The relationship between frailty, BMI, and mortality in older adults: results from the CLHLS

Frailty is a useful indicator of the rate of biological and is also associated with mortality in Older adults. Overweight and obesity has been suggested to act as protective factors in the elderly. However, the combined effect of frailty and body mass index (BMI) on mortality is less clear. This study aimed to investigate the association between frailty, BMI and mortality in Chinese Older adults. We included participants aged 65 years and older in 2008 at baseline and 2011/2012, 2014, and 2017/2018 follow-up surveys of the Chinese Longitudinal Healthy Longevity Survey (CLHLS). The frailty index (FI) was calculated using 39 baseline variables, all of which were deficits in health status measured by questionnaire and physical examination. Frailty was defined as an FI ≥ 0.25. BMI (kg/m2) was categorized as underweight (BMI < 18.5), normal body weight (18.5 ≤ BMI < 24.0) and overweight (BMI ≥ 24.0). A Cox proportional hazards model was used to analyze the association of the combined frailty and BMI categories with all-cause mortality, adjusting for sociodemographic and health-related factors. A total of 8,825 (79.45%) participants were categorized as nonfrail, and 2,282 (20.55%) participants as frail. By Cox proportional hazards model, the nonfrailty-overweight group exhibited a significantly lower mortality risk (HR = 0.881[0.805, 0.963]) compared to nonfrailty-normal group. In the frail subgroup analysis, overweight was associated with a significantly lower risk of death (HR = 0.838; 95% CI: 0.716, 0.980; p-trend < 0.001). Mortality in the nonfrail population decreased with increasing BMI and the risk of death was lowest in the nonfrailty-overweight group (HR = 0.901; 95% CI: 0.824,0.986; p-trend < 0.001). The mortality risk is elevated among frail elderly individuals, particularly those who are underweight. Overweight exhibits a protective role in both frail and non-frail elderly persons.