The effect of time of feeding on plasma amino acids during exercise and recovery in horses

Feeding management in horses suggests feeding horses in advance of exercise, particularly the grain portion of the diet. Plasma amino acids (AA) peak at 3-6 hours post feeding depending on the amino acid. The timeframe between feeding and exercise may affect the availability of amino acids during and after exercise. The purpose of this study was to observe the differences in plasma amino acids in horses fed prior to exercise or after exercise. Eight light type horses were fed a diet with adequate protein and amino acids for horses in light to moderate exercise. After an adjustment period, horses completed a standardized exercise test (SET). Relative to the SET, horses were fed either 2hr prior (PRE horses) to the SET, 1hr after completing the SET (POST horses) or horses remained fasted throughout the sampling period (FASTED horses). Plasma was drawn prior to exercise, at the peak of exercise as well as at 1, 2, 4 and 7 hours post exercise. Plasma was analyzed for amino acids, glucose, lactate, creatinine, creatine kinase, ammonia, urea-N and 3-methylhistdine. After completion of the SET and sampling period, horses entered a one-week recovery period which was followed by another SET. The protocol repeated until horses rotated through all feeding protocols in the study (3 SETs). The majority of the plasma amino acids were elevated in PRE horses compared to POST horses prior to the SET until 2hr post exercise where POST horses’ plasma amino acid concentrations became elevated and remained elevated until the end of the sampling period. In that same time frame, plasma amino acids for the PRE group decreased out to the end of the sampling period. The elevation of plasma amino acids in POST horses would be expected as they were fed at 1hr post exercise while PRE horses were reaching a 4hr post feeding time frame at this point. This elevation was not observed for plasma concentrations of isoleucine, leucine, methionine and histidine. Concentrations of these amino acids initially were greater for POST horses in the post exercise period however they declined more rapidly than the other amino acids. The rapid decrease of some of the plasma amino acid concentrations may suggest uptake by muscle for recovery. This in conjunction with a decrease in plasma creatine kinase concentrations for POST horses suggests that feeding post exercise may facilitate better muscle protein balance (synthesis vs. breakdown) in the recovery period following exercise.