Being overweight and physically inactive may account for 25% to 30% cases… (Joe Raedle / Getty Images )
Two out of three adult Americans are at greater risk for getting cancer — and for dying of it — than they need to be. Not because of smog in their air or radon in their basements. Not because of tobacco in their cigarettes or mutations in their genes.
No, the particular cancer risk shared by these 150 million or so Americans comes from having too many calories in their diet and too little exercise in their daily lives.
In other words, from being overweight.
It's widely known that simply being overweight, let alone obese, dramatically increases the risk for high blood pressure, heart attacks, strokes and diabetes. But according to a 2009 survey by the American Institute for Cancer Research, only about 50% of Americans know that size also matters when it comes to cancer.
The risk is not trivial. The same institute estimates that every year about 100,000 Americans get a cancer they wouldn't have gotten if they had kept their weight in check. And researchers have estimated that about 14% of cancer deaths in men and 20% in women could be avoided by this same restraint.
Obesity can raise the risk for a number of major cancers — colon, postmenopausal breast, endometrial, kidney and esophageal — the National Cancer Institute says, and when paired with physical inactivity, it can be held liable for 25% to 30% of cases of those cancers. Obesity has also been linked to a number of other cancers, including liver, gallbladder, pancreatic and ovarian.
"Obesity is almost like the new smoking," says Dr. Anne McTiernan, director of the Prevention Center at the Fred Hutchinson Cancer Research Center in Seattle. "The effect isn't as big for most cancers, but it's so prevalent that it will have a huge impact."
Indeed, the National Cancer Institute estimates that smoking accounts for 37.5% cancer deaths in men and 22.8% in women. But smoking does most of its dirty work in lung cancer victims. When lung cancer is taken out of the picture, smoking can only be blamed for 12% of cancer deaths in men and 6% in women — fewer than can be chalked up to excess pounds.
No one knows for sure exactly how weight increases cancer risk, but it's likely that it does so in multiple ways, with the precise mechanism differing from cancer to cancer. High levels of estrogen, insulin and inflammatory compounds are among the suspects that have been implicated in research to date.
A more precise understanding of the biology behind all this may someday lead to drugs that can mitigate the damage. In the meantime, of course, there's an excellent way to avoid the obesity risk, and that's to never become obese at all.
That would require major lifestyle changes for many of us, and making such changes is exceedingly hard, says Dr. John Glaspy, an oncologist at UCLA's Jonsson Comprehensive Cancer Center. "Sure, we could make it a death penalty offense to sell sugared drinks," he says. But short of such extreme modes of encouragement, a widespread thinning of America is not to be expected anytime soon.
Not only is there strong evidence that if you're overweight, you're more likely to die of cancer. It's also been shown that the more overweight you are, the more deadly the trend gets, according to a landmark study published in the New England Journal of Medicine in 2003.
Scientists at the American Cancer Society in Atlanta looked at cancer death rates for men and women in five weight categories: healthy (body mass index of 18.5 to 24.9); overweight (BMI, 25 to 29.9); and three levels beyond: "obese" (BMI, 30 to 34.9), "very obese" (BMI, 35 to 39.9) and "very, very obese" (BMI, 40 or more).
Compared with death rates for men and women of healthy weight, death rates from all cancers lumped together as a group rose consistently along with BMI: Rates were 52% higher for very, very obese men and 62% higher for very, very obese women.
But weight doesn't affect all cancers equally. For some cancers the researchers examined, such as bladder cancer, weight had no significant effect on the death rate. At the other extreme, the death rate from liver cancer was 350% higher for very obese men than for men of healthy weight (though only 68% higher for very obese women than for women of healthy weight).
And the death rate from uterine (endometrial) cancer was 525% higher for very, very obese women than for women of healthy weight.
How do those numbers stack up against other risks? Infection with the hepatitis C virus increases the death rate for liver cancer by 1,600%, which makes the 350% increase from obesity sound pretty measly. But only about 1.5% of Americans have a chronic hepatitis C virus infection, whereas about 30% are obese, notes Michael Karin, distinguished professor of pharmacology and pathology at UC San Diego. "So the overall contribution to liver cancer deaths from obesity far exceeds what hepatitis C infection does," Karin says.