Association between body mass index and all-cause mortality among oldest old Chinese

Objectives: To examine the association between BMI and all-cause mortality in the oldest old (≥80 years). Design: The study used a prospective cohort study design. Setting: Chinese Longitudinal Healthy Longevity Survey (CLHLS) between 1998/99 and 2011. Population: 8026 participants aged 80 years and older were followed every two to three years. Measurements: Body weight and knee height were measured. Height was calculated based on knee height using a validated equation. Deaths were ascertained from family members during follow-up. Results: The mean BMI was 19.8 (SD 4.5) kg/m². The prevalence of underweight, overweight and obese was 37.5%, 10.2% and 4.4%, respectively. There were 5962 deaths during 29503 person-years of follow-up. Compared with normal weight, underweight was associated with a higher mortality risk (HRs: 1.20 (95%CI 1.13-1.27) but overweight (HR 0.89 (95%CI 0.81-0.99)) were associated with a lower risk. Obesity had a HR 0.91 (95%CI 0.78-1.05) for mortality. Conclusion: Among oldest old Chinese, underweight is associated with an increased risk of all-cause mortality but overweight is associated with a reduced risk. Interventions to reduce undernutrition should be given priority among the oldest old Chinese.