Association Between Body Mass Index and Physical Fitness Among University Students in a Sub-Plateau Region of Gansu Province, China: A Cross-Sectional Study

While the relationship between body mass index (BMI) and physical fitness in youth often follows an inverted U-shape, populations-based data from sub-plateau settings remain scarce. This study characterized the cross-sectional BMI–fitness association among 14,744 students (9931 females) at a single university in Gansu Province, China (altitude ~1483 m). Fitness was assessed across seven components (vital capacity, speed, power, flexibility, endurance, and muscular strength/endurance) and summarized as a standardized Physical Fitness Index (PFI). Overweight/obesity prevalence was 11.2% (males 18.3%, females 7.7%)—lower than the 14.0% national average for same-aged university students. BMI displayed a monotonic inverse association with PFI; underweight and normal-weight groups performed comparably, while overweight and obese groups showed progressively lower PFI, with a stronger association in males (β = −0.46) than females (β = −0.36). The weight-normalized vital capacity index (VCWI)—a derived indicator of pulmonary function—showed the strongest inverse correlation (males r = −0.47; females r = −0.40). Associations were most pronounced for VCWI and endurance, whereas flexibility exhibited a negligible BMI-related gradient. These findings describe a cohort-specific pattern, emphasizing the need for broader investigation of regional variations, and should not be interpreted as evidence of an altitude-mediated effect.