Introduction Many epidemiological studies have demonstrated that body mass index (BMI) is associated with the risk of developing age-related cataracts. These reports have suggested that high and low BMIs can affect the onset or progression of age-related visual impairment. However, few prospective studies have examined this relationship in a general Asian population. Therefore, in this study, we investigated whether BMI was associated with increased risk of age-related cataracts by performing a 5-year prospective population-based study among a middle-aged Japanese population.

Methods This 5-year population-based study included 35 365 men and 40 825 women (aged 45–74), who were recruited onto the Japan Public Health Center-based Prospective Study (JPHC Study) and had not reported cataracts in baseline survey. The self-reported diagnosis of age-related cataracts was used in the analysis of this study.

Results At follow-up, 1004 men (2.84%) and 1807 women (4.43%) reported new diagnoses of age-related cataracts. The multivariate ORs for those in the lowest and the highest BMI category, compared with a BMI category of 21.0–22.9 as a reference point (OR, 1.00), were 1.29 (95% CI 0.93 to 1.79) and 1.15 (95% CI 0.96 to 1.39) in men, and 1.23 (95% CI 0.97 to 1.55) and 1.19 (95% CI 1.04 to 1.36) in women.

Conclusion High and low BMIs have been suggested previously as the risk of age-related cataracts for Caucasian population in developed countries and the population living in developing countries respectively. However, the present large-cohort study showed that a U-shaped association between BMI and incidence of cataracts in Japanese men and women.