Current issues of ACP Journal Club are published in Annals of Internal Medicine


Etiology

Physical activity was associated with lower risk for breast cancer

ACP J Club. 1997 Nov-Dec;127:78. doi:10.7326/ACPJC-1997-127-3-078

Related Content in this Issue
• Companion Abstract and Commentary: Physical activity was associated with a reduced risk for death among postmenopausal women


Source Citation

Thune I, Brenn T, Lund E, Gaard M. Physical activity and the risk of breast cancer. N Engl J Med. 1997 May 1;336:1269-75.


Abstract

Objective

To determine whether an association exists between everyday physical activity and the risk for breast cancer.

Design

Cohort analytic study with a mean follow-up of 14 years.

Setting

Community-based study in Norway.

Participants

25 624 women (age range 20 to 54 y at study entry) who were invited to participate in 2 health surveys, one during 1974 to 1978 and the other during 1977 to 1983, to determine risk factors for cardiovascular disease.

Assessment of risk factors

Self-reported level of physical activity during leisure hours and work hours was assessed at study entry for the previous year and was reassessed 3 to 5 years later. Physical activity was graded 1 for sedentary; 2 for moderate activity; 3 for more strenuous leisure physical activity, including recreational athletics; and 4 for regular strenuous activity, including competitive athletics. Physical activity at work was also rated on the following 4-point scale: 1 for sedentary work, 2 for work involving much walking, 3 for work involving much lifting and walking, and 4 for heavy manual labor. At baseline, age, body mass index (BMI), height, parity, smoking and dietary habits, and county of residence were also recorded.

Main outcome measure

Incidence of breast cancer.

Main result

351 incident cases of breast cancer occurred during follow-up. After adjustment for age, BMI, height, parity, and county of residence and through use of the sedentary group as the reference group, the relative risk (RR) for breast cancer among women who exercised regularly during leisure time (leisure activity level 3) was 0.63 (95% CI 0.42 to 0.95) and 0.48 (CI 0.25 to 0.92) among women with heavy manual labor jobs (work activity level 4). Further adjustments for smoking and dietary habits did not influence the estimates. The reduction in risk was not seen among women who exercised moderately (leisure level 2) during their leisure time (RR 0.93, CI 0.71 to 1.22), among women who walked regularly (RR 0.84, CI 0.63 to 1.12), or among women who lifted regularly at work (work level 3) (RR 0.74, CI 0.52 to 1.06). A nonsignificant declining risk for breast cancer was found with an increasing level of physical activity among premenopausal women (P = 0.10). The reduction in risk for breast cancer was greater among younger women (age < 45 y at baseline) than among older women (age ≥ 45 y at baseline). In stratified analysis, the risk for breast cancer was lowest in women with a BMI < 22.8 who exercised ≥ 4 hours per week (RR 0.28, CI 0.11 to 0.70).

Conclusion

Physical activity during leisure time and at work was associated with a reduced risk for breast cancer.

Source of funding: Norwegian Cancer Society.

For article reprint: Dr. I. Thune, Institute of Community Medicine, University of Tromsø, N-9037 Tromsø, Norway. FAX 47-7764-4831.


Commentary

Physical activity has been increasingly on the minds of patients and physicians, most notably because of the 1996 Summer Olympics and the Surgeon General's Report on physical activity and health (1). The editorial accompanying the article by Thune and colleagues asked whether exercise was beneficial and how much was necessary to obtain a benefit (2). I would adapt this to the following 3 questions: 1) What benefits can women expect from exercise? 2) How much exercise is necessary for women to obtain these benefits? and 3) How can we get our sedentary patients to exercise?

To the first question, I would answer that women who are more active experience a variety of benefits. As shown in the study by Kushi and colleagues, women who were more active had lower overall and cardiovascular mortality. In the study by Thune and colleagues, a lower incidence of breast cancer was found among women who exercised regularly. In other studies, exercise was shown to prevent hypertension, diabetes, obesity, osteoporosis, and many other conditions. Physical activity is an effective treatment for hypertension, hyperlipidemia, and depression (1).

The answer to the second question about how much exercise is enough depends on which benefit is being considered. Kushi and colleagues found that postmenopausal women who exercised moderately once a week had a 24% reduction in total mortality compared with sedentary women. It was unclear whether the group of women who exercised daily did substantially better. It is important to note that the amount of vigorous exercise seemed to have no effect because the benefit was more dependent on the amount of moderate activity. On the other hand, Thune and colleagues found a reduced incidence of breast cancer only in the women who spent ≥ 4 hours a week exercising vigorously to keep fit or those who participated in competitive sports but not in the women who spent ≥ 4 hours a week walking, bicycling, or doing other moderate exercise. This exercise classification scheme resulted in over two thirds of the study sample being in the moderate exercise group. It is unclear whether the lack of benefit observed for moderate exercise is because of this classification (which makes it more difficult to show a difference) or because there was, in fact, no benefit.

Finally, the crucial issue is the answer to the third question: How do we as clinicians get our patients to exercise? Patients usually know that they should exercise, and almost all physicians think that exercise is important. A useful model is called the "Stages of Change" or "Transtheoretical Model," (3) which posits that people move through a series of 5 stages. For exercise, the first would be lack of interest in exercise (precontemplation), followed by interest in exercise but not doing any (contemplation). Next would be interest in exercise and doing so occasionally (preparation), followed by exercising regularly (initial action and then maintenance). Our goal as clinicians is to do 2 things: 1) assess the stage where our patients are and 2) give them advice to help move to the next stage. Practically speaking, patients who are interested in exercise but not currently active can be helped to develop a plan and a start date. Those who are not interested in exercise can be given literature to read to help convince them that it is beneficial and feasible. For patients who do not exercise because they see no benefit, these 2 articles provide additional evidence that it is indeed good for them.

Scott E. Sherman, MD, MPH
Ann L. Mai, MD
Veterans Affairs Center for the Study of Healthcare Provider BehaviorSepulveda, California, USA

Scott E. Sherman, MD, MPH
Veterans Affairs Center for the Study of Healthcare Provider Behavior
Sepulveda, California, USA

Ann L. Mai, MD
Veterans Affairs Center for the Study of Healthcare Provider Behavior
Sepulveda, California, USA


References

1. U.S. Department of Health and Human Services. Physical activity and health. A report of the Surgeon General. Atlanta, GA: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Chronic Disease Prevention and Health Promotion, 1996.

2. McTierman A. Exercise and breast cancer—time to get moving? N Engl J Med. 1997; 336:1311-2.

3. Prochaska JO, DiClemente CC, Norcross JC. In search of how people change. Applications to addictive behaviors. Am Psychol. 1992;47:1102-14.