Smoking is the number one cause of preventable cancer for decades and continues to kill more than half a million people a year in the United States.
But obesity is poised to take the lead, as Americans continue to decline with the decline in tobacco use.
Otis Brawley, a Johns Hopkins oncologist and former medical director of the American Cancer Society, said the change could take five to ten years.
He said that high rates of obesity could threaten the steady decline in cancer mortality rates since the early 1990s in America.
Obesity and overweight are associated with an increased risk of at least 13 types of cancer, including stomach cancer, colorectal and liver cancer, as well as postmenopausal breast cancer.
Researchers at the American Cancer Society say that excess body weight is associated with about 8% of all cancers in the United States and about 7% of cancer deaths.
The exact relationship between cancer and overweight is unknown, but researchers are focusing on the visceral fat that surrounds the internal organs.
These fats are an active member of metabolism and produce hormones such as estrogen, which is associated with an increased risk of breast cancer and some other cancers, according to the American Institute for Cancer Research.
"It's a complex interaction between metabolism, inflammation and immunity, which creates a more cancerous environment," said Jennifer Leighbeil, a breast cancer doctor at the Dana Farber Cancer Institute.
The type of cancer that is strongly linked to obesity is endometrial cancer, and obese and overweight women are two to four times more likely to develop the disease than women with normal weight.
Overweight or obese people are about twice as likely to develop liver and kidney cancer, and are 1.5 times more likely to develop pancreatic cancer.