Surprise! Obesity (and Inactivity) Can Spur Cancers

[Guest guest]

" you see a progressive increase in cancer rates for each unit

increase in BMI above 21, " he told Science News Online.

Surprise! Obesity (and Inactivity) Can Spur Cancers

Janet Raloff

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 of the

London-based International Obesity Task Force.

It's ironic, 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 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

, 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, " 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

ls 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, 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, 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.

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. "

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