L-Tryptophan supplementation increases serotonin blood levels in dogs fed a dissociated carbohydrate-based diet

The aim of the present study was to investigate the effect on blood serotonin (5-HT) concentrations of the addition of L-Tryptophan (L-TRP) to a dissociated diet, in the meal consisting only of the carbohydrate component. Six sheltered dogs (5 males and 1 neutered females), weighing between 15 and 30 kg and living in the shelter for more than six months, were recruited. The dogs were fed two daily meals (at 8.00 A.M. and 4.00 P.M.). The morning meal was composed of puffed rice with the addiction of1mg/kg of L-TRP, whereas the evening meal was composed of the commercial diet previously eaten. Blood was collected the day before the first administration of the diet (T0) and the 30th day of the treatment (T1). A further blood sample was collected 14 days after the discontinuation of the diet (T2). To prevent possible influence of circadian rhythm, all three samples were collected at 4.00 P.M., just before the second meal. The results were compared using the Wilcoxon test (p < 0.05). The statistical analysis revealed a significant difference between the blood concentrations of TRP at T1 and those determined at T2 (p = 0.028). No statistical differences between T0 and T1, and between T0 and T2 was found. Significant difference between the blood 5-HT concentration at T1 and T0 (p = 0.028), and between T1 and T2 (p = 0.028) were found. On the contrary,no statistical difference between T0 and T2 was found. The data obtained, albeit carried out on a small number of animals showed that the blood levels of TRP could be correlated with those of 5-HT. Indeed, the determination of blood 5-HT concentrations seems to provide a faithful image of the cerebral metabolism of this amine, as increases in circulating TRP concentrations is followed by significant increases in 5-HT.