Effects of a commercial dose of L-tryptophan on plasma tryptophan concentrations and behaviour in horses.
Authors: Noble G K, Brockwell Y M, Munn K J, Harris P A, Davidson H P B, Li X, Zhang D, Sillence M N
Journal: Equine veterinary journal
Summary
# Effects of L-tryptophan on plasma concentrations and behaviour in horses L-tryptophan is widely incorporated into equine calmative supplements on the premise that it crosses the blood–brain barrier and increases serotonin synthesis, yet empirical evidence supporting its efficacy in horses has remained absent. Noble and colleagues administered a commercial dose of L-tryptophan (6.3 g) or placebo to 12 Thoroughbreds in a crossover design, measuring plasma tryptophan via gas chromatography and observing behavioural responses during neutral, socially challenging, and novel object exposure conditions, with trials conducted following both forage and concentrate meals. Plasma tryptophan increased threefold within 1.5–2 hours post-dosing, though its duration of elevation was diet-dependent: concentrations remained elevated for several hours after hay feeding but declined sharply after oats, mirroring the respective glycaemic profiles; critically, the tryptophan-to-competing large neutral amino acid ratio—the factor determining blood–brain barrier transport efficiency—increased comparably and persisted similarly with both diets. Despite these pharmacokinetic changes, neither the presence of an unfamiliar handler nor exposure to novel stimuli produced behavioural modifications attributable to tryptophan administration, regardless of dietary context. For practitioners and product formulators, these findings suggest that the commercial dose tested achieves measurable systemic tryptophan elevation but fails to translate into observable calming effects under the test conditions, prompting questions about optimal dosing protocols and whether conventional behavioural assessment may lack sufficient sensitivity to detect subtle neurophysiological changes in equine temperament.
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Practical Takeaways
- •Commercial L-tryptophan products may increase plasma tryptophan levels, but this does not reliably translate to observable calming effects in horses
- •The meal composition (roughage vs. concentrate) significantly affects how long tryptophan remains elevated in the blood, yet neither diet produced behavioral benefits
- •Current evidence does not support using standard commercial doses of L-tryptophan for calming purposes; higher doses or alternative formulations may need investigation
Key Findings
- •Commercial dose of L-tryptophan (6.3 g) increased total plasma tryptophan 3-fold, peaking at 1.5-2 hours post-administration
- •Tryptophan remained elevated longer after hay feeding than after oat feeding, correlating with glycemic responses
- •The tryptophan to large neutral amino acid ratio increased similarly with both diets despite different plasma tryptophan kinetics
- •L-tryptophan supplementation produced no significant behavioral changes despite physiological effects, even when horses were exposed to novel stimuli or strangers