Objective Based on the data from the 5^th National Physical Fitness Surveillance in China, this study aimed to explore the relationship between abnormal body weight and physical fitness levels in older adults.

Methods The study included adults aged 60-79 years from the 5^th National Physical Fitness Surveillance database. Body weight was categorized into four groups—underweight, normal weight, overweight, and obesity—based on body mass index (BMI), with underweight, overweight, and obesity classified as abnormal weight. Physical fitness indicators such as cardiorespiratory endurance (number of 2-minute high-knee steps), lower limb muscle strength (number of 30-second chair stands), flexibility (sit-and-reach test), balance ability (eyes-closed single-leg stance time), and reaction ability (choice reaction time) were collected. Each physical fitness indicator was divided into four levels (excellent, good, pass, and fail) based on quartile cutoffs. After adjusting for demographic factors (including age, sex, urban/rural residence, education level, pre-retirement occupation, marital status, exercise habits, smoking, sleep quality, and self-rated fitness), binary Logistic regression was used to analyze the association between physical fitness indicators and abnormal weight.

Results A total of 39 927 older adults aged 60-79 years were included, comprising 19 777 men (49.5%) and 20 150 women (50.5%), with a mean age of 69.2±5.6 years. The prevalence rates of underweight, overweight, and obesity were 2.3%, 41.8%, and 16.7%, respectively, with underweight prevalence significantly increasing with age (P < 0.001). Both obesity and underweight older adults had significantly lower physical fitness levels than the normal-weight group. Regression analysis revealed a significant association between cardiorespiratory endurance and abnormal weight. Compared with the excellent group, older men in other levels had a 51.6%-68.9% higher risk of underweight (women: 31.8%-41.3%), a 4.7%-23.6% higher risk of overweight (women: 5.0%-25.8%), and a 5.6%-57.5% higher risk of obesity (women: 15.2%-66.6%). Lower limb muscle strength was significantly associated with abnormal weight in older women, with overweight risk increasing by 22.1%-44.5% and obesity risk by 38.1%-61.1% compared with the excellent group, whereas no clear association was observed in older men. Additionally, the relationships between flexibility, balance ability, reaction ability, and abnormal weight remained inconclusive.

Conclusions Chinese older adults aged 60-79 years face a dual burden of underweight and overweight/obesity. Poor cardiorespiratory endurance and muscle strength (in women) are significantly associated with abnormal weight. Maintaining good cardiorespiratory endurance and muscle strength may help improve abnormal weight status in older adults.