Interaction of Ingested Leucine with Glycine on Insulin and Glucose Concentrations

The majority of individual amino acids increase insulin and attenuate the plasma glucose response when ingested with glucose. Objective. To determine whether ingestion of two amino acids simultaneously, with glucose, would result in an additive effect. Leucine (Leu) and glycine (Gly) were chosen because they were two of the most potent glucose-lowering amino acids when given individually. Materials and Methods. Nine subjects received test items on four separate days. The first was a water control, then 25？g glucose, or Leu + Gly (1？mmol/kg fat-free mass each) ±25？g glucose, in random order. Glucose, insulin, and glucagon were measured frequently for 2.5 hours. Net areas were calculated. Results. The glucose area response decreased by 66%. The insulin area response increased by 24% after ingestion of Leu + Gly + glucose compared to ingestion of glucose alone. The decrease in glucose response was not additive; the increase in insulin response was far less than additive when compared to previously published individual amino acid results. The glucagon concentration remained unchanged. Conclusion. There is an interaction between Leu and Gly that results in a markedly attenuated glucose response. This occurred with a very modest increase in insulin response. Changes in glucagon response could not explain the results. The mechanism is unknown. 1. Introduction In a series of studies our laboratory has demonstrated that single amino acids when ingested at 1.0？mmol/kg fat free mass with or without 25？g of glucose stimulate a rise in insulin and glucagon. When ingested with glucose they are capable of attenuating the glucose rise integrated over a 2.5-hour period of time. Sixteen amino acids were studied. The potency of the individual amino acids in regard to their effect on the insulin, glucagon, and glucose concentrations varied greatly between amino acids and the specific responses could not be predicted based on the structure of the amino acids [1]. Leucine is well recognized to strongly stimulate insulin secretion and to reduce the glucose rise when ingested with glucose [2]. When ingested with glucose, glycine similarly reduced the glucose rise, but only weakly stimulated insulin secretion. [2, 3]. Therefore, we were interested in determining if the glucose-attenuating effect would be greater if these two amino acids were ingested together with glucose. We were particularly interested in whether the effects would be additive since glycine produced a major decrease in glucose response in the presence of only a very modest increase in insulin; that is, it may