect on insulin secretion is observed. The most potent protein is gelatin, which consists of 30% glycine residues.

Objective: The objective of the present study was to determine whether glycine per se stimulates insulin secretion or reduces the glucose response when ingested with glucose.

Design: Nine healthy subjects were tested on 4 separate occasions. Plasma glucose, insulin, glucagon, and glycine concentrations were measured at various times during a 2-h period after the ingestion of 1 mmol glycine/kg lean body mass, 25 g glucose, 1 mmol glycine/kg lean body mass + 25 g glucose, or water only, given in random order.

Results: Plasma concentrations of glycine and glucagon were elevated after the ingestion of glycine, as expected. The serum insulin concentration also was slightly elevated after the ingestion of glycine alone. When glycine was ingested with glucose, the plasma glucose area response was attenuated by > 50% compared with the response after the ingestion of glucose alone. The dynamics of the insulin response after the ingestion of glycine plus glucose were modestly different from those after the ingestion of glucose alone, but the area response was not significantly different.

Conclusion: The data are compatible with the hypothesis that oral glycine stimulates the secretion of a gut hormone that potentiates the effect of insulin on glucose removal from the circulation.

Key Words: Glycine • insulin • glucose • glucagon • gut hormones • incretin • amino acids

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