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Here's one more reason for women to stick with their walking, jogging or gym routine: Physical activity will greatly reduce the risk of breast cancer.
Two new studies this week show that women between the ages of 12 and 35 who exercise regularly are 23 per cent less likely to get breast cancer before menopause, and post-menopausal women benefit even more from physical activity.
At a time when there aren't many preventive strategies for breast cancer, this is one way women can take charge, the researchers say.
"You don't have to be a marathon runner to get the risk-reducing benefits of exercise," said Graham Colditz, one of the authors of the study that looked at young women and cancer, and a professor in the department of surgery at the Washington University School of Medicine in St. Louis, Mo.
Recreational activity beats out household chores and work-related tasks when it comes to reducing breast cancer risk, researchers found. (Seth Wenig/Associated Press)
Dr. Colditz's study, published yesterdayin the U.S.-based Journal of the National Cancer Institute, is the largest analysis to date of the effects of exercise on pre-menopausal breast cancer. Researchers from Washington University and Harvard University evaluated nearly 65,000 women who filled out questionnaires on their level of physical activity at various periods of their lives, starting from age 12. The researchers followed up over six years and found that 550 women had been diagnosed with breast cancer.
Women whose activity was equivalent to 3.25 hours a week of running or 13 hours a week of walking had a 23 per cent reduced risk of the disease, compared with those who had been less active, the study found.
The researchers concluded that the benefits of exercise were not linked to a particular sport or the intensity of a workout, but rather to the total activity.
"I think the study clearly supports our encouraging girls and young women to be physically active to lower their risk of breast cancer and to sustain that activity on through the pre-menopausal years," Dr. Colditz said.
He explained that the risk of cancer is lower probably because active women are exposed to less estrogen. If girls are more active during adolescence, they are likely to begin menstruating later and, as a result, have a lower level of estrogen in their bodies.
Other studies have shown that the more estrogen some women are exposed to, the greater their breast-cancer risk.
A quarter of all cases of breast cancer are diagnosed in women before menopause. Previous studies that looked at the influence of exercise on breast-cancer risk during this period produced conflicting results.
But studies on breast-cancer risk post-menopause have reached the same conclusion: Physical activity reduces the risk of the disease.
The second study, published earlier this week in the online edition of the British Journal of Sports Medicine, said that women who were physically active throughout their lives had the lowest risk of breast cancer.
And because most cases of breast cancer arise in post-menopausal women, females in this group who exercised had their risk reduced even more than pre-menopausal women.
"It's never too late to start," said Christine Friedenreich, one of the study's authors and a research scientist at the Alberta Cancer Board.
Dr. Friedenreich and her colleagues reviewed 62 studies that looked at the impact of physical activity on breast-cancer risk. They found that physically active women had a 25 per cent reduced risk of developing the disease compared with women who were the least active, findings that were consistent with Dr. Colditz's results.
All types of activity, including household chores and work-related tasks, reduced the risk, but recreational activity had the greatest impact, the study found.
Women with a body mass index (an approximation of body fat) under 22 benefited the most. But even women with a higher BMI, one above 25 (which is considered to be overweight), saw benefits from being physically active. The researchers did not see much of an impact among obese women (a BMI over 30).
Dr. Friedenreich said women generally gain weight after menopause, and that is a risk factor for breast cancer. Being physically active not only controls the weight gain, but brings that risk down, she said.
The research paper also looked at studies of different racial groups, and found that black, Asian and Hispanic women had stronger risk reductions if they were physically active compared to their counterparts who were more sedentary.
Physical activity not only reduces the threat of breast cancer, but also helps in the prevention of cardiovascular disease and diabetes, and improves mental health, she added.
"We've now got very strong evidence that physical activity reduces the risk of breast cancer. Because there are very few modifiable lifestyle risk factors for breast cancer, this is one where women actually have control. That's actually a very strong and empowering message that you can give to women," Dr. Friedenreich said.
