Dietary supplementation with tryptophan increases the plasma concentrations of tryptophan, kynurenine, and melatonin in Yili mares

Context Tryptophan (Trp) is the precursor of melatonin (MT) and the latter plays vital physiological roles in mares.Aims The purpose of this experiment was to investigate the effects of dietary Trp supplementation on the plasma Trp, kynurenine (Kyn), 5-hydroxytryptophan (5-HT), and melatonin (MT) concentrations in female Yili horses.Methods Twenty Yili mares aged 2years with mean bodyweight (BW) of 263.5±14.77kg and of similar stature were selected and randomly allocated to the control (CON; basal diet), basal diet plus Trp at 20mg/kg BW (TRP1), basal diet plus Trp at 40mg/kg BW (TRP2), or basal diet plus Trp at 60mg/kg BW (TRP3) group.Key results The plasma total Trp, Kyn, and MT concentrations in all Trp groups steadily increased, reached their peak values, and gradually decreased after Trp supplementation between 0h and 12h. However, the plasma 5-HT concentration displayed the opposite trend. Peak plasma total Trp and 5-HT concentrations were attained between 1h and 3h, while those of KYN and MT appeared between 4h and 6h after Trp supplementation. The plasma total Trp and Kyn concentrations were significantly higher in TRP2 and TRP3 than in CON between 1h and 12h (P<0.05) after Trp supplementation. The plasma 5-HT concentration was significantly (P<0.05) lower in TRP1 than in CON at 3h, 4h, 6h, 9h, and 12h after Trp supplementation. The plasma MT concentrations in TRP1 and TRP2 were significantly (P<0.05) higher than in CON at 3h, 4h, and 12h, and at 0h, 1h, and 12h after Trp supplementation (P<0.05).Conclusions Dietary Trp supplementation can increase the plasma total Trp, Kyn, and MT concentrations in Yili mares and the optimal Trp dosage was 20mg/kgBW.Implication The addition of Trp to a basal diet or feed may increase the plasma total Trp, Kyn, and MT concentrations in female horses.