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Smoking has been the No. 1 preventable cause of cancer for decades and still kills more than 500,000 people a year in the United States. But obesity is poised to take the top spot, as Americans’ waistlines continue to expand while tobacco use plummets.

The switch could occur in five or 10 years, said Otis Brawley, a Johns Hopkins oncologist and former chief medical officer of the American Cancer Society. The rise in obesity rates could threaten the steady decline in cancer death rates since the early 1990s, he said.

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Yet only about half of Americans are aware of the link between excess weight and cancer. And researchers are struggling to answer such fundamental questions as how surplus weight increases the risk of the disease and whether, conversely, losing weight helps prevent cancer or a recurrence.

Being obese and overweight — long implicated in heart disease and diabetes — has been associated in recent years with an increased risk of getting at least 13 types of cancer, including stomach, pancreatic, colorectal and liver malignancies, as well as postmenopausal breast cancer. Researchers at the American Cancer Society say that excess body weight is linked to about 8 percent of all cancers in the United States and about 7 percent of cancer deaths.