2016 Spring.Includes bibliographical references.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 tryptophan supplementation impacted reactive behavior and physiological stress measurements in the horse. Eleven horses (9 geldings, 2 mares) were given four treatments—0 mg Trp/kg bodyweight (CON), 20 mg Trp/kg bodyweight (LOW), 40 mg Trp/kg bodyweight (MED), and 60 mg Trp/kg bodyweight (HIGH)—in a randomized crossover design. Each treatment lasted three days. On Days 1 and 3 of each treatment, horses underwent a behavior test to measure startle response. Heart rate measurements and the speed at which the horses fled from startling stimuli were recorded. In addition, serum glucose, lactate, and cortisol levels were analyzed both immediately before the startle test and again 15 minutes after the test. Significant sedative effects were seen at LOW Day 1 on heart rate increase 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). Some 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 HIGH Day 1 or Day 3. A subset of blood samples was analyzed for serum free Trp and the ratio of Trp to other large neutral amino acids, which verified treatment effect
