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(CNN) It's not going to come as a surprise that obesity shortens your life by contributing to heart disease, strokes, diabetes and other diseases. But how many years are lost?

Researchers from the National Cancer Institute compared normal weight smokers to extremely obese non-smokers. They found the smokers lived longer on average.

Healthy weight smokers lost about nine years of their lives. Non-smoking adults who were extremely obese -- defined as having a BMI of 55 to 55.9 -- lost nearly 14 years on average.

Researchers analyzed 20 large studies, which included more than 9,500 extremely obese adults and more than 300,000 people with a normal BMI. Participants came from the United States, Sweden and Australia.

The findings, published Tuesday in PLOS Medicine, highlight the need to develop more effective ways to combat the growing public health problem of extreme obesity, the authors write.

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