Randomised clinical trial on the effect of a single oral administration of l-tryptophan, at three dose rates, on reaction speed, plasma concentration and haemolysis in horses.
Authors: Noble Glenys K, Li Xiuhua, Zhang Dagong, Sillence Martin N
Journal: Veterinary journal (London, England : 1997)
Summary
# Editorial Summary L-tryptophan is widely marketed as an oral calming supplement for horses, yet evidence supporting its efficacy remains limited. In a randomised, blinded, cross-over trial involving 60 mature horses, researchers administered placebo or single doses of tryptophan at 30, 60, or 120 mg/kg bodyweight and assessed reaction speed using standardised startle tests, whilst monitoring plasma tryptophan concentrations and haematological safety over 96 hours via 432 blood samples. Although oral tryptophan administration successfully elevated plasma concentrations in a dose-dependent manner (P<0.001) and proved safe with no evidence of haemolysis or clinical pathology abnormalities, the supplement produced no measurable improvement in reaction speed or apparent calming effect when horses were startled. These findings suggest that whilst single-dose tryptophan supplementation poses no safety concerns at the doses tested, practitioners cannot rely on it as an evidence-based anxiolytic strategy for horses, and further investigation into alternative dosing schedules, formulations, or chronic administration protocols may be warranted before dismissing tryptophan's putative benefits entirely.
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Practical Takeaways
- •L-tryptophan marketed as a calmative for horses does not demonstrate effectiveness in reducing startle or reaction speed responses, despite industry marketing claims
- •While tryptophan is safe at recommended doses with no evidence of haemolysis or adverse clinical pathology, practitioners should not rely on it as a behavioral management tool
- •Consider alternative evidence-based calming strategies and behavioral management approaches rather than supplementing with tryptophan for anxious or reactive horses
Key Findings
- •Oral L-tryptophan administration at 30, 60, or 120 mg/kg significantly increased plasma tryptophan concentrations (P<0.001) in horses
- •Despite elevated plasma tryptophan levels, there was no significant effect on reaction speed or startle response in horses
- •No evidence of acute haemolytic anaemia or clinical pathology changes in 432 blood samples collected over 96 hours post-administration
- •Single-dose oral tryptophan paste appears safe at tested doses but lacks efficacy as an equine calmative