The skinny on BMI and mortality.

Evidence Various systematic reviews of observational studies in the general population evaluate all-cause mortality and BMI.1-10 • The review with the most studies1 (97 studies, 2.88 million participants) found the following relative risks (RRs) compared with normal BMI (18.5-24.9 kg/m2): -overweight (25.0-29.9 kg/m2), RR = 0.94; -obese, grade I (30.0-35.0 kg/m2), RR = 0.95; and -obese, grade II or higher (BMI > 35 kg/m2), RR = 1.29. • The review with the most participants2 (8 studies, 5.8 million participants) found these hazard ratios (HRs) for men compared with high-normal BMI (22.5-24.9 kg/m2): -low BMI (< 18.5 kg/m2), HR = 1.88; -low-normal BMI (18.5-19.9 kg/m2), HR = 1.39; -mid-normal BMI (20.0-22.4 kg/m2), HR = 1.15; -low overweight (25.0-27.4 kg/m2), HR = 0.97; -high overweight (27.5-29.9 kg/m2), HR = 1.04; and -obese, grade I (30.0-35.0 kg/m2), HR = 1.18. • The third largest study3 (19 studies, 1.46 million participants) found the following HRs for women compared with high-normal BMI (22.5-24.9 kg/m2): -BMI below 18.5 kg/m2, HR = 1.25; -BMI 20.0 to 27.4 kg/m2, very similar risk throughout range (HR = 1.03-1.05); and -above 27.5 kg/m2, mortality increases with BMI (27.530.0 kg/m2, HR = 1.14; 40.0-50.0 kg/m2, HR = 2.13). • Other studies had similar findings.4-6 Meta-analyses had similar findings for those with diabetes. For the elderly (≥ 65 years),1,8 being overweight lowered risk (best at 27.5 kg/m2).8 For those with pre-existing cardiovascular disease,9,10 chronic obstructive pulmonary disease,11 or hemodialysis,12 overweight and grade I obesity conferred similar risk9 or reduced risk10-12 relative to normal BMI.