Newsletter 85, Fall 2004
Research supports the link between regular physical activity and lower cancerrisk. For breast, prostate and colon cancers, even moderate activity - suchas brisk walking - clearly aids prevention.
Scientists are getting a more complete picture of how physical activity may lowercancer risk. Results that are now emerging from a number of large, long-termhuman studies show that physical activity, hormones, dietary fat and cancer developmentare all connected.
"Three types of cancer - colon, breast and prostate - seem clearly tobe influenced by physical activity levels," says Dr. Ritva Butrum, SeniorScience Advisor to AICR.
As far back as 1984, researchers in California found that men with desk jobshad a 60 percent greater risk of colon cancer than men whose jobs involved strenuousphysical activity. As of 2002, 43 out of 51 studies had shown that exercisingmore is associated with a lower risk of colon cancer.
Some researchers think exercise may reduce colon cancer risk by suppressing levelsof insulin and a protein known as insulin-like growth factor 1 (IGF-1). "Severalstudies have shown that people with higher levels of insulin and IGF-1 have ahigher risk of colon cancer," says Dr. Edward Giovannucci, Associate Professorof Medicine at Harvard Medical School. "Chronically high levels of insulinand IGF-1 seem to stimulate cells to divide. When more cells are dividing, thereis a greater chance of a cancer-causing genetic mutation."
Scientists also suspect that exercise speeds the movement of food through thecolon and decreases secretion of bile acids, two factors that are associatedwith lower colorectal cancer risk.
Exercise at Any Age CouldLower Breast Cancer Risk
Many studies have also shown that exercise may reduce risk of breast cancer.For example, girls ages 14-16 who were physically active for two to three hoursa week had about a 30 percent reduced risk of getting breast cancer later inlife. Another study found that premenopausal women who exercised four hours ormore per week reduced their risk of breast cancer approximately 50 percent.
High levels of the hormones estrogen and progesterone may contribute to breastcancer development. Physical activity appears to lower levels of these hormones.
"Women who are physically active during adolescence and young adulthood are exposedto lower levels of hormones," says Dr. Leslie Bernstein, Professor ofPreventive Medicine at the University of Southern California. "We thinkthis explains why exercise reduces breast cancer risk in younger women. Thereis still a lot we need to learn about how exercise reduces risk in older women."
One possibility is that exercise helps to avoid weight gain. Older women whoare overweight are at higher risk for breast cancer, partly because having largerfat cells leads to higher levels of both insulin and estrogen. Also, physicalactivity may strengthen the body's immune system.
Among studies that show positive results of exercise for older women is the government-fundedWomen;s Health Initiative, which recently reported that 74,000 women ages50-79 who did exercise such as brisk walking for 1 1/4-2 1/2 hoursa week reduced their risk of breast cancer by 18 percent.
Vigorous Is Better,But Moderate Works Too
For some cancers, exercise may need to be vigorous to provide a protective effect.One study found that men who exercised vigorously for four to five hours a weekdid not reduce their risk of prostate cancer but were less likely to get an aggressiveform of the disease. A more recent study published last year concluded that menand women who exercised vigorously had a 40 percent lower risk of rectal cancerthan people who got less exercise.
But most research shows a benefit even from moderate exercise. "Frequent,short bouts of activity - such as four 10-minute walks a day - are justas effective as longer exercise sessions and may be easier to fit more ofteninto a busy life," advises Ruth Ann Carpenter, M.S., R.D., Director ofthe Center for Research Dissemination at the Cooper Institute in Dallas, Texas.
If you are getting regular, moderate exercise, but want to burn more calories,then intensify and/or extend your physical activity after checking with yourdoctor. Weight-bearing exercise (including walking, aerobics and tennis) to reduceosteoporosis, weight lifting for strength and stretching for flexibility areall important as well.
AICR recommends working toward the goals of one hour of vigorous activity (toincrease heart and breathing rates) per week in addition to one hour of moderatedaily exercise the rest of the week. Be sure to check with your doctor beforestarting any physical activity program.
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