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Severe obesity shortens life by up to 10 years, which is comparable with the effects of lifelong smoking.  Moderate obesity is having a BMI of 30 to 35, and severe obesity is having a BMI of 40 to 50. Rates of death were lowest among men and women whose BMI was 23 to 24. Continuing to smoke is as dangerous as doubling your body weight, according to the study, and changing your diet but continuing to smoke is not the way to increase lifespan. For smokers the key thing is that stopping smoking works.

Obesity kills by increasing the risk of heart disease and stroke.  BMI is a strong predictor of overall mortality both above and below the apparent optimum of about 22·5 - 25.