Association of the newly proposed dietary index for gut microbiota and depression: the mediation effect of phenotypic age and body mass index.

Background: Gut microbiota and depression have garnered attention. The dietary index for gut microbiota (DI-GM) is a newly proposed index that reflects the diversity of gut microbiota, yet its association with depression remains unstudied.

Methods: Data from the National Health and Nutrition Examination Survey were analyzed. Depression was assessed using Patient Health Questionnaire (PHQ-9). Dietary recall data were used to calculate the DI-GM (including components beneficial and unfavorable to gut microbiota). Multivariable weighted logistic and linear regression were employed to investigate the association of DI-GM with depression and total PHQ-9 score. The potential mediating role of phenotypic age and body mass index (BMI) was explored. Secondary analyses included subgroup analyses, restricted cubic spline (RCS), and multiple imputation.

Results: A higher DI-GM and beneficial gut microbiota score were associated with a lower prevalence of depression (DI-GM: OR = 0.94, 95% CI = 0.89, 0.99; beneficial gut microbiota score: OR = 0.88, 95% CI = 0.82, 0.94) and lower total PHQ-9 score (DI-GM: β=-0.09, 95% CI=-0.14, -0.04; beneficial gut microbiota: β=-0.15, 95% CI=-0.21, -0.08). RCS indicated a non-linear relationship between DI-GM and depression. A significant mediating effect of phenotypic age (proportion of mediation: 19.81%, 95% CI: 12.86-63.00%) and BMI (proportion of mediation: 16.49%, 95% CI: 12.87-62.00%) was observed.

Conclusions: The newly proposed DI-GM was negatively associated with the prevalence of depression and total PHQ-9 score. Mediation analyses demonstrated a significant mediating effect of phenotypic age and BMI.