Effect of diet on plasma tryptophan and serotonin in horses: a link to behaviour?

This study investigated the effect of two different diets on plasma tryptophan (TRP) and serotonin (5-HT) concentrations in 36 Dutch Warmblood horses, examining their relationship with behavior. Subjects were divided into two groups: a high-forage diet (HF,n=18) and a high-starch diet (HS, n=18). During the eighth week of the trial, following a protocol of moderate exercise, blood samples were collected three hours post-feeding. Plasma 5-HT and pH levels were significantly higher in the HF group compared to the HS group (P<0.05 and P<0.01, respectively), while mares exhibited significantly higher TRP levels than geldings (P<0.05). Although no initial dietary effects on behavior were observed, horses in the HS group displayed increased excitability and reduced manageability during exercise, and further developed crib-biting behavior after an additional four weeks of the trial. These results confirm that plasma 5-HT is influenced by diet and sex; however, further research is required to determine the validity of plasma 5-HT as a marker of serotonergic activity and its specific relationship with equine behavior.