Conclusion:
Physical activity was associated with a reduced risk of BC.

STUDY

1. Entered over 25 000 women age 20-54 in 1977-87.
2. Health surveys asked about leisure-time and work activity.

RESULT

1. During a median of 14 years follow-up, 351 cases of invasive BC were identified—100 premenopausal; 251 postmenopausal.
2. After adjustment for multiple risk factors including body mass index, women who exercised regularly had a relative risk of BC of 0.6 compared with sedentary women.
3. In regularly exercising women, the reduction in risk was greater in premenopausal women (at entry) than in postmenopausal women, and in women under age 45 at entry compared with those over 45 (RR=0.4).
4. The risk of BC was lowest in lean women (BMI < 23) who exercised at least 4 hours per week (RR=0.3).
5. Risk was also reduced in women with higher levels of activity at work.

DISCUSSION

1. "Our results support the idea that physical activity protects against breast cancer, particularly among premenopausal and younger post menopausal women."
2. The projective effect was notable among lean women who were consistently active during their leisure time.

CONCLUSION
Physical activity during leisure time and work was associated with a reduced risk of breast cancer.
NEJM May 1, 1997; 336: 1269-75 Original investigation from Univ. Of Tromso, Norway.
An editorial in the same issue of NEJM (pp 1311-12) comments: The preponderance of breast cancer in women as opposed to men, the reproductive patterns common to women in whom breast cancer develops (early menarche, late menopause, low parity, delayed childbearing, and short duration of lactation), and increased risk associated with obesity (particularly truncal obesity) suggest that female hormones contribute to the cause of breast cancer.
There is considerable evidence that obesity and elevated concentrations of endogenous estrogens increase the risk of cancer among postmenopausal women. Physical activity may act by reducing obesity and thus reducing fat stores, which provide the substrate for conversion of androgens to estrogens.
How should the physician answer a woman who asks whether she should exercise and whether it will help prevent breast cancer? "I recommend a resounding yes to the first question." Regular physical activity in women reduces overall mortality and the incidence of coronary heart disease, diabetes mellitus, stroke, osteoporosis, obesity, and disability. It also lessens the impact of such chronic ailments as arthritis and cognitive decline. "With respect to whether exercise reduces the risk of breast cancer, however, too many questions remain for women and their doctors to make informed decisions on whether, how, and how much to exercise."

Comment:
The editors of NEJM considered this study provocative enough to make it the lead article. RTJ