Research has shown that milk has an insulinotropic effect that cannot be explained by the presence of lactose. Evidence suggests that milk is able to stimulate the release of insulin because of the amino acids it contains. In particular, the whey protein fraction of milk seems to account for most of this insulinotropic effect. Whey protein is rapidly absorbed when ingested in isolation, but this rapid absorption is not responsible for the release of insulin because the same effect is still seen when whey is ingested in a mixed meal. Instead, it may be that the insulinotropic effects of whey can be mainly accounted for by the presence of the branched chain amino acids (BCAA) iosleucine, valine, and particularly leucine. Glucose-dependent insulinotropic polypeptide (GIP) may also stimulate insulin release following amino aid ingestion, but it is not clear as to the extent of this effect.
Research published in the American Journal of Clinical Nutrition in 20071 investigated different amino acid mixtures for their insulinotropic effect, and compared them to whey protein. Twelve healthy subjects were given a drink of pure glucose (25 mL in 250 water mL) as a reference or glucose drink (25 mL) combined with free amino acids or whey protein (in 250 mL water). The test drinks consisted of lysine and threonine; leucine, isoleucine and valine (BCAA); leucine, isoleucine, valine, threonine and lysine; or 18g whey protein. The branched chain amino acid drink containing leucine, isoleucine and valine resulted in a significantly higher insulin response than the glucose reference. However, the test drink containing leucine, isoleucine, valine, threonine and lysine had a similar insulinotropic effect to that seen after whey protein but only the reference glucose drink and whey protein drink stimulated a GIP response.
These results suggest that the BCAA are the main reason for the insulinotropic effects of whey protein. However, the fact that only the whey protein caused a significant release of GIP, suggests that GIP only plays a small role in the insulinotropic effect of amino acids. Whey protein is a good source of BCAA, and it would appear that their appearance in plasma is the stimulatory mechanisms for insulin release. In other studies, the insulinotropic effects of leucine have been studied and found to be particularly strong. However in this study, despite the plasma area under the curve for leucine being higher for the branched chain test drink and the branched chain plus threonine and lysine test drinks compared to the whey protein drink, no significant differences were seen in the insulin stimulating effects. This suggests that the insulinotropic effects of whey are not due solely to leucine.
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