Dietary Tryptophan Supplementation Causes Distinct Hepatic Transcriptional and Fecal Microbial Responses in Dairy Calves

Dietary tryptophan may influence early host and microbial development, but responses to different supplementation levels in dairy calves remain unclear. This study examined the effects of tryptophan supplementation on organ development, hematological parameters, hepatic gene expression, and fecal microbial and tryptophan metabolic profiles. Thirty-six dairy calves were assigned to four treatments in a randomized complete block design, receiving 0, 0.25, 0.50, or 0.75 g tryptophan/calf/d, with nine calves per treatment. Fecal metagenomics and targeted tryptophan metabolite profiling were performed in all calves, and organ measurements and liver transcriptomics were conducted in six calves per treatment. No significant overall treatment effects were detected for organ weights, intestinal lengths, or hematological parameters. Fecal kynurenic acid showed an overall treatment effect (p = 0.026) and a negative linear contrast across supplementation levels (p = 0.005), with lower concentrations at 0.50 and 0.75 g/calf/d than in controls. Relative to controls, the 0.25, 0.50, and 0.75 g/calf/d groups had 197 upregulated and 170 downregulated, 447 upregulated and 315 downregulated, and 1691 upregulated and 267 downregulated hepatic genes, respectively. Upregulated genes were enriched in olfactory transduction, whereas downregulated genes were enriched in cytokine–cytokine receptor interaction at all supplementation levels and IL-17 signaling at the two higher levels. Fecal microbial alpha diversity did not differ among treatments, whereas community composition showed an overall treatment effect (p = 0.046). More differentially abundant microbial taxa and functional pathways were detected at the two lower supplementation levels than at the highest level. These findings describe distinct hepatic transcriptional and fecal microbial responses to tryptophan supplementation, but no causal link between these responses was established.