Equine veterinary journal2007; 40(1); 51-56; doi: 10.2746/042516407X238503

Effects of a commercial dose of L-tryptophan on plasma tryptophan concentrations and behaviour in horses.

Abstract: L-tryptophan is a common ingredient in equine calmative products, but its effectiveness has not been demonstrated in horses. Objective: To determine whether a commercial dose of L-tryptophan increases plasma tryptophan and alters behaviour in horses fed a roughage or concentrate meal. Methods: L-tryptophan (6.3 g) or placebo (water) was administered per os in a cross-over design, to 12 Thoroughbred horses (503 +/- 12.1 kg bwt), just before a meal of lucerne hay or oats. Plasma tryptophan was measured by gas chromatography. Horse behaviour was observed in an empty enclosure, then in the presence of an unfamiliar person and a novel object. Results: Total plasma tryptophan increased 3-fold in both studies, peaking 1.5-2 h after dosing. After the peak, tryptophan remained high for several hours if the horses had been fed hay, but fell sharply if fed oats, consistent with the glycaemic responses to these meals. However, the ratio of tryptophan to 4 large neutral amino acids (phenylalanine, tyrosine, leucine and isoleucine) increased in the tryptophan-treated horses to a similar extent and for a similar duration, with both diets. The presence of a stranger or novel object increased heart rate (P<0.05), but caused no behavioural effects that were altered by tryptophan, regardless of the diet. Conclusions: Plasma tryptophan increases when tryptophan is administered at a dose used in some commercial products, but this is not reflected by marked behavioural changes in the horse. Conclusions: Further work is required to refine behavioural tests and identify an effective dose of L-tryptophan in the horse.
Publication Date: 2007-12-18 PubMed ID: 18083660DOI: 10.2746/042516407X238503Google Scholar: Lookup
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  • Journal Article
  • Randomized Controlled Trial
  • Research Support
  • Non-U.S. Gov't


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This research investigates the impact of the common equine supplement L-tryptophan on horse behavior and plasma tryptophan levels. The study found that while plasma tryptophan levels increased after administration, it did not result in any significant changes in horse behavior, regardless of the type of diet fed.


  • The study used a cross-over design, which means each participant received both the treatment (L-tryptophan) and the placebo at different points.
  • 12 Thoroughbred horses were used, with an average body weight of 503 +/- 12.1 kg.
  • The dose of L-tryptophan or a placebo was administered orally, followed by a meal of either lucerne hay or oats.
  • The plasma tryptophan – an essential amino acid – in the horse after administering the dose, was measured by gas chromatography.
  • The behavior of the horses was observed in various scenarios: in an empty enclosure, in the presence of an unfamiliar person and with a novel object.


  • Following the L-tryptophan dosage, plasma tryptophan in the horses increased three-fold, peaking 1.5-2 hours after administration.
  • The level of tryptophan remained high for several hours in horses fed with hay, while it dropped rapidly in horses fed with oats. This corresponds with the glycaemic responses to the respective meals.
  • The ratio of tryptophan to four large neutral amino acids rose similarly in both diets, reflecting that tryptophan increases regardless of the type of meal.
  • The presence of an unfamiliar person or the introduction of a novel object increased the heart rate of the horses, but there was no significant change in their behavior that could be linked to L-tryptophan.


  • While the administration of a commercial dose of L-tryptophan increased plasma tryptophan level in horses, it did not substantially influence their behavior.
  • Further research is needed to refine the behavioral tests and determine an effective dose of L-tryptophan that could potentially alter the behavior of horses.

Cite This Article

Noble GK, Brockwell YM, Munn KJ, Harris PA, Davidson HP, Li X, Zhang D, Sillence MN. (2007). Effects of a commercial dose of L-tryptophan on plasma tryptophan concentrations and behaviour in horses. Equine Vet J, 40(1), 51-56. https://doi.org/10.2746/042516407X238503


ISSN: 0425-1644
NlmUniqueID: 0173320
Country: United States
Language: English
Volume: 40
Issue: 1
Pages: 51-56

Researcher Affiliations

Noble, G K
  • School of Agricultural and Veterinary Sciences, Charles Sturt University, Wagga Wagga, New South Wales 2678, Australia.
Brockwell, Y M
    Munn, K J
      Harris, P A
        Davidson, H P B
          Li, X
            Zhang, D
              Sillence, M N

                MeSH Terms

                • Amino Acids / metabolism
                • Animal Feed
                • Animals
                • Area Under Curve
                • Behavior, Animal / drug effects
                • Behavior, Animal / physiology
                • Chromatography, Gas / veterinary
                • Cross-Over Studies
                • Heart Rate / physiology
                • Horses / blood
                • Horses / physiology
                • Male
                • Tryptophan / blood
                • Tryptophan / pharmacology


                This article has been cited 2 times.
                1. Staniszewska M, Kowalik S, Sadok I, Ku0119dzierski W. The Influence of Exercise Intensity on Tryptophan Metabolites in Thoroughbred Horses.. Pharmaceuticals (Basel) 2023 Jan 11;16(1).
                  doi: 10.3390/ph16010107pubmed: 36678604google scholar: lookup
                2. Agar C, Gemmill R, Hollands T, Freeman SL. The use of nutritional supplements in dressage and eventing horses.. Vet Rec Open 2016;3(1):e000154.
                  doi: 10.1136/vetreco-2015-000154pubmed: 26925239google scholar: lookup