Printer-Friendly May 15, 2008 — That exercise reduces the risk for postmenopausal breast cancer is well known, but a new study shows that it can also protect against premenopausal breast cancer.
The study, published online May 13 in the Journal of the National Cancer Institute, also provides some details on the amount of exercise that is involved. The most active women, who showed a 23% reduction in the risk for premenopausal breast cancer, reported exercise that was equivalent to running 3.25 hours a week or walking 13 hours a week.
The strongest association for a lowered risk for premenopausal breast cancer was with activity between the ages of 12 and 22 years; the next strongest association was with activity between the ages of 23 and 34 years. No association for reduced risk for premenopausal breast cancer was apparent after age 35 years. However, many other studies have shown that exercise during adulthood reduces the risk for postmenopausal breast cancer.
"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," commented lead 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," he told journalists.
Dr. Colditz and colleagues analyzed data from 64,777 premenopausal women involved in the Nurses' Health Study II. During the 6 years of follow-up, 550 premenopausal women developed breast cancer.
The women completed detailed questionnaires about their occupational and leisure-time activities. Because self-reporting can be unreliable, the researchers used data from an earlier validation study on past-year adult activity to correct for measurement error. When they did this, Dr. Colditz and colleagues found a 39% lower breast cancer risk for total lifetime physical activity in the most active women compared with the least active women. This suggests that "our original estimate of a 23% lower risk was an underestimate," they commented.
The most important association with risk for premenopausal breast cancer was total activity. It did not seem to matter much what the activity was; the differences between strenuous, moderate, and walking activities were not statistically significant. "You don't have to be a marathon runner to get the risk-reducing effects of exercise," Dr. Colditz commented.
Exactly how exercise protects against breast cancer is unclear, but it is thought to work through hormone-related mechanisms. Physical activity can delay the start of menstruation and lengthen the time between periods, and strenuous exercise can stop ovulation. All of these effects would reduce a woman's lifetime exposure to estrogen and its mitogenic effects.
"Although the underlying mechanisms require further study, this research support the benefits of regular exercise during all ages among women," Dr. Colditz and colleagues commented. Physical activity is 1 of the very few risk factors for breast cancer that can be modified, and the finding that exercise appears to protect against breast cancer has public health implications, they added.
More Evidence of Protective Effect of Exercise
Further evidence that exercise protects against breast cancer comes from a review of 62 studies published online May 13 in the British Journal of Sports Medicine.
Nearly half of these studies showed a dose-response effect, researchers Dr. Christine Friedenreich, from the Alberta Cancer Board, in Calgary, Canada, and Dr. Anne E. Cust, from the School of Population Health, University of Melbourne in Australia, note, and overall, they show that exercise reduces the risk for breast cancer by about 25%.
Women who had undertaken a lot of physical activity throughout their life had the lowest risk for breast cancer, the researchers found. All types of activity reduced the risk for breast cancer, but recreational activity had a greater effect than occupational or household activity. Moderate and vigorous activity had equal benefits.
This review adds a few details to what is already known about the effect of exercise on reducing the risk for breast cancer, the researchers write. "Consistent and strong observational epidemiologic evidence" already exists to show that physical activity reduces the risk for breast cancer, and this evidence has been classed as "convincing," they point out.
Dr. Maruti is supported by the National Cancer Institute, National Institutes of Health. Dr. Colditz is supported by the American Cancer Society. Dr. Friedenreich was supported by career awards from the Canadian Institutes of Health Research and the Alberta Heritage Foundation for Medical Research, and Dr. Cust is supported by a Research Scholar Award from the Cancer Institute NSW, Australia.
J Natl Cancer Inst. 2008;100:728-737.
Br J Sports Med. Published online May 13, 2008.
Clinical Context
Many studies have investigated the link between physical activity and reduced risk for breast cancer. Possible biological mechanisms include reduced exposure to sex steroid hormones, changes in insulin-related factors and adipocytokines, modulation of inflammation and immune system, and alterations in estrogen metabolism and intracellular signaling pathways.
A systematic review by Monninkhof and colleagues published in Epidemiology in January 2007 indicated an association between physical activity and reduced risk for breast cancer in women who are postmenopausal. However, Rockhill and colleagues reported in December 1998 in the Journal of the National Cancer Institute that leisure-time activity during adolescence was not associated with breast cancer in premenopausal women. Subsequently, Colditz and colleagues reported in September 2003 in the British Journal of Cancer that leisure-time activity during adulthood was not associated with breast cancer in premenopausal women.
