People with a body mass index, or BMI, above 40 are robbed of an average 6.5 years of expected life span, a study has found. And the toll in years lost rises with the degree of obesity, reaching nearly 14 years for the most obese -- those with a BMI above 55, researchers said.

The study found that the reduction in life expectancy associated with being extremely obese was similar to that seen in adults who smoke. And as a person's obesity rises to higher levels, his or her expected life span falls below that of smokers.

The findings come from a project that aggregated the results of about 20 long-term studies on obesity in the United States, Australia and Sweden. They were published in PLoS Medicine, in what is believed to be the largest study to date of the health consequences of severe obesity.

Compared with their normal-weight peers, the extremely obese are more likely to succumb early to heart disease, cancer and diabetes. For men with "class III obesity," the rate of death attributable to heart disease and diabetes is especially elevated compared with normal-weight males. For women in the same obesity category, cancer deaths dramatically outstripped those among normal-weight women.

But premature deaths attributable to all causes, from injury to chronic lower respiratory infections, were consistently higher in those with severe obesity, the study found.

The extremely obese -- those who generally would need to lose 100 lbs. or more to attain a "normal healthy weight" -- are a fast-growing segment of the U.S. population, now representing about 6% of American adults.

The ranks of those with a BMI over 40 (for example, a 5-foot-6 person weighing 250 pounds or more) have grown fourfold since the 1980s. The population with a BMI over 50 (say, a 5-foot-10 person weighing more than 350 pounds) has grown by 10% in the same period.

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