Preliminary evaluation on the effectiveness of varying doses of supplemental tryptophan as a calmative in horses

Abstract

Tryptophan (Trp), the amino acid precursor to serotonin, is a common ingredient in many commercial equine calming supplements. However, there is little scientific research to support the efficacy of tryptophan at modifying horse behavior. The objective of this study was to examine how various doses of supplemental tryptophan impact reactive behavior and physiological stress measurements in the horse. Eleven horses (9 geldings, 2 mares) were assigned a series of four treatments in a randomized crossover design. Supplemental tryptophan was provided orally at rates of 0 (CON), 20 (LOW), 40 (MED), and 60 (HIGH) mg/kg bodyweight for three days. Horses received behavior tests to measure startle response on Days 1 and 3 of treatment. Heart rate measurements and the speed at which horses fled from startling stimuli were recorded. Serum glucose, lactate, and cortisol levels were compared before and after startle tests. Additionally, a subset of samples was analyzed for tryptophan content and the ratio of tryptophan to other large neutral amino acids. Significant sedative effects were seen at LOW Day 1 on heart rate during the startle test (P=0.05) and on change in serum lactate levels (P=0.03). At MED Day 1, sedative effects were seen on change in serum cortisol levels (P=0.01). Excitatory effects were seen at MED Day 3 on the time for heart rate to return to baseline after the startle test (P=0.03). No significant effects were seen at the HIGH dose. Amino acid analysis verified treatment effect on Day 1. Across treatments, levels were similar on Days 1 and 3 (P=0.9) while there were no significant increases in amino acids on Day 3 (P=0.98). Overall, tryptophan supplementation had both sedative and excitatory effects on physiological stress measurements but did not change reactive behavior of horses in this study.