Body mass index and physical fitness in university students: sex differences and nonlinear patterns in a large cross-sectional study.

BACKGROUND: Body mass index (BMI) is routinely used in university health surveillance, but its sex- and grade-specific association with multidimensional physical fitness, including nonlinear patterns, remains unclear. METHODS: Data from 16,292 university students (8,505 females; 7,787 males; aged 17-23 years) who completed the national standardized fitness assessment were analyzed in this cross-sectional study. Vital capacity, 50-m sprint, standing long jump, sit-and-reach, 800/1,000-m run, and muscular endurance were standardized within sex to derive a Physical Fitness Index (PFI), with time-based tests reverse-scored. Two-way analyses of variance examined associations of sex, academic grade, and BMI category with PFI. Sex- and grade-specific quadratic models were used to characterize BMI-PFI curves and estimate the model-derived BMI value associated with peak predicted PFI. Pearson correlations assessed associations between PFI and individual fitness components. RESULTS: Academic grade was associated with PFI, whereas no overall sex effect was observed (sex: F (1, 16,284) = 1.51, p = 0.219, partial η2 = 0.000093; grade: F (3, 16,284) = 74.54, p < 0.001, partial η2 = 0.0136). The sex × grade interaction was statistically significant but small (F (3, 16,284) = 20.44, p < 0.001, partial η2 = 0.00375). Within both sexes, normal-weight students showed the highest PFI; underweight students had higher PFI than overweight and obese peers, while PFI was lower in the overweight and obese groups. The BMI-PFI association was stronger in males (partial η2 = 0.1061) than in females (partial η2 = 0.0287). All quadratic models showed a concave BMI-PFI pattern, with lower predicted PFI at higher BMI values and model-derived peak estimates varying across sex-grade subgroups. These estimates should not be interpreted as physiological optima or recommended BMI targets. Correlations indicated that PFI integrated cardiorespiratory fitness, lower-limb power, flexibility, muscular endurance, and running performance. CONCLUSION: BMI was associated with multidimensional physical fitness in this large cohort of university students, with lower PFI observed at both low and high BMI values. BMI showed a stronger association with PFI than academic grade, particularly in males. These findings support BMI-informed and sex-sensitive fitness monitoring, but should be interpreted as cross-sectional associations because body-composition and behavioral measures were unavailable.