Nutritional Modulation of the Microbiota-Gut-Brain Axis in Cancer-Related Fatigue: A Narrative Review of Mechanisms, Biomarkers, and Interventions

Background/Objectives: Cancer-related fatigue (CRF) is one of the most prevalent and disabling symptoms among patients with cancer, but its underlying mechanisms and effective management strategies have not yet been fully elucidated. Increasing evidence suggests that the microbiota-gut-brain axis (MGBA) may contribute to the pathogenesis of CRF by integrating nutritional status, intestinal barrier integrity, microbial metabolites, immune responses, and host energy metabolism. However, direct causal evidence in human CRF remains limited, and several mechanistic insights are extrapolated from other disease models. This narrative review aims to synthesize evidence for nutritional MGBA-CRF regulatory mechanisms, focusing on microbiota-associated biomarkers and microbiome-targeted dietary interventions, and to distinguish directly observed in a CRF population from indirect hypothetical mechanisms.