Vigorous and even moderate exercise may affect the menstrual cycle, perhaps by suppressing gonadotropin-releasing hormone and lowering a woman's cumulative exposure to estrogen and progesterone. Energy balance and caloric restriction might also inhibit carcinogenesis. Thune and colleagues conducted a survey to investigate the relationship between everyday exercise and the risk of breast cancer in women.

Norwegian women from 35 to 49 years of age and a random sample of 10 percent of women from 20 to 34 years of age in three counties were invited to participate in the study. A total of 25,624 women were surveyed from 1974 to 1978 and from 1977 to 1983. All of these women were asked to complete a questionnaire and bring it with them when they attended a screening clinical examination. Participants were also asked to complete a food frequency questionnaire that was used to calculate energy and fat intake.

The women were asked to self-report their physical activity during leisure and work hours using a scale of 1 to 4. A grade of 1 was assigned to those whose leisure time was spent reading, watching television or in other sedentary activities and to those whose work was described as sedentary. A grade of 2 was given to women who spent at least four hours per week walking, bicycling or engaging in other types of physical activity or had jobs that involved a great deal of walking. A grade of 3 was given to women who spent at least four hours a week exercising to keep fit and participating in recreational activities, and whose jobs involved both walking and lifting. A grade of 4 was given to women who engaged in regular, vigorous training or participated in competitive sports several times a week or whose jobs required heavy manual labor.

Women were followed for a mean of 14 years. Women who developed cancer or who died within the first year of the study were excluded from the analyses. Breast cancer was diagnosed in 351 women. Two thirds of women reported participating in moderate activity during leisure time. Fifteen percent exercised regularly. Only 14 percent reported being sedentary at work. Energy intake was positively correlated with physical activity. The association was more pronounced with work activity than with leisure time activity.

After adjustment for the factors of age, body mass index, height, parity and county, greater activity during leisure time was associated with a reduced risk of breast cancer. In women who exercised regularly, the risk reduction was greater in premenopausal women and in women younger than 45 years of age. Women who exercised at least four hours a week during their leisure time had a 37 percent reduction in the risk of breast cancer. The risk of breast cancer was lowest in lean women (body mass index less than 22.8) who exercised at least four hours per week. Risk was also reduced in women with higher levels of activity at work, again with a more pronounced effect among premenopausal than postmenopausal women.

The authors conclude that physical activity during both leisure time and work reduced the overall risk of breast cancer in women, particularly among premenopausal and younger postmenopausal women.