Objective To analyze the trend of changes in the ranges of body mass index (BMI) among students at a university in Tianjin from 2021 to 2024, and to propose intervention strategies based on the key influencing factors.

Methods A total of 1000 first-year students enrolled in 2021 at a university in Tianjin were randomly selected, and their physical fitness test data over the 4 years of undergraduate study were collected. Differences in physical fitness test results were compared among students with varying BMI ranges. Linear regression analysis was used to identify the key factors affecting changes in BMI ranges in the students.

Results The proportion of underweight students remained relatively stable, decreasing slightly from 2.5% in the first year to 2.0% in the senior year. The proportion of students with normal weight showed a fluctuating downward trend, from 71.5% at college enrollment to 69.0% by the fourth year. In contrast, the proportion of overweight/obese students showed a fluctuating upward trend. In physical fitness tests for the first year, overweight/obese students showed the best performance in the standing long jump test (2.15 ± 0.42 m, P = 0.08), normal-weight students showed the best performance in the sit-and-reach test (20.1 ± 7.5 cm, P = 0.03), and underweight students performed best in the sit-up test (1 min 48.7 ± 19.5 repeats, P = 0.02). Simple linear regression analysis revealed that students who changed from being underweight to having normal weight by the senior year showed associations with sit-up performance (β = 0.03, 95% CI, 0.018 to 0.041) and 1000 m run (β = 0.73, 95% CI, 0.601 to 0.877). Students who transitioned from having normal weight to being overweight/obese were showed negative associations with their performance in 800 m run (β = －0.08, 95% CI, －0.102 to －0.047) and 1000 m run (β = －16.9, 95% CI, －19.110 to －13.826). Students who changed from being overweight/obese to having normal weight showed associations with their performance in standing long jump (β = －0.41, 95% CI, －0.631 to －0.227) and 1000 m run (β = －0.15, 95% CI, －0.215 to －0.073).

Conclusion Male students of different BMI ranges should focus on improving their 1000 m run performance, and overweight/obese male students should emphasize standing long jump training as well. Underweight female students should strengthen sit-up training, normal-weight female students should focus on 800 m run training, and overweight/obese females students should strengthen standing long jump training.