Menopause Live (13 September, 2010) from IMS

Although the underlying mechanisms remain unclear, physical activity has been consistently shown to reduce breast cancer risk in postmenopausal women. More recently, there is evidence that the risk reduction is even seen in premenopausal women [1]. However, it is still unknown whether the reduction in risk of breast cancer is similar in Asian women, as most of the previous studies were performed in Western people. Suzuki and colleagues [2] have recently conducted a hospital-based, case–control study in Japanese women that considered age and intensity of physical activity. They found that strenuous but not moderate physical activity at the age of 12 years was inversely associated with both pre- and postmenopausal breast cancer risk across estrogen receptor (ER) and progestogen receptor (PR) subtypes (overall odds ratio 0.24; 95% confidence interval 0.14–0.43), and that moderate physical activity in the preceding 5 years was associated with a decrease in risk for postmenopausal breast cancer, but only for ER+ and PR+ tumors, suggesting that the time and optimal intensity of physical activity are involved in the reduction in risk of breast cancer. They also showed that physical activity at the age of 20 years was weakly associated with breast cancer risk, but only when the physical activity was of moderate intensity.

Comment
During the past 50 years, the incidence of breast cancer in Japanese women has increased, with a peak incidence at around 45 years of age [3], in remarkable contrast to the incidence seen in European countries, where breast cancer is more common in postmenopausal women and the rate of the disease is about two-fold higher than that of Japanese women. Because the incidence of breast cancer in Japanese women living in the USA is much higher than that of those born and growing up in Japan, it is clear that the difference in lifestyle, particularly in diet, is deeply involved in the incidence of breast cancer. Therefore, the results of this study strongly suggest that physical activity is associated with breast cancer risk of both pre- and postmenopausal women, regardless of ethnicity and lifestyle. 

The results from Suzuki and colleagues are compatible with the results of the Nurses’ Health Study II, in which strenuous physical activity during the ages 12–22 years strongly contributed to a reduction in premenopausal breast cancer risk [1]. The authors, however, have revealed that physical activity, in particular a high level of activity, during the pubertal phase is very important in reducing the risk of breast cancer in pre- and postmenopausal women. They infer that the delayed menarche caused by strenuous physical activity might be one of the underlying mechanisms, although they could not find a significant difference in age at menarche between the case and control groups. Physical activity increases serum levels of sex hormone binding globulin, decreases the level of insulin and bioactive IGF-I and causes positive effects on non-estrogen-related mechanisms such as immune function and antioxidant enzymes [4]. Thus, diverse mechanisms must function in combination with each other, but it still remains unclear how physical activity during puberty affects breast cancer risk in later life. 

It is also interesting to consider the effect of physical activity on risks of malignancies in other organs. Convincing evidence exists that physical activity is associated with colorectal cancer [5] and possibly with endometrial cancer [6]. On the other hand, no association has been shown between physical activity and lung or pancreatic cancer. As well as affecting the risk of cancer, physical activity strongly influences risk in several other aspects of women’s health such as cardiovascular disease, metabolic syndrome and osteoporosis. Therefore, clinicians should focus on recommending physical activity for many aspects of women’s health care.

Hideki Mizunuma
Department of Obstetrics & Gynecology, Hirosaki University, Japan

References
1. Maruti SS, Willett WC, Feskanich D, Rosner B, Colditz GA. A prospective study of age-specific physical activity and premenopausal breast cancer. J Natl Cancer Inst 2008;100:728–37.
http://www.ncbi.nlm.nih.gov/pubmed/18477801
2. Suzuki R, Iwasaki M, Kasuga Y, et al. Leisure-time physical activity and breast cancer risk by hormone receptor status: effective life periods and exercise intensity. Cancer Causes Control 2010 Jul 4. Epub ahead of print.
http://www.ncbi.nlm.nih.gov/pubmed/20607384
3. Katanoda K, Qiu D. Comparison of time trends in female breast cancer incidence (1973–1997) in East Asia, Europe and USA, from Cancer Incidence in Five Continents, Vols IV–VIII. Jpn J Clin Oncol 2007;37:638-9.
http://www.ncbi.nlm.nih.gov/pubmed/17878229
4. Neilson HK, Friedenreich CM, Brockton NT, Millikan RC. Physical activity and postmenopausal breast cancer: proposed biologic mechanisms and areas for future research. Cancer Epidemiol Biomarkers Prev 2009;18:11-27.
http://www.ncbi.nlm.nih.gov/pubmed/19124476
5. Halle M, Schoenberg MH. Physical activity in the prevention and treatment of colorectal carcinoma. Dtsch Arztebl Int 2009;106:722-7.
http://www.ncbi.nlm.nih.gov/pubmed/19997551
6. John EM, Koo J, Horn-Ross PL. Lifetime physical activity and risk of endometrial cancer. Cancer Epidemiol Biomarkers Prev 2010;19:1276-83.
http://www.ncbi.nlm.nih.gov/pubmed/20406960