The gut-brain axis is a dynamic network linking gut microbiota, metabolism, and brain function. This study explores the role of Nicotinamide Adenine Dinucleotide (NAD+), a key coenzyme in energy metabolism, synthesized de novo from dietary tryptophan (Trp). Trp also regulates neurotransmission and immunity. Gut microbiota influence Trp metabolism, impacting neurological and immune responses. This bidirectional relationship is critical for understanding conditions such as inflammatory bowel disease (IBD), highlighting the interplay between microbial composition, inflammation, and metabolism.