To evaluate the association between dietary fiber intake and obesity in Japanese outpatients with type 2 diabetes, as well as the demographic and dietary factors influencing this association.

Cross-sectional study with nationwide registry data on Japanese type 2 diabetes outpatients (2014-2019). Diet was self-reported via a Food Frequency Questionnaire. Obesity was defined as BMI ≥25 kg/m2.

Clinics throughout Japan.

1,565 Japanese outpatients with type 2 diabetes (63.1% men; age range 30–89 years).

Multivariate analysis revealed that a higher dietary fiber intake was associated with a lower risk of obesity in all participants (95% CI = 0.439–0.795, p < 0.001). Stratified analysis showed a significant inverse association between fiber intake and obesity in men and the older age groups (59–89 years), but not in women or the younger age group (30–58 years). In men, higher fiber intake was associated with healthier lifestyle behaviors, including increased physical activity (p < 0.001) and non-smoking (p < 0.001), with stronger associations compared to women. Vegetables, fruits, and soybeans/soy products were strongly correlated with fiber, while grains had a weak correlation. Folate was the micronutrient most strongly correlated with fiber.

Higher fiber intake was associated with lower obesity risk, particularly in men and older age groups. These findings emphasize the need for targeted public health initiatives promoting the intake of diverse fiber-rich foods to effectively manage obesity. Further research is needed to understand sex- and age-specific factors in the fiber-obesity relationship in diverse populations.