People who are severely obese may lose as many as 14 years off their life, a new study suggests.

U.S. researchers pooled data from 20 previous studies and found that a body mass index (BMI) greater than 40 -- considered severe obesity -- raises the odds of dying early from heart disease, cancer and diabetes compared to people of normal weight.

"We found that the death rates in severely obese adults were about 2.5 times higher than in adults in the normal weight range," said lead investigator Cari Kitahara, a research fellow at the U.S. National Cancer Institute.

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Severe obesity accounts for an excess 509 deaths per 100,000 men each year, and 382 excess deaths per 100,000 women, she said.

Whether losing weight would improve lifespan isn't clear, Kitahara said. But not becoming obese in the first place will extend your life, she added.

Kitahara's team calculated that, compared with normal-weight people, severely obese people were cutting their lives short by 6.5 to 13.7 years. That's similar to the toll taken by smoking, she said.

BMI is a calculation of body fat based on height and weight. As an example, if you stand 5 feet 4 inches tall and weigh 235 pounds, your BMI is 40, which is considered severely obese. Similarly, if you're 280 pounds and 5-feet-10, your BMI is 40. By comparison, a BMI between 18.5 and 24.9 is considered a healthy weight.

About 6 percent of U.S. adults are severely obese, according to background information in the report, published online July 8 in PLOS Medicine.

Dr. David Katz, director of the Yale University Prevention Research Center in New Haven, Conn., said the study findings underscore existing concerns.

"We have long had clear and compelling evidence that obesity is related to the major chronic diseases that plague modern societies: heart disease, cancer, stroke, diabetes, dementia and more," Katz said.

Severe obesity is more dangerous than lesser degrees of obesity, and rates of severe obesity are rising steeply, he added. "We also have data to show that the death toll of obesity is rising," he said.

Effective treatments for advanced obesity can help somewhat, Katz said. "But to a far greater extent, it can and must be addressed with prevention, since severe obesity rarely has to happen in the first place," he said.

For the study, the researchers homed in on previous research on nearly 10,000 severely obese people who had never smoked or had any chronic disease. They compared these folks with about 304,000 normal-weight adults.

Over the 30-year study period, the severely obese men and women were more likely to die early compared with normal weight people, they found.

Heart disease was the major factor linked with death among the severely obese, followed by cancer, diabetes, kidney and liver disease.

Moreover, the risk of dying from any of these conditions rose along with weight.

However, the findings are limited because people reported their own height and weight to calculate BMI and also because BMI was the only measure of obesity used, the researchers said.