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Background: Ingested proteins are known to stimulate a rise in insulin and glucagon concentrations. In our effort to explain this effect, we have begun to measure the effect of individual amino acids. Objectives: The objectives were to determine the effect of lysine ingestion on insulin and glucagon concentrations and whether the effect is moderated by glucose ingestion. Design: Thirteen healthy subjects were studied on 4 occasions. Water, 25 g glucose, 1 mmol lysine/kg lean body mass, or lysine plus glucose was given on separate occasions at 0800 after a 12-h fast. Serum lysine, glucose, insulin, and glucagon were measured during a 2.5-h period. The amount of lysine provided was equivalent to that present in a 672-g (24-oz) steak. Results: Lysine ingestion resulted in an '3-fold increase in lysine concentration and in a small decrease in glucose concentration. When lysine was ingested with glucose, the 2.5-h glucose area response decreased by 44% (P , 0.02). Lysine alone increased the insulin area response modestly; the insulin increase when lysine was ingested with glucose was similar to that when only glucose was ingested. Lysine stimulated an increase in glucagon (P , 0.02), whereas glucose decreased glucagon. Conclusions: Lysine ingestion results in a small decrease in serum glucose and an increase in glucagon and insulin concentrations. Lysine ingested with glucose dramatically attenuated the glucosestimulated glucose response, but there was no change in insulin response. Whether similar effects will be observed with more physiologic doses of lysine remains to be determined. 2009;90:314-20. Am J Clin Nutr