May 25, 2016
BMJ : British Medical Journal
AKE-HOME MESSAGE
The authors of this systematic review and meta-analysis of 230 cohort studies evaluated the association between BMI and mortality. For every 5-point increase in BMI, the summary relative risk of all-cause mortality was 1.05 for all individuals, 1.18 for never smokers, and 1.21 for healthy never smokers. The relationship between BMI and all-cause mortality was a J-shaped curve among never smokers and healthy never smokers, with nadirs at BMI 23–24 and 22–23, respectively. The nadir decreased to 20–22 in studies with ≥20 years of follow-up. The relationship between BMI and all-cause mortality was U-shaped in lower-quality studies with greater potential for bias and shorter follow-up.
There appears to be a J-shaped relationship between BMI and risk of all-cause mortality, with the lowest risk in individuals with BMI between 20 and 24.
Abstract
OBJECTIVE
To conduct a systematic review and meta-analysis of cohort studies of body mass index (BMI) and the risk of all cause mortality, and to clarify the shape and the nadir of the dose-response curve, and the influence on the results of confounding from smoking, weight loss associated with disease, and preclinical disease.
DATA SOURCES
PubMed and Embase databases searched up to 23 September 2015.
STUDY SELECTION
Cohort studies that reported adjusted risk estimates for at least three categories of BMI in relation to all cause mortality.
DATA SYNTHESIS
Summary relative risks were calculated with random effects models. Non-linear associations were explored with fractional polynomial models.
RESULTS
230 cohort studies (207 publications) were included. The analysis of never smokers included 53 cohort studies (44 risk estimates) with >738 144 deaths and >9 976 077 participants. The analysis of all participants included 228 cohort studies (198 risk estimates) with >3 744 722 deaths among 30 233 329 participants. The summary relative risk for a 5 unit increment in BMI was 1.18 (95% confidence interval 1.15 to 1.21; I(2)=95%, n=44) among never smokers, 1.21 (1.18 to 1.25; I(2)=93%, n=25) among healthy never smokers, 1.27 (1.21 to 1.33; I(2)=89%, n=11) among healthy never smokers with exclusion of early follow-up, and 1.05 (1.04 to 1.07; I(2)=97%, n=198) among all participants. There was a J shaped dose-response relation in never smokers (Pnon-linearity <0.001), and the lowest risk was observed at BMI 23-24 in never smokers, 22-23 in healthy never smokers, and 20-22 in studies of never smokers with ≥20 years’ follow-up. In contrast there was a U shaped association between BMI and mortality in analyses with a greater potential for bias including all participants, current, former, or ever smokers, and in studies with a short duration of follow-up (<5 years or <10 years), or with moderate study quality scores.
CONCLUSION
Overweight and obesity is associated with increased risk of all cause mortality and the nadir of the curve was observed at BMI 23-24 among never smokers, 22-23 among healthy never smokers, and 20-22 with longer durations of follow-up. The increased risk of mortality observed in underweight people could at least partly be caused by residual confounding from prediagnostic disease. Lack of exclusion of ever smokers, people with prevalent and preclinical disease, and early follow-up could bias the results towards a more U shaped association.