About natural supplements · Jun 13, 2016
Does tryptophan calm horses or not? The story is complicated
Some papers say it doesn’t work, but the biochemistry tells you exactly why those trials were always going to struggle, and what that actually means for your horse.
Tryptophan supplementation reliably raises plasma tryptophan concentrations in horses, but controlled trials have not demonstrated a measurable calming effect on behaviour. The critical bottleneck is the blood-brain barrier: tryptophan competes with other large neutral amino acids for the same LAT1 transporter, so what a horse has eaten before supplementation largely determines how much tryptophan actually reaches the brain. Only around 5% of metabolised tryptophan is converted to serotonin, and tryptophan hydroxylase is the rate-limiting enzyme in that pathway.
Common misconceptuion – Tryptophan does not calm horses
The role of Tryptophan in your horse’s diet
L-Tryptophan is an amino acid, which, if lacking in the diet, can cause anxiety. Serotonin is made from L-Tryptophan and serotonin is the neurotransmitter in the horse’s brain that helps your horse feel calm. You can see why a horse lacking in L-Tryptophan might be spooky and tense.
Doubts raised on how beneficial Tryptophan is
A friend sent me this review of L-Tryptophan as a calmer for horses: http://bit.ly/1A8hRaS.
Why we think supplementing Tryptophan for horses makes sense
This is my response to her email:
“Thank you for sending this – it’s an excellent article and the authors must’ve spent weeks gathering all this info together. For me, this article was very biased against Tryptophan. Which is probably why it appeared where it did. But they do still put all the relevant facts forward, which is commendable.”
Their points:
“There are zero studies that confirm Tryptophan (Trp) as a calmer in horses.” This is true. It is also true of a huge number of other nutraceuticals. Research into naturally occurring, easily available ingredients is limited, because of the lack of financial return to big pharmaceuticals. Especially in horses. It doesn’t mean that Trp isn’t a valid calmer.
“Of the work that has been done, very low doses (25 and 50mg per day) cause mild excitement. (Bagshaw study.)” As mentioned here, this study was very flawed. Only a few horses, only a few hours, too low a dose, breed differences. IMHO the poor horse probably just got a bit bored of standing around having their heart rate measured. I think this dose of Trp did absolutely nothing.
“Very high oral doses (350 and 600mg/kg bodyweight) lead to anaemia, increased respiration and haemolysis.” This also doesn’t mean that Trp doesn’t work as a calmer. It just means that very high doses are dangerous. Very high doses of carrots are also dangerous. That doesn’t mean that it is bad to feed horses carrots.
The daily dose of Trp included in our Calm Mix is 2000 mg per day. This is above the dose that Bagshaw found to cause slight excitement (25 or 50mg per day) and way below the dose that Paradis found to cause anaemia (175 000 mg or 300 000 mg per day).
They then make the point that other factors (diet, exercise, gender etc.) influence the uptake of Trp and therefore the effectiveness. This is also true (in fact I think it’s a bit of a no-brainer), but also doesn’t mean that Trp doesn’t work.
The one point they make which is that Trp might decrease endurance is valid. But I guess if you have a horse that leaps around like a maniac for half of the endurance ride, that might also decrease his ability to stay the distance. It’s a judgement call.
Then there is the also very valid point that excitability in horses is not exactly well defined. Is it hysteria? Or aggression? Or hyperactivity? Or fearfulness? Because Trp seems (in studies in other species) to have varying effectiveness in controlling these varied symptoms. Aggression, fearfulness and hysteria are decreased by Trp. Whereas hyperactivity isn’t.
The one symptom which has strongly been shown to improve with Trp supplementation in other species is depression. Which these authors immediately disregarded because they felt it was irrelevant to horses. I disagree. I think horses can and do get depressed. And that anything that improves mood is bound (IMHO) to improve behaviour.
Also, in our version of Calm Mix for the UK we have added Vitamin D. In a recent study, Vit D has been found to increase the production of the enzyme Tryptophan Hydroxylase 2 (TPH2). This is the enzyme which drives the conversion of Trp to 5-HTP, which in turn is converted to Serotonin. The conversion of Trp to 5-HTP is the rate limiting step (slows the whole process down). So anything that can increase this step is invaluable in getting a higher Serotonin level in the brain.
Whenever I look at other studies like this, I am more than a bit sceptical. Because, every single day, we get feedback from our customers that Calm Mix works.
What we’d recommend
Calms horses, using Tryptophan and other ingredients.
See the product
Frequently asked questions
Does tryptophan actually calm horses, or is it just a myth?
The honest answer is: the equine clinical evidence is not there yet. Randomised trials, including one on 60 horses given doses up to 120 mg/kg, found no effect on reaction speed when horses were startled. However, elevated plasma tryptophan did not cause harm either, and the absence of equine trials is not the same as proof of no effect. Evidence from other species supports tryptophan’s role in reducing aggression, fearfulness and low mood.
Does the horse’s diet affect how well tryptophan supplementation works?
Yes, significantly. A carbohydrate-rich, low-protein meal triggers insulin release, which clears competing branched-chain amino acids from the blood, giving tryptophan a clearer run at the blood-brain barrier transporter. One study found that feeding oats sharply reduced plasma tryptophan elevation after supplementation, while hay feeding sustained it. What your horse eats alongside a tryptophan supplement will influence whether any meaningful amount reaches the brain.
Are high doses of tryptophan dangerous for horses?
Very high doses carry real risk, but the danger threshold is far above typical supplement levels. A randomised trial using 30, 60, and 120 mg/kg found no haemolysis or clinical pathology changes. Haemolytic anaemia has been associated only with extreme doses, far beyond what any responsible commercial supplement provides. The key is staying within evidence-supported ranges and not conflating toxicity at extreme doses with risk at supplemental doses.
Can tryptophan reduce cribbing in horses?
Based on the available trial evidence, no. A dedicated study testing tryptophan combined with calcium carbonate found no effect on the total number of cribbing bouts per day, duration of bouts, or crib-bites per bout over three weeks. Cribbing is a complex stereotypy with multiple drivers, and tryptophan alone does not appear to address the underlying mechanism.
Sources
- 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
- Effects of a commercial dose of L-tryptophan on plasma tryptophan concentrations and behaviour in horses
- The Influence of Exercise Intensity on Tryptophan Metabolites in Thoroughbred Horses
- Basic Neurochemistry – Serotonin
- Large neutral amino acids: dietary effects on brain neurochemistry and function
- Influence of Tryptophan and Serotonin on Mood and Cognition with a Possible Role of the Gut-Brain Axis
- Carbohydrate craving, obesity and brain serotonin
- Reducing cribbing frequency in horses through dietary supplementation of tryptophan and calcium carbonate