Surprise! Obesity (and Inactivity) Can Spur Cancers

Some 60 percent of U.S. adults say they’re worried at the prospect of developing cancer, yet only 6 percent recognize that being overweight is a leading predisposing factor.

That’s one finding from a June survey, commissioned by the American Institute for Cancer Research in Washington, D.C. The survey was unveiled on July 11 at a meeting in Washington by Philip James of the London-based International Obesity Task Force.

It’s ironic, James notes, that so few people recognize obesity’s role in cancer. Accumulating data indicate, he says, that “you can probably ascribe over 100,000 new cases of cancer in the United States each year directly to excessive weight”–and that’s independent of the role of poor diet or too little exercise.

Obesity’s cancer risk traces largely to fat, observes George A. Bray of the Pennington Biomedical Research Center in Baton Rouge, La. He’s not referring to the type that we gobble down so much as the fat that accumulates on us as cushiony padding, especially throughout our torsos.

“Most of us look at our guts and our hips and our love handles and think of fat as an inert substance that merely collects and hangs off of us,” he says. Instead, the endocrinologist notes, “fat is a remarkably active substance.”

Fat-containing cells–especially those in the abdomen–behave like little hormone factories, he says, secreting estrogen, insulin, and a host of other so-called growth factors. As they circulate throughout the bloodstream, the substances signal certain tissues to increase in size and divide.

That’s normal.

What’s not normal are today’s burgeoning waistlines.

Researchers gauge heft via a body mass index, or BMI. Generally defined as an individual’s weight in kilograms divided by the square of height in meters, BMI offers some accounting for height in evaluating what the bathroom scale says. (For persons who prefer to work with standard U.S. units, use the calculator on a Web site. Otherwise, calculating BMI takes a few steps more. Divide one’s weight in pounds by 2.2, then divide one’s height in inches by 39.37. Next, divide the first number by the square of the second.)

International guidelines classify people with BMIs 25 and over as overweight. Individuals with a BMI of 30 and over are termed obese.

According to such designations, an estimated 750 million people, globally, are overweight and some 300 million more are obese, reports James, a consultant to the World Health Organization.

Earlier research showed convincingly that overweight and obese people face an elevated risk of diabetes, heart disease, and stroke. More recent studies have indicated that rates for those disorders begin to climb at a BMI of 21, well below the threshold for the overweight classification.

Emerging data “now look as though the very same [lower cutoff] applies for cancer,” James says. After accounting for all other known risk factors, “you see a progressive increase in cancer rates for each unit increase in BMI above 21,” he told Science News Online.

For perspective, a 5′ 10″ person weighing 150 pounds would tip the BMI scales at 21.5.

The good news? The same lifestyle recommendations to cut risks of other types of chronic disease will pay off in cancer protection as well. The bad news: Record numbers of people are ignoring those recommendations.

Expanding waistlines

Even as popular culture extols the virtues of thin, more Americans are plumping out and doing it at younger ages than at any time in recent history.

According to a study by researchers at the University of North Carolina in Chapel Hill, the share of U.S. adults who are obese has doubled during the past 40 years. By their mid-30s, 26 percent of U.S. men and 28 percent of women are now obese, they report in the June 18 Annals of Internal Medicine.

As a result of this lateral growth spurt, 61 percent of U.S. adults are overweight–many of them excessively, according to the Surgeon General’s office. So are 13 percent of elementary-school kids and 14 percent of adolescents.

Contrary to popular belief, James points out, North America’s penchant for fast and fatty foods does not make it the heaviest population. That dubious distinction, he says, goes to Middle Easterners, followed closely behind by residents of Central and Eastern Europe.

Clearly, Bray argues, “we are in the midst of an obesity epidemic.”

As our bodies’ stores of fat grow, so does their production of cellular growth factors. This creates an environment in which cell growth and division accelerate, increasing the chance that random mutations will occur and lead to cancer. At the International Research Conference on Food, Nutrition, and Cancer meeting in Washington in July, Bray summarized recent data from several labs contributing to this view.

The cancers fostered by obesity tend to be ones most responsive to estrogen, other steroid hormones, and related compounds that burgeoning fat cells secrete. Many obesity-induced cancers originate in the prostate, colon, breast, uterine endometrium, and kidney, Bray says.

