But obesity is poised to take the lead, said Otis Brawley, an oncologist at Johns Hopkins and former medical director of the American Cancer Society, that the change could happen within five to ten years.
He noted that high rates of obesity may threaten the continuing decline in cancer mortality rates since the early 1990s in the United States.
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.
Jennifer Leighbeil, a breast cancer specialist at the Dana Farber Cancer Institute, said the interaction is complex between metabolism, inflammation and immunity, creating a more cancer-prone environment.
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.