Researchers from the Clinical Trial Service Unit (CTSU) analyses of nearly 60 long term studies covering almost a million people worldwide concluded that moderate obesity, which is widely spread and common, shortens life expectancy by up to 4 years and that severe obesity, which is still not that common, shortens it by up to 10 years, which is comparable with the effects of lifelong smoking.

Most of the study participants were based in Europe and North America, over an average of 10 to 15 years. During that time about 100,000 of them died, making this the largest investigation of how obesity affects mortality.

The researchers used body mass index (BMI) to assess obesity. Moderate obesity is having a BMI of 30 to 35, and severe obesity is having a BMI of 40 to 50. Although not regarded as a perfect measure of obesity as it doesn't take into account waist circumference, BMI is still a useful way of assessing the link between fatty tissue and ill health.

The results showed that rates of death were lowest among men and women whose BMI was 23 to 24.

When researchers compared the effect of obesity with that of smoking they concluded that:

Continuing to smoke is as dangerous as doubling your body weight, and three times as dangerous as moderate obesity.
Changing your diet but keeping on smoking is not the way to increase lifespan.

Weighing a third more than the optimum, shortens lifespan by about 3 years and for most people, a third more than the optimum means carrying 20 to 30 kilograms [50 to 60 pounds, or 4 stone] of excess weight.

For those who becoming overweight or obese, stopping now could add years to their life.

The analysts also found there was a higher rate of death among people with a BMI much lower than the optimum 23 to 24 but they couldn't pinpoint why this was so, except that the risk was considerably greater for smokers than non smokers.

Obesity does increase the rate of death for some types of cancer, but usually it kills by increasing the risk of heart disease and stroke.