In a stunning new example of how Americans are eating themselves to death, a landmark study concludes that one of every five cancer deaths in women and one of every seven in men are due to excess pounds.
The study, by the American Cancer Society, is by far the largest on the subject and the first to quantify the risk for all forms of cancer.
More than 900,000 people were followed for 16 years to see how their weight at the start of the study affected their risk of later dying of the disease. The heaviest women had cancer death rates 62 percent higher than those of normal weight, and overall men had rates 52 percent higher.
Applied to obesity patterns in the entire population, it means that 20 percent of cancer deaths in women and 14 percent in men could be due to excess weight, according to the study, which was published today in the New England Journal of Medicine.
"The public health implications for the United States are profound: more that 90,000 deaths per year from cancer might be avoided" if people could maintain normal weight, the author writes.
The study "firmly establishes that cancer is a major consequences of obesity," said Michael Thun, the cancer society's chief epidemiologist.
The damage done by fat isn't limited to a few types of cancer, and instead "represents the rule rather than the exception," experts from Harvard University and Sweden's Karolinska Institute wrote in a commentary.
Obesity is especially important as a cancer-risk factor because it's so common - about 30 percent of American adults are obese and more than half are overweight, by federal health standards.
Carrying around too many pounds has long been known to raise the risk of heart disease and diabetes, but there's less awareness of its cancer hazard, Thun said.
Smoking remains the biggest cancer risk, accounting for 30 percent of all such deaths. Diet accounts of another 30 percent, but it's a mixed bag - it depends on what people et (how many fats, fruits vegetables) and how much they eat.
This study focuses squarely on the how-much part. It also adjust for other non-dietary factors that affect cancer risk, such as smoking, so the toxic effect of too man calories can be isolated.
A BMI of 25 and above is considered overweight, and 30 and above is obese. A 5-and-4 inch person weighing 174 pounds has a BMI of 30; a 6-foot person weighing 184 pounds has a BMI of 25.
The heaviest women in the study were more than twice as likely to die of breast cancer that women with BMIs of 25 of less, and were more than six times as likely to die of uterine cancer.
The heaviest men were 34 percent more likely to die of prostate cancer than normal-weight men.
"That's a new association," Thun said. "Thee's been very little data on obesity and prostate cancer."
Overall, though, fat was more of a cancer hazard for women than men. Extra pounds mean extra hormones - estrogen and insulin - which are produced in fatty tissue and which spur cell growth, setting the stage for the overgrowth that's typical of tumors.
"Women develop cancers that are more sensitive to hormones and obesity," such as breast and uterine cancers, said Patrick Remington, an epidemiologist and associate director of the University of Wisconsin Comprehensive Cancer Center.
"Men don't have the same estrogen-dependent tumors," and the male hormone testosterone isn't made in the same way female hormones are, he explained.
Obesity also makes it tougher to detect cancers, making lumps harder to find, mammograms and other diagnostic tests more difficult to do, and patients less likely to go to doctors.
"Obesity has sort of a double effect on breast cancer. It increases the risk of getting the disease and it increases the risk of dying from the disease," because overweight women often are diagnosed when cancer is more advanced, Thun said.
Thun said more has to be done to fix "toxic environments" - cities not conducive to walking or biking, transportation systems heavily dependent on cars, too much television, too much high-calorie convenience food, and desk jobs and longer workdays that leave little time or opportunity for exercise.
Studies are important, but "what is more important, ultimately, is policies and social factors that make it easier for people to maintain a healthy body weight," he said.