A cohort study by Maruti and colleagues in the Journal of the National Cancer Institute evaluates whether leisure-time physical activity is linked to reduced risk for breast cancer in premenopausal women.
In addition, a literature review by Friedenreich and Cust in the British Journal of Sports Medicine evaluates the effects of activity dose, type, and timing on the risk for breast cancer in women and whether the link between activity and breast cancer risk reduction is modified by population subtypes.
Study Highlights
Maruti and colleagues (Journal of National Cancer Institute)
64,777 women from the 1989 Nurses' Health Study II cohort were followed up for 6 years, starting in 1997, at ages 33 to 51 years.
Exclusion criteria were death or cancer (except nonmelanoma skin cancer) before 1997, noninvasive breast cancer, lack of youth activity data, and postmenopausal status.
In 1997, subjects reported average hours per week spent in strenuous, moderate, and walking leisure activity for ages 12 to 13 years, 14 to 17 years, 18 to 22 years, 23 to 29 years, and 30 to 34 years.
In 1997 and 2001, subjects reported activity in prior year.
Activity data were available for ages 12 to 55 years.
Metabolic equivalent (MET) value described the effort required for the activity compared with resting metabolic rate.
550 subjects were diagnosed with invasive premenopausal breast cancer.
Greater total average lifetime activity had the strongest association for breast cancer risk reduction.
Strenuous, moderate, or walking types of activity were not linked to risk.
Greater activity during ages 12 to 22 years was most strongly associated with cancer risk reduction
Friedenreich and Cust (British Journal of Sports Medicine)
Literature search of PubMed, Medline, Embase, CINAHL, and BIOSIS Previews databases in any language were conducted up to September 2007 for studies that measured physical activity with breast cancer as the main outcome.
Exclusion criteria were reanalysis of previous studies, male subjects, lack of point estimates with confidence intervals, different study design, and method or analysis problems.
62 studies included 28 cohort and 34 case-control studies.
47 studies (76%) showed that most active vs least active women had a breast cancer risk reduction (significant in 30 studies, borderline in 9 studies, and nonsignificant in 8 studies).
Overall risk reduction was 25%.
Dose-response relationship was noted in 28 of 34 studies.
14 studies showed no effect, and 1 study found nonsignificant increased cancer risk for most active women.
Recreational vs walking/cycling for transportation, household, and occupational activity was linked to greater decrease in risk (20% vs 14% vs 14% vs 13% decrease).
Vigorous vs moderate activity was linked to slightly greater risk reduction (26% vs 22% decrease).
Lifetime (vs 1 - 2 years before cancer diagnosis) and activity at or after age 50 years (vs adolescence and early adulthood) were linked to greater risk reduction.
Factors associated with greater risk reduction with activity:
Premenopausal and postmenopausal women showed reduced risk with activity, but the effect was greater and occurred in more studies in postmenopausal women.
Lower body mass index (BMI): lean BMI (< 22 kg/m2; risk reduction, 0.73) vs normal BMI (22 - 25 kg/m2; risk reduction, 0.76) vs high BMI (25 to less than 30 kg/m2; risk reduction, 0.81) vs very high BMI (at least 30 kg/m2; risk reduction, 1.04).
Nonwhite populations: Asian (50% risk decrease), black (40% risk decrease), and Hispanic (30% risk decrease) vs white (26%).
No family history of breast cancer vs family history of breast cancer (risk reduction, 0.94 vs 1.2).
Negative estrogen and progesterone hormone receptor tumors vs positive hormone tumors (risk reduction, 0.61 vs 0.86).
Parous vs nulliparous women (risk decrease, 28% vs 22%).
Energy intake had no effect.
Pearls for Practice
Premenopausal women who engage in leisure-time physical activity equivalent to running 3.25 hours per week or walking 13 hours per week have 23% reduced risk for breast cancer, with the strongest association for activity between ages 12 to 22 years.
Increased physical activity, especially recreational and lifetime or later adulthood activity, is linked to 25% reduced risk for breast cancer in women. Modifying factors that have the strongest effect include postmenopausal status, normal BMI, nonwhite ethnicity, no family history of breast cancer, negative hormone receptor tumors, and parity.
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Exercise Protects Against Premenopausal Breast Cancer CME/CE
News Author: Zosia ChusteckaCME Author: Penny Murata, MD DisclosuresRelease Date: May 15, 2008; Valid for credit through May 15, 2009
Credits Available
Physicians - maximum of 0.25 AMA PRA Category 1 Credit(s)™ for physicians;Family Physicians - up to 0.25 AAFP Prescribed credit(s) for physicians;Nurses - 0.25 nursing contact hours (None of these credits is in the area of pharmacology)