The relationship between Body Mass Index and Physical Fitness Index among college students: a cross-sectional study in Yangzhou, China

Background Obesity and insufficient physical activity are major public health concerns among college students. Body Mass Index (BMI) and physical fitness levels are key indicators of functional health, but large-scale, objective measurement studies among Chinese college students suggest their correlation remains limited. This study aims to investigate the relationship between BMI and fitness index, and to explore gender differences. Methods This study collected physical fitness test data from 28,861 undergraduate students in their first through fourth years at a university in Yangzhou, China, during the 2024 academic year. According to national standards, BMI is categorized into underweight, normal weight, overweight, and obese. Physical fitness is assessed using standardized tests covering lung capacity, flexibility, speed, explosive power, endurance, and muscular strength, with an overall fitness score calculated. The associations between BMI, gender, and physical fitness tests and scores were analyzed using two-way analysis of variance, correlation analysis, and quadratic regression models. Results Body Mass Index is significantly correlated with most physical fitness test indicators. Students with normal BMI achieved higher overall fitness scores and better performance in most physical fitness tests, while underweight, overweight, and obese students performed worse. A higher BMI is associated with poorer performance in speed, endurance, and muscle strength, while lung capacity increases with rising BMI. Quadratic regression analyses revealed a significant nonlinear association between Body Mass Index and physical fitness, with optimal performance observed within the normal BMI range, and declines evident at both lower and higher BMI levels. This non-linear pattern differed by gender. Among males with higher Body Mass Index, the decline in physical fitness is more pronounced. Female students demonstrated significantly higher overall physical fitness levels than male students. Conclusion Body Mass Index exhibits a distinct nonlinear relationship with physical fitness. College students with BMIs within the normal range demonstrate optimal physical fitness levels, while deviations from this range are associated with overall declines in physical fitness, particularly among male students. These findings provide a scientific basis for physical fitness monitoring and stratified intervention strategies for college students. However, due to the cross-sectional design of this study, causal relationships between BMI and physical fitness cannot be established, and future longitudinal studies are warranted.