Throughout women’s reproductive years, their ovaries regularly flood their bodies with tidal surges of estrogen that can fuel the growth of reproductive tissues, such as the breast and uterus. At menopause, when the ovaries shut down, this major source of estrogen dries up, but fat cells continue to make estrogen. In men and in postmenopausal women, those fat cells will be the primary source of the hormone. The more fat, the more estrogen those cells will shed.

Indeed, Bray told Science News Online, “the main reason for increased breast and endometrial cancer in postmenopausal women is the estrogen production from their increased fat tissue.”

Obesity is far from the only factor responsible for these tumors and cancers of the prostate, colon, and kidney. Radiation, pollution, poor diet, and other factors can spawn them as well. However, James notes, a review of data by the World Health Organization’s International Agency for Research on Cancer last February concluded that between a quarter and third of all colon, breast, endometrial, kidney, and esophageal cancers globally may trace to excess weight, especially obesity.

Get your motor running

People gain weight only by eating more calories than their bodies burn. One way to burn more calories is to rev up metabolism with activity.

As an individual’s participation in exercise climbs, pounds will drop–provided that food intake remains stable. Increase exercise and also decrease calorie consumption and the pounds will disappear more quickly.

Until recently, researchers had assumed that any anticancer benefits of increasing activity came from reining in weight gain. New data suggest it’s more complicated than that–and potentially more encouraging for the legions of heavyweights.

Christine Friedenreich of the Alberta Cancer Ward in Calgary, Canada, has been probing physical activity’s role in cancer. At the July meeting in Washington, she reviewed trends emerging from analyses of some 170 studies conducted over the past 15 years. “All used different definitions of physical activity, many looked at different cancer sites, and many measured physical activity in different ways,” she notes. However, despite the divergent methods, designs, and focus, “we still get fairly consistent results,” she told Science News Online.

For colon cancer, the most widely studied malignancy in this regard, 51 studies have investigated possible impacts of activity–and 43 reported cancer-risk reductions with increasing exercise. Similarly, 31 of the 45 breast cancer studies found such a trend. For these two sites, the anticancer benefits of increased physical activity “are now convincing,” she says. The case for prostate cancer appears “probable,” she adds, and for malignancies of the uterine endometrium and lung “possible”.

In some cases, men who exercised heavily–perhaps 30 to 45 minutes of vigorous activity five or more days a week–experienced just half the prostate-cancer incidence of couch potatoes. “That’s a huge risk reduction,” she notes. Among breast cancer studies finding an exercise benefit, average risk reductions were in the range of 30 to 40 percent.

These effects were seen in people who were of a healthy weight as well as those who were overweight or obese.

Though the mechanism for physical activity’s protection remains unknown, Friedenreich notes there are a number of possibilities. For instance, activity speeds the transit of foods through the gut and decreases the secretion of bile acids during digestion. Slow transit and high acid production both increase risk of colon cancer. What’s more, exercise tends to moderate hormone production, which could limit overexposure of vulnerable tissues. Regular activity might even strengthen the immune system.

In her own studies, the epidemiologist has been focusing on lifetime physical activity trends in men and women up to 85 years old–not just recreational exercise but also occupational tasks and work around the home.

The biggest benefits against breast cancer seem to stem from activity after menopause. Compared to the perpetually inactive, she found, “if a woman had been inactive prior to menopause and then became active afterward, she could still experience a 40 percent decrease in breast cancer risk.” Women who were consistently active throughout adulthood showed a slightly stronger benefit still. The findings appeared in a series of three specialty journals last fall.

Her team is now crunching their data for prostate cancer and beginning a new study investigating possible exercise benefits against endometrial cancer.

Most provocative, a study that her team published in the May 20 International Journal of Cancer found that for women who regularly engaged in comparable physical activity, those who had the biggest waist-to-hip ratio–indicative of preferentially storing abdominal fat–had the highest risk of breast cancer. “So,” she says, “there seems to be an effect of obesity on breast cancer, independent of physical activity.”

In another study, moderate-intensity occupational and household activities decreased breast cancer risk.

How much activity is enough? No one knows, Friedenreich admits, though many health groups advocate a minimum of 30 minutes four to five times a week of any activity that raises your heart rate and or works up a sweat. That could amount to something as simple as brisk walks or heavy housework. “The key,” she says, “is to get moving and keep moving.”

Janet Raloff is the Editor, Digital of Science News Explores, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.

More Stories from Science News on Health & Medicine