Growing Evidence Clarifies Numerous Cancer-Fighting Benefits of Exercise, Say Experts

American Institute for Cancer Research
Thursday, 16 December 1999

According to experts at the American Institute for Cancer Research (AICR), regular physical exercise prompts a series of changes within the human body that actively fight cancer. By studying these changes, researchers have begun to isolate how and why physical activity seems to lower cancer risk.

"For almost fifty years we've known that regular exercise protects against heart disease," said Edward Giovannucci, M.D., Sc.D., of the Harvard Medical School and Harvard School of Public Health. "The studies coming in now point to a similar protective effect against cancer, especially colon cancer. Laboratory studies and clinical trials are helping us understand the physiological changes that occur in the body of a person who exercises regularly."

For the past decade, researchers studying large populations have noted that cancer rates decrease as physical activity increases. A 1997 AICR report that analyzed over 4,500 studies into the link between diet and cancer concluded that exercise may be generally protective against the overall risk of cancer.

The report, Food, Nutrition and the Prevention of Cancer: A Global Perspective, went on to review the effect of exercise upon cancers occurring at specific body sites. Studies are underway into the possible link between exercise and lowered risk of colon, breast, prostate, and other cancers.

The evidence that physical activity protects against colon cancer - the fourth most common cancer worldwide - is "particularly strong and convincing," said Dr. Giovannucci. Scientists investigating the exercise-cancer link have developed several theories to explain why.

One hypothesis centers on the fact that food moves more quickly through the bodies of people who exercise. This means that bile acids and dietary carcinogens spend less time in contact with the cells lining the colon. The kind of cellular damage that can lead to cancer is thereby reduced.

Another theory is that regular physical activity keeps the body's levels of triglycerides, glucose and insulin in check. High levels of these blood compounds have been associated with a syndrome that promotes cellular growth in general and growth of tumor cells specifically.

Similarly, regular exercise may decrease levels of prostaglandins, hormonelike substances that stimulate cell proliferation. Cells that reproduce rapidly are more likely to give rise to cancer.

Obesity is a Separate Risk Factor

Some cancer-fighting benefits of exercise are linked with its effect on overall body mass. According to the Surgeon General's Report on Nutrition and Health, obese men are 33% more likely to die of cancer than individuals of healthy weight. Obese women, however, are 55% more likely to die of cancer.

This difference may in part be due to a biological environment that is unique to postmenopausal women. Before menopause, formation of a hormone called estradiol primarily takes place within the ovaries. Estradiol is the most potent form of human estrogen.

After menopause, however, estradiol is formed exclusively within the body's fatty tissues. In obese postmenopausal women, greater deposits of fat produce high amounts of estradiol, which some studies link to increased risk of breast cancer.

A regular exercise program helps reduces fat deposits. Women with less body fat can better keep the estradiol in their systems in line.

"As more research comes in," said Dr. Giovannucci, "we'll learn more about biological mechanisms like that one. In particular, we'll uncover new ways that physical activity bolsters our systems, and helps our bodies to police themselves."

How Much is Enough?

"Any activity helps," said Dr. Giovannucci. "Different levels of activity may prove effective against different cancers, but in general, some regular exercise is better than none. Even in studies among obese individuals, we're starting to see that moderate physical activity can decrease cancer risk."

Colon cancer studies indicate that even 3-4 hours of brisk walking per week can produce anti-cancer benefits. "Now, we're not talking about a saunter, or a leisurely stroll," Dr. Giovannucci cautioned. "According to the data, walking at a brisk enough pace to get the heart rate up and break a sweat is the sort of activity that may help prevent colon cancer."

Data from prostate cancer studies, however, suggest that significantly higher levels of exercise may be needed to afford similar levels of protection. Only individuals who jogged or ran 15-20 miles per week, for example, consistently showed lower risk of prostate cancer in epidemiological studies.

Intensity of exercise is not the only factor currently under investigation. Some researchers are trying to determine how age affects the amount of protection afforded by exercise.

"Some studies seem to indicate that physical activity at an early age - while the breast is forming - may prove particularly effective in lowering breast cancer risk," Dr. Giovannucci said. He added that this is only one example.

"The thing to remember is that even a moderate amount of regular exercise seems to fight cancer on several fronts. This looks to be the case for all individuals, at any age."

According to a survey commissioned by AICR, 84% of Americans know that eating more fruits and vegetables decreases the risk of cancer. Surprisingly fewer Americans - only 60% of those surveyed - knew about the link between exercise and lower cancer risk. The survey was conducted by International Communications Research (ICR) and involved 1012 adults, 18 years or older, chosen at random.

AICR offers the free brochure "Getting Active, Staying Active," which contains advice on making the kind of daily changes that can help reduce cancer risk. It provides information to help you assess your present fitness level, offers tips on setting realistic personal goals, and reviews several strategies for long-term success. To order this information, call AICR at 1-800-843-8114, or visit the Institute online at www.aicr.org

For more information, or to contact American Institute for Cancer Research, see their website at: www.aicr.org