Effects of leucine or whey protein addition to an oral glucose solution on serum insulin, plasma glucose and plasma amino acid responses in horses at rest and following exercise

Reasons for performing study: Providing protein or amino acid mixtures in combination with glucose to post exercise in man has resulted in increases in the post feeding insulin response and in muscle glycogen and protein synthesis rates. However, whether protein and/or amino acids can modify the post exercise insulin responses in horses remains to be fully elucidated.Objectives: To determine whether whey protein or leucine addition to a glucose solution affects the post gavage plasma insulin, glucose and amino acid responses in horses and whether these responses are different following a period of exercise vs. rest. Methods: Six mature, conditioned Thoroughbreds received a nasogastric gavage containing either 1 g/kg bwt glucose (G), G + 0.3 g/kg bwt whey protein (GW) or G + 0.3 g/kg bwt leucine (GL), following a period of either rest (R) or an exercise test on a high speed treadmill (EX). Each horse was studied under all 6 treatment conditions, separated by 10 day intervals. Blood samples were collected pre-exercise/rest, pregavage and at regular intervals up to 300 min post gavage. Plasma was analysed for glucose and amino acid concentrations and serum insulin concentrations were determined. Results: There was a significantly (P<0.05) greater insulin response in GL-R and GL-EX when compared to the other treatments. When compared to rest, post exercise plasma glucose responses were lower in G and GW but unchanged following GL administration. Plasma alanine concentrations were elevated post exercise in all EX treatments. With the exception of markedly elevated plasma leucine concentrations after GL-R and GL-EX, the plasma concentrations of all indispensable amino acids decreased during the post gavage period.Conclusions: Leucine but not whey protein augmented the serum insulin response to an oral glucose load. Leucine supplementation warrants further investigation as a means to increase the rate of post exercise muscle glycogen synthesis in horses.