Adherence to the dietary index for gut microbiota and the 5-year incidence of metabolic dysfunction-associated steatotic liver disease in Iranian adults: a prospective cohort study

Background Diet is a key modulator of gut microbiota and may influence the development of metabolic dysfunction-associated steatotic liver disease (MASLD). The Dietary Index for Gut Microbiota (DI-GM) has been proposed to capture the overall capacity of diet to promote a favorable gut microbial profile. Prospective evidence linking DI-GM to MASLD risk remains limited. Methods This prospective analysis included 5,058 adults without MASLD at baseline from the Monitoring of Metabolic Diseases Risk Factors in Tehran (MMRT) study. Dietary intake was assessed using a validated 125-item food frequency questionnaire. The five-year incidence of MASLD was evaluated using multivariable logistic regression models, and associations were expressed as odds ratios (ORs) with 95% confidence intervals (CIs). Subgroup analyses were conducted to assess potential effect modification. Sensitivity analyses examined the robustness of results after excluding participants with substantial weight gain and after additional adjustment for metabolic and dietary factors. Mediation analyses were performed to explore potential pathways underlying the observed associations. Results Over five years of follow-up, 562 participants developed MASLD. Higher DI-GM scores were associated with a lower likelihood of incident MASLD. In the fully adjusted model, individuals in the highest quartile of DI-GM had 42% lower odds of MASLD compared with those in the lowest quartile (OR:0.58; 95%CI:0.42–0.80; P-trend < 0.01). Each one-standard-deviation increment in DI-GM score was associated with reduced odds of MASLD (OR:0.72; 95%CI:0.65–0.81;P-value < 0.001). The inverse association was more pronounced among women and participants aged ≥ 45 years (P-interaction < 0.01). Mediation analyses suggested that CAP, HOMA-IR, and serum vitamin D partially explained the association. Conclusions Greater adherence to a diet supportive of gut microbiota, as reflected by higher DI-GM scores, was associated with a lower five-year risk of MASLD. These findings highlight the potential role of microbiota-related dietary patterns in MASLD prevention.