Invasive Breast Cancer Less Common After Lifetime of Strenuous Exercise

MADISON, Wis., Feb. 15 -- The risk of invasive breast cancer is associated with a 23% reduction for women who spend a lifetime exercising strenuously more than six hours a week, according to a population-based case-control study

And even six hours or more a week of strenuous recreational physical activity in the past 10 years is associated with a 13% reduction in the risk, found Amy Trentham-Dietz, Ph.D., of the University of Wisconsin's Comprehensive Cancer Center here.

But there was no similar association between physical activity and breast carcinoma in situ, Dr. Trentham-Dietz and colleagues reported in the February issue of Cancer Epidemiology Biomarkers & Prevention.

Also, the risk reduction only applied to women without a family history of breast cancer, the researchers said.

The findings came from the population-based case-control study in Massachusetts, New Hampshire, and Wisconsin, involving 7,630 controls, 1,689 women with carcinoma in situ, and 6,391 women with invasive breast cancer, all between the ages of 20 and 69.

There were 332 women who said they worked out six hours or more a week who were diagnosed with invasive breast cancer, 95 women with that exercise history who had carcinoma in situ, and 461 controls who reported the strenuous life style.

The mean age of diagnosis was 54.

The study aimed to investigate the association between cancer and both recreational and occupational activity, using 40-minute telephone interviews to assess lifetime activity, the researchers said.

Strenuous recreational activity included jogging or running; bicycling; calisthenics, aerobics, dance and racquet sports; swimming; and walking or hiking.

Occupational activity was assessed using the physical demand strength ratings of the U.S. Department of Labor's Dictionary of Occupational Titles, the researchers said.

Compared with women who do not exercise, the study found:

• After adjusting for other risk factors, women who had more than six hours a week of strenuous physical activity had an odds ratio for invasive breast cancer of 0.77, with a 95% confidence interval from 0.65 to 0.92.
• Women averaging more than six hours a week of strenuous physical activity during the past 10 years had a 13% reduction in invasive breast cancer risk. The odds ratio was 0.87, with a 95% confidence interval from 0.77 to 0.99.
• The associations did not apply to women who had a first-degree relative with breast cancer.
• There was no apparent link between the risk of carcinoma in situ and recreational exercise. The odds ratio was 0.94, but the 95% confidence interval included unity.
• Strenuous occupational activity also had no significant association with reduction in breast cancer risk, although there was a trend toward a lower risk for carcinoma in situ.

"A woman's hormone levels naturally fluctuate throughout her life, and we have found that exercise likely offers protection against breast cancer regardless of a woman's stage in life," according to lead author Brian Sprague, a research assistant at the Carbone cancer center.

"The take-home message for women should be that it is never too late to begin exercising," Sprague said in a statement.

Dr. Trentham-Dietz noted that this study is just the latest to find a link between exercise and a reduced risk of cancer, adding that several biological mechanisms have been proposed, including a reduction in estrogen levels because of exercise, prevention of weight gain, regulation of insulin sensitivity and alterations in immune function.

A next step would be "further studies of population subgroups" to clarify the link and identify women most likely to benefit, she said in a statement.

The researchers noted that the study design allows the possibility of both selection and recall bias. On the other hand, they said, the study's strengths include its large sample, its population-based sampling, and extensive data about potentially confounding variables.