Tryptophan metabolism has been demonstrated to play a role in many disease states and changes in the kynurenine pathway metabolites are being linked to depression, cancer, and neurodegenerative disorders. Like other metabolic pathways, diet can affect the observed levels of downstream metabolites. Therefore, it is critical to understand how common laboratory diets can alter levels of these metabolites in experimental animals.
A recent study published by Dr. Yuhei Yajima et al. in the International Journal of Tryptophan Research investigated how common laboratory diets can alter kynurenine metabolite levels. In this study three experimental laboratory diets (MF, AIN-93G, and AIN-93M) were fed to male C57BL/6J mice. Mice fed the MF diet had greater food and water intake compared to those on either of the AIN diets however, there was no statistical differences in weight gain. Interestingly, mice on the MF diet had significantly more plasma kynurenine. Furthermore, there were marked metabolic changes in brain tissue. MF mice had significantly lower levels of tryptophan in the hippocampus and striatum and lower kynurenine levels in the hippocampus as compared to both AIN diets. The researchers next investigated amino acid levels in the plasma and found that several including asparagine, glutamine, serine, and lysine were significantly lower and ornithine was significantly higher in the MF fed animals. Not surprisingly, PCA analysis of urinary metabolites demonstrated that MF animals were separated from both AIN diet groups which clustered together. This study highlights the importance of choosing an appropriate diet for laboratory animals and an understanding that metabolite levels can be affected by feed choice, thereby potentially confounding study outcome. Further studies should look into metabolite differences in other mouse strains, laboratory feeds, as well as in females.