Jun 25, 2012

Timing of Physical Activity Key to Reduce Breast Cancer Risk

June 25, 2012 — It is increasingly well-established that physical activity may reduce the risk for breast cancer; now a new study indicates that the timing and extent of exercise are key to gaining benefit.

According to the study, published online June 25 in Cancer, any level of physical activity during the reproductive and postmenopausal years appeared to decrease this risk, but the highest level of exercise was associated with the largest decrease in risk.

The authors found that although all levels of exercise were beneficial, women who exercised 10 to 19 hours per week experienced the greatest benefit, with an approximately 30% reduced risk.

“We found that recreational physical activity over the life-course, particularly during the reproductive and postmenopausal years, among parous women decreases breast cancer risk,” lead author Lauren E. McCullough, MSPH, a doctoral student in the Department of Epidemiology at the University of North Carolina, Chapel Hill, said in an interview. “The observed inverse associations found in our study are consistent with most other studies that have examined the effect of physical activity on breast cancer risk, where an average 25% risk reduction is reported.”

McCullough explained that the term “reproductive period” is an arbitrary term that was selected to capture this very important time during a women’s life, from when she experiences her first birth until menses ceases. “One could define the term ‘early adulthood’ or ‘adolescence’ as being a part of the ‘reproductive period,’ but what we really mean by using the phrase ‘reproductive period’ is the time from which a women had her first child until menopause,” she said. “Exercise during the time period preceding this milestone event of a first birth did not appear to be associated with breast cancer risk.”

Weight Gain and Weight Loss

However, a substantial gain in weight after menopause could negate the benefits of regular activity.

A high weight gain during adulthood was associated with a 28% increased risk for breast cancer among women who had no physical activity over the life-course. Conversely, exercise did not appear to mitigate the effects of weight gain, as null associations were observed among high gainers reporting high levels of physical activity during the same period (odds ratio [OR], 1.02).

However, it is yet unknown whether weight loss among overweight women will help enhance the effects of exercise. “The 3-way interaction between body size, weight change, and exercise is of interest,” said McCullough, “but unfortunately, we were unable to explore this question with only 2000 women. Studies with larger samples may be able to [explore this further] in the future.”

“With regard to weight loss, exercise, and breast cancer risk, one would intuitively think that women who exercise and lose weight may be at lowest risk of breast cancer,” she added. “However, this too is difficult to study, as so few women actually experience weight loss, especially during the postmenopausal period.”

McCullough pointed out that in their study, there were only 131 case women and 137 control women who lost weight after menopause. Comparatively, there were 525 case women and 436 control women who gained weight after menopause. “So although our data show a large breast cancer risk reduction for weight loss and high physical activity, there are so few women in this category (33 cases and 49 controls), the estimate is not very precise,” she said.

Growing Evidence in Support of Exercise

As previously reported by Medscape Medical News, a growing body of evidence has found that physical exercise reduces the risk for cancer, as well as reduces mortality in cancer patients and survivors. For example, a literature review published online May 8 in the Journal of the National Cancer Institutereported that physical activity is associated with improved survival in people with breast and colon cancer.

“Many treatments may increase survival, but at a cost of quality of life; physical activity may not only extend life but may also enhance its quality,” notes Edward L. Giovannucci, MD, ScD, from the Department of Nutrition at the Harvard School of Public Health, Boston, Massachusetts, in an accompanying editorial. “Even though direct effects of physical activity on cancer are not definitely proven, given that physical activity is generally safe, improves quality of life for cancer patients, and has numerous other health benefits, adequate physical activity should be a standard part of cancer care.”

Specifically, the association between breast cancer risk and exercise has already been documented.One paper reported that postmenopausal women who maintain a regular, moderate to vigorous exercise program reduce their risk for breast cancer, even if they did not exercise in the past. Women who maintained a high level of activity for more than 7 hours a week during the 10 years before the study reduced their risk for breast cancer by 16% vs more sedentary women in age-adjusted and multivariate (each relative risk, 0.84; 95% confidence interval, 0.76 – 0.93) models.

Another study also reported that exercise could reduce the risk for breast cancer specifically in premenopausal women. There was a 39% lower breast cancer risk for total lifetime physical activity in the most active women compared with the least active women, according to the article.

“We don’t have a lot of prevention strategies for premenopausal breast cancer, but our findings clearly show that physical activity during adolescence and young adulthood can pay off in the long run by reducing a woman’s risk of early breast cancer,” said investigator Graham Colditz, MD, DrPH, from the Siteman Cancer Center, Washington University School of Medicine, in St. Louis, Missouri. “This is just 1 more reason to encourage young women to exercise regularly.”

Strongest Association in Reproductive Years

In the current study, McCullough and colleagues investigated the relationship between recreational physical activity and breast cancer risk. They sought to characterize the joint effects of activity level, weight gain, and body size in a population-based sample of 1504 cases of breast cancer (n = 233 in situ; n = 1271 invasive) and 1555 control patients (aged 20 – 98 years) who participated in the Long Island Breast Cancer Study Project.

They found that in the entire cohort, the risk for breast cancer was only slightly decreased with any recreational exercise (OR, 0.94). It varied modestly by menopausal status (OR, 1.15 for premenopausal women; OR, 0.87 for postmenopausal women).

Compared with other groups, recreational physical exercise during a woman’s reproductive years showed the strongest association. Parous postmenopausal women who exercised 10 to 19 hours per week had an age-adjusted OR of 0.67 vs those who were inactive. The association was also observed for postmenopausal women who exercised at that level when it was done after menopause (OR, 0.70).

“Even upon restricting our analyses to parous women creating comparable groups across time periods, we find that the most relevant period for breast cancer risk reduction is during the reproductive period and following menopause,” write the authors. “Similarly, we found comparable effect estimates for cases of invasive and in situ breast cancer, and thus report findings for total breast cancer.”

Exercise also appeared to preferentially reduce the risk for hormone receptor–positive breast cancer. Among parous postmenopausal women, those who exercised during the reproductive period showed a 25% risk reduction of hormone receptor–positive breast cancer (OR, 0.75) and a 4% risk reduction among hormone receptor–negative cases (OR, 0.96).

The authors estimated the association between exercise and estrogen receptor status and found little difference between the 2 groups. Their data indicated that there was a 15% risk reduction for estrogen receptor–negative breast tumors and a 20% risk reduction for estrogen receptor–positive cancers among postmenopausal women who had high rates of exercise during their reproductive period.

“Collectively, these results suggest that women can still reduce their breast cancer risk later in life by maintaining their weight and engaging in moderate amounts of activity,” the authors conclude.

The study was supported in part by awards from the Department of Defense Breast Cancer Research Program and the National Cancer Institute and the National Institute of Environmental Health Sciences. The authors have disclosed no relevant financial relationships.

Cancer. Published online June 25, 2012. Abstract

— Roxanne Nelson



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