Equine Science Update
Reports from the world of  equine research.
Does tryptophan calm horses?
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Many feed supplements are marketed for calming excitable horses. A common ingredient is L-tryptophan, often combined with other “calming” ingredients such as magnesium and thiamine.

Tryptophan is an essential amino acid, which the horse is unable to manufacture  itself and needs to obtain in the diet. The body uses tryptophan to make serotonin, a neurotransmitter. Work in other species and humans shows that the concentration of serotonin varies during the day. It has the lowest concentration in the morning, and rises to a peak in the evening. Increased levels of serotonin in the brain have been associated with sedation, increased sleepiness, reduced aggression, and reduced fearfulness.

The reasoning behind giving extra tryptophan is that it should lead to an increase in serotonin in the brain, which in turn should have a calming effect. But it’s not quite as simple as that. A number of factors may influence the uptake of tryptophan by the brain, including the type and amount of fat, protein and carbohydrates in the diet.

A diet containing low levels of fat would reduce the availability of tryptophan by increasing the amount of tryptophan bound to the proteins in the blood. Conversely, horses on high carbohydrate diets may be more likely to take tryptophan into the brain. Horses on such diets tend to be more excitable anyway - so if the tryptophan has an effect it might be more noticeable.

Although tryptophan has been shown to have a calming effect in some species, there is little evidence that it is effective in horses. There have been few studies of the effect of tryptophan in horses, and none has produced evidence that it has a calming effect in horses. Indeed, high doses may have toxic effects. Shetland ponies given high doses (600mg/kg) of tryptophan by stomach tube became restless, had an increased respiratory rate, and haemolysis.  Another study found that tryptophan supplementation caused reduced stamina.

A recent behavioural study found that a commercial tryptophan product, fed at the recommended dose rate, had no calming effect on horses subjected to standardised fear and handling tests. Dr Jens Malmkvist and Dr Janne Winther Christensen, at the Danish Institute of Agricultural Sciences Tjele, in Denmark, assessed the effect of tryptophan on the response of young horses to frightening stimuli.

Twenty-eight two-year-old Danish Warmblood horses were accustomed to wearing a heart monitor, and being separated from their fellows and fed individually. All of the horses received a similar diet and exercise regime. For the test, the horses were fed either the recommended dose of a commercial tryptophan-containing supplement, or a placebo.  Neither the handler nor the observer knew which treatment each horse had been given.

The horses were tested 2-3 hours after being treated. Two experiments were carried out. The first one assessed the response of stallions to white noise. Stallions were individually turned into a pen to see if they were put off eating by a novel stimulus (white noise emitted from a CD player close to the feed container). The researchers recorded heart rate and assessed the horses’ behaviour.

In the second experiment mares were introduced to a pen that contained a red and white plastic curtain close to a feed container. The horses’ responses were recorded, and they were also assessed while an experienced handler tried to lead them through the plastic curtain.

The scientists found that a single dose of tryptophan, at the recommended dose rate, failed to have a calming effect on the horses. They found no significant differences in response to the frightening stimuli between the tryptophan or placebo groups. Neither did the tryptophan make the horses easier for the handler to lead.

Rather than rely on tryptophan to calm fearful horses, Malmkvist and Christensen recommend that a better approach is to use effective habituation methods, combined with a better understanding of the causes of fearful behaviour.

For more details see:

A note on the effects of a commercial tryptophan product on horse reactivity.
J Malmkvist, JW Christensen
App Anim Behav Sci (2007) 107, 361- 366


For a review of  L-tryptophan- see also
Calmatives for the excitable horse: A review of L-tryptophan.
A Grimmett, MN Sillence.
Vet J (2005) 24 - 32.

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Written by Mark Andrews. Last updated: 28.10.07.
© Copyright Equine Science Update  2007