Being mildly obese takes years off your life

What is your BMI? Find it here.

Body-mass index (BMI) and cause-specific mortality in 900,000 adults: collaborative analyses of 57 prospective studies

What's their BMI?

What are their BMIs?

Background: The main associations of body-mass index (BMI) with overall and cause-specific mortality can best be assessed by long-term prospective follow-up of large numbers of people. The Prospective Studies Collaboration aimed to investigate these associations by sharing data from many studies.

Methods: Collaborative analyses were undertaken of baseline BMI versus mortality in 57 prospective studies with 894 576 participants, mostly in western Europe and North America (61% [n=541 452] male, mean recruitment age 46 [SD 11] years, median recruitment year 1979 [IQR 1975—85], mean BMI 25 [SD 4] kg/m2). The analyses were adjusted for age, sex, smoking status, and study. To limit reverse causality, the first 5 years of follow-up were excluded, leaving 66 552 deaths of known cause during a mean of 8 (SD 6) further years of follow-up (mean age at death 67 [SD 10] years): 30 416 vascular; 2070 diabetic, renal or hepatic; 22 592 neoplastic; 3770 respiratory; 7704 other.

Findings: In both sexes, mortality was lowest at about 22·5—25 kg/m2. Above this range, positive associations were recorded for several specific causes and inverse associations for none, the absolute excess risks for higher BMI and smoking were roughly additive, and each 5 kg/m2 higher BMI was on average associated with about 30% higher overall mortality (hazard ratio per 5 kg/m2 [HR] 1·29 [95% CI 1·27—1·32]): 40% for vascular mortality (HR 1·41 [1·37—1·45]); 60—120% for diabetic, renal, and hepatic mortality (HRs 2·16 [1·89—2·46], 1·59 [1·27—1·99], and 1·82 [1·59—2·09], respectively); 10% for neoplastic mortality (HR 1·10 [1·06—1·15]); and 20% for respiratory and for all other mortality (HRs 1·20 [1·07—1·34] and 1·20 [1·16—1·25], respectively). Below the range 22·5—25 kg/m2, BMI was associated inversely with overall mortality, mainly because of strong inverse associations with respiratory disease and lung cancer. These inverse associations were much stronger for smokers than for non-smokers, despite cigarette consumption per smoker varying little with BMI.

Interpretation: Although other anthropometric measures (eg, waist circumference, waist-to-hip ratio) could well add extra information to BMI, and BMI to them, BMI is in itself a strong predictor of overall mortality both above and below the apparent optimum of about 22.5—25 kg/m2. The progressive excess mortality above this range is due mainly to vascular disease and is probably largely causal. At 30—35 kg/m2, median survival is reduced by 2—4 years; at 40—45 kg/m2, it is reduced by 8—10 years (which is comparable with the effects of smoking). The definite excess mortality below 22.5 kg/m2 is due mainly to smoking-related diseases, and is not fully explained.

Journal reference: Lancet 2009; 373: 1083-96

New Scientist article.

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