Abstract & Commentary
Synopsis: Exercise reduces the risk of breast cancer after menopause. Although the exercise need not be strenuous, longer duration of exercise confers greater benefit.
Source: McTiernan A, et al. JAMA. 2003;290:1331-1336.
This study was part of the Women’s Health Initiative (WHI). A total of 93,676 women aged 50-79 were enrolled between October 1993 and December 1998. Those with serious underlying illness, known breast cancer, or missing data were excluded. Women were recruited from 40 different centers, and about 15% were of color. They were followed by medical questionnaires annually, with about 95% response rates for each of the 6 subsequent years. Historical exercise was ascertained by asking (yes or no) if they did exercise that was long enough to work up a sweat and make their heart beat fast at least 3 times a week when they were 18, 35, 50, and current age at study entry. Current exercise was further assessed by asking how often they did the aforementioned exercise at study enrollment, as well as how often they currently walked outside the home for more than 10 minutes without stopping; they could choose between rarely/never, 1-3 times per month, weekly, 2-3 times per week, 4-6 times per week, and 7 or more times per week. Information about speed and duration was also gathered. They were also queried about moderate- and low-intensity exercise, with examples provided. Using midpoint values for ranges of frequency and duration of exercise reported, McTiernan and colleagues assigned metabolic equivalent (MET) values to the reported weekly activity and calculated MET~hours/week. Reproducibility and validation of this system was tested on a subset of more than 500 participants; test-rest reliability was very good. Detailed information about behavioral risks, age at menopause, menstrual and reproductive history, family risk factors, hormone replacement therapy, and diet was also collected. The women were followed up a mean of 4.7 years later, and there were 1780 newly diagnosed cases of breast cancer.
Past exercise: Women who engaged in strenuous physical activity at least 3 times a week at ages 35 and 50 had a reduction in overall breast cancer risk that was statistically significant only for the "exercise at 35" bracket. However, invasive cancer risk was markedly reduced for those who exercised at either 35 or 50. Exercise at 18 did not appear to confer much benefit.
Current exercise: There was an inverse "dose response" relationship between exercise at enrollment and breast cancer risk: those expending > 5-10 MET/week had an 18% reduction in breast cancer compared with sedentary women, and those expending > 40 MET/week had a 22% reduction. Similarly, more than 7 hours/week of strenuous exercise was associated with a 21% reduction in risk of breast cancer. These relationships held except for the most obese, even when controlling for Body Mass Index (BMI), the most obvious covariate. They also held when controlling for waist circumference, and behavioral and biologic risk factors.
Comment by Barbara A. Phillips, MD, MSPH
This is the largest, most carefully done study of the relationship between exercise and breast cancer to date. Previous studies have shown that exercise is associated with a reduced risk of breast cancer,1-4 but the current study is larger, makes an attempt to quantify exercise, and controls for most known important variables.
You know who needs to hear this? Your "worried well" women, who don’t smoke, take vitamins, and worry about their health. Exercise is a potent defense against most of the ills associated with aging, including cancer, diabetes, obesity, and arthritis. And the odds are that your female patients will need this message more than your male patients. Data from the 2000 National Health Interview Survey indicate that fewer women than men exercise regularly.5
The editorial accompanying this article6 attempts to reconcile (and apologize) for 2 conflicting reports on the need for exercise: that from the Centers for Disease Control (CDC)7 and that from the Institute of Medicine (IOM).8 The CDC recommends 30 minutes a day of moderate physical activity. The IOM is more focused on weight control and recommends 60 minutes a day of moderate-intensity exercise. This reminds me a little bit of the prune commercial, ". . . a half a dozen too many? Six not enough?" Any exercise at all would be a big step for most Americans; in fact, 72% of women and 65% of men do not engage in regular leisure time physical activity.5 The people writing these reports have not been to a mall or taken public transportation lately. A minority of people in America exercise. Our job is to get our patients moving! Any exercise at all is more than most are getting.
Dr. Phillips, Professor of Medicine, University of Kentucky; Director, Sleep Disorders Center, Samaritan Hospital, Lexington, KY, is Associate Editor of Internal Medicine Alert.
1. Thune I, et al. N Engl J Med. 1997;336:1269-1275.
2. Luoto R, et al. Eur J Epidemiol. 2000;16:973-980.
3. Breslow RA, et al. Cancer Epidemiologic Biomarkers Prev. 2001;10:805-808.
4. Lee IM, et al. Cancer Causes Control. 2001;12:137-145.
5. Barnes PM. Schoenborm CA. Physical activity among adults: United States 2000. Advance data from vital and health statistics. Hyattsville, Md. National Center for Health Statistics, 2003.
6. Lee IM. JAMA. 2003;290:1377-1379.
7. Pate RR, et al. JAMA. 1995;273:402-407.
8. Food and Nutrition Board, Institute of Medicine. Dietary References Intakes of Energy, Carbohydrates, Fiber, Fat, Protein, and Amino Acids (Macronutrients) Washington, DC. National Academy Press. 2002.
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