Host–Gut Microbiota Interactions: Exploring the Potential Role of Vitamin B1 and B2 in the Microbiota–Gut–Brain Axis and Anxiety, Stress, and Sleep Quality

<i>Background:</i> The microbiota-gut-brain axis plays a key role in regulating mental health, including anxiety, stress, and sleep quality. Vitamins B1 and B2 may influence these outcomes by modulating gut microbiota. This study aimed to examine the relationships among mental health indicators, gut microbiota, and levels of vitamins B1 and B2. <i>Methods</i>: This study conducted a cross-sectional analysis to explore associations between mental health status, gut microbiota composition and function, and circulating vitamin B1/B2 levels. Ten representative microbes were selected for analysis. Mediation models were used to assess whether gut microbiota mediate the effects of vitamins on mental health. <i>Results:</i> Vitamin B1 and B2 levels were significantly associated with stress, sleep quality, and sleepiness (<i>p</i> < 0.05). The abundance of specific gut microbiota also showed significant inter-correlations (<i>p</i> < 0.05). Specific gut microbiota abundances are correlated with host anxiety, stress, sleep, and sleepiness levels (<i>p</i> < 0.05). We did not observe significant differences in the abundance of specific gut microbiota associated with different vitamin B1 and B2 nutritional levels in the host (<i>p</i> > 0.05). Gut microbial diversity and composition varied notably between different vitamin level groups and anxiety, stress, sleep quality, and sleepiness groups. Although both vitamin B2 and <i>Bacteroides</i> had significant direct effects on sleep quality (<i>p</i> < 0.05), no mediating effect of <i>Bacteroides</i> was observed (<i>p</i> > 0.05). <i>Conclusions:</i> These findings suggest potential associations between vitamins B1 and B2 and mental health, as well as between gut microbiota and host psychological outcomes; no significant mediating effect of the microbiota was observed. These exploratory results offer preliminary insights for future research on microbiota-targeted interventions and precision nutrition strategies.