BMI moderates the association between physical fitness and spherical equivalent in Chinese college students: a cross-sectional study

Background Myopia has become a global public health crisis, with young adults representing a priority population for prevention. While physical fitness is recognized as a protective factor, whether this protection is uniformly distributed across different BMI subgroups remains unclear. This study examined the moderating role of BMI in the relationship between physical fitness and spherical equivalent (SE) among Chinese university students. Methods A cross-sectional study was conducted with 300 college students (100 normal weight, 100 overweight, 100 obese) from Zhejiang Province, China. Physical fitness and SE were measured using standardized protocols. Multiple linear regression, adjusted for sex, was performed to test the moderating effect of BMI. Results Physical fitness was independently associated with SE (Model 1: β = 0.042, p = 0.001), while BMI showed no main effect (Model 1: p = 0.766). A significant negative interaction emerged between fitness and BMI (Model 2: β = −0.009, 95% CI: −0.013 to −0.004, p < 0.001). Simple slope analysis (post-hoc analysis of Model 2) revealed that the protective association of fitness was stronger at low BMI ( β = 0.071, p < 0.001), moderate at mean BMI ( β = 0.032, p = 0.011), and absent at high BMI ( β = −0.007, p = 0.523). Johnson-Neyman analysis (post-hoc analysis of Model 2) identified a threshold at BMI_c = 0.744 (raw BMI ≈ 26.45 kg/m 2 ), after which the association was no longer significant. Conclusion Physical fitness protects against myopia in college students, but this benefit is significantly diminished among those with higher BMI, revealing heterogeneity in intervention effectiveness across weight subgroups. From a public health perspective, this finding suggests that universal physical activity promotion alone may not confer equitable vision benefits across all BMI groups. Stratified intervention strategies—combining weight management with structured exercise for overweight/obese subgroups—are warranted to ensure equitable outcomes in population-level myopia prevention.