Researchers analyzing results from nearly 60 long term studies covering almost a million people worldwide concluded that moderate obesity, which is now common, shortens life expectancy by up to 4 years and severe obesity, which is still uncommon, shortens it by up to 10 years, which is comparable with the effects of lifelong smoking.

The analysis was coordinated by researchers at the Clinical Trial Service Unit (CTSU), a medical research center based at Oxford University, and is published as a paper in the 18 March online issue of The Lancet.

For the analysis the researchers brought together the results of 57 prospective studies that followed about 895,000 people, most of them 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, said the researchers in a press statement.

The researchers used body mass index (BMI, the ratio of a person’s weight to the square of their height, measured as kilograms per metre squared or kg/m2) 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 (for instance 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. This would be for example the BMI of a person 1.70 m tall (5 ft 7 in) who weighs about 70 kg (154 lbs or 11 stone).

Professor Sir Richard Peto from Oxford University was the main statistician behind the analysis. He said that when they compared the effect of obesity with that of smoking they found 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. For smokers the key thing is that stopping smoking works,” he added.

Dr Gary Whitlock, an epidemiologist at Oxford University led the analysis. He said they found that in countries like Britain and America, carrying excess weight shortens human lifespan:

“Weighing a third more than the optimum shortens lifespan by about 3 years,” said Whitlock, 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.”

He suggests that if you are gaining weight and becoming overweight or obese, stopping yourself getting even more overweight could add years to your 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; they couldn’t say why this was so exactly, 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.

In the UK, one in four deaths among the middle aged are from heart attack or stroke, and 1 in 16 cancer deaths are due to being overweight or obese.

In the US, where middle aged people tend to be heavier, these figures are even higher: one in three deaths from heart attack or stroke and 1 in 12 cancer deaths among middle aged Americans is due to being overweight or obese, said the researchers.

The researchers also concluded that although other measures such as waist circumference and waist-to-hip ratio could add more information to BMI, and BMI to them, BMI itself is a “strong predictor of overall mortality both above and below the apparent optimum of about 22·5-25 kg/m2”.

The Medical Research Council, the British Heart Foundation and Cancer Research UK sponsored the Clinical Trial Service Unit at Oxford University to coordinate the study.

“Body-mass index and cause-specific mortality in 900 000 adults: collaborative analyses of 57 prospective studies.”
Prospective Studies Collaboration.
The Lancet, Early Online Publication, 18 March 2009.
doi:10.1016/S0140-6736(09)60318-4

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Sources: Journal Abstract, Oxford University.

Written by: Catharine Paddock, PhD