The citation: Lahmann PH, Friedenreich C, Schuit AJ, Salvini S, Allen NE, Key TJ, Khaw KT, Bingham S, Peeters PH, Monninkhof E, Bueno-de-Mesquita HB, Wirfalt E, Manjer J, Gonzales CA, Ardanaz E, Amiano P, Quiros JR, Navarro C, Martinez C, Berrino F, Palli D, Tumino R, Panico S, Vineis P, Trichopoulou A, Bamia C, Trichopoulos D, Boeing H, Schulz M, Linseisen J, Chang-Claude J, Chapelon FC, Fournier A, Boutron-Ruault MC, Tjonneland A, Johnson NF, Overvad K, Kaaks R, Riboli E. Physical Activity and Breast Cancer Risk: The European Prospective Investigation into Cancer and Nutrition. Cancer Epidemiol Biomarkers Prev. 2006 Dec 19; [Epub ahead of print]
The abstract is online, but you can also download the PDF for free.
Let's take a look at the details, since you're unlikely to get those in the media reports of "Ladies! Do more housework! Forget about your careers! It's good for you, we swear!"
Who was included: Women from several European countries. Most were invited from the "general population" (although it's not clear how or where they were recruited) and were 25-70 years old.
Who was excluded: Women with any kind of existing cancer, missing questionnaire data, or on the extreme ends of energy intake, or who were surgically menopausal.
How the study was done: Women completed questionnaires (in person or self-administered) about food and lifestile. "Anthropometric" measurements were obtained at enrollment in the study. The women were followed to see who got breast cancer, via the countries' population cancer registries, or by active follow-up (health insurance claims, contact with woman or next of kin, cancer/pathology registries). Women were followed from the time they entered the study until their first breast cancer diagnosis, death, emigration, or end of the follow-up period.
What the questionnaires asked about: current occupational activity, employment status, and level of physical activity done at work; frequency and duration of nonoccupational physical activity during the past year, including housework, home repair, gardening, stair climbing, and recreational activities such as walking or cycling; vigorous physical activity; reproductive, sociodemographic, and lifestyle characteristics; age at menarche; age at first pregnancy; education; smoking status; alsohol consumption, BMI; oral contraceptive and hormone replacement therapy use. According to the authors, "Housework, home repair, gardening, and stair climbing were combined to obtain an overall estimate of household activity. Walking (including walking to work, shopping, and leisure time), cycling (including cycling to work, shopping, and leisure time), and sports activities were combined to derive overall recreational activity. Because the intensity of recreational and household activities was not directly recorded, a MET value was assigned to each reported activity according to the Compendium of Physical Activities."
Findings: Data was analyzed for 218,169 women, ages 20-80, followed for an average period of 6.4 years.
For total physical activity (i.e., combined occupational, recreational, and household activities), increasing activity level was associated with an overall decrease in risk of breast cancer in all women. Stronger and significant risk reductions became apparent for combined recreational and household activities and for household activity alone. Among postmenopausal women, compared with women in the lowest quartile of combined recreational and household activities, women in the top quartile had a 17% reduced risk of breast cancer after adjusting for multiple covariates. A similar risk reduction was observed in premenopausal women, but none of the individual categorical risk estimates reached statistical significance when adjusted for other risk factors. Household activity on its own was significantly inversely related to breast cancer risk in premenopausal and postmenopausal women; the HRs for the highest versus the lowest quartile of household activity were 0.71 (95% CI, 0.55-0.90) for premenopausal women and 0.81 (95% CI, 0.70-0.93) for postmenopausal women. Recreational activity alone was not significantly associated with risk.Keep in mind that "household activity" was defined not just as sweeping and dusting, but also to include home repair, stair climbing, gardening, etc. What the authors are asserting here is that postmenopausal women with the most household activity (among four levels) had a lower risk of breast cancer than those in the group with the least amount of household activity. They are also saying that this difference was not found to be significant in premenopausal women.
The individual activities, housework, home repair, gardening, stair climbing, walking, cycling, and sports activities, in MET-hours per week, were each inversely associated with breast cancer risk with nonsignificant trends, except for housework (Ptrend = 0.002, premenopausal women; Ptrend = 0.016, postmenopausal women) and sports activities (Ptrend = 0.01, postmenopausal women). Housework was the predominant component of household activity. On average, premenopausal women spent a mean (SD) of 17.7 (14.3) h and postmenopausal women 16.1 (13.2) h on housework chores. Vigorous activity, defined as MET-hours per week, was not significantly associated with breast cancer risk in either of the menopausal groups. Overall, <40% of all women engaged in vigorous activity (data not shown).What this tells you is that of the types of household activity, only housework and sports were associated with reduction in breast cancer risk. Vigorous activity was not associated with a decreased risk. Housework made up the biggest part of the household activity.
Limitations of the study: The researchers only asked women about their activity levels for the past year compared to whether they got breast cancer in some relatively short-term later year. Not only is this subject to women's recollection or reporting of the events, it means that the authors cannot make statements about physical activity over a lifetime or longer period of time and breast cancer risk. The authors also report that studies in the U.S. and China that examined hours of household activity in a lifetime physical assessment found no association with breast cancer risk. In the French group of this study, women with 14 or greater weekly hours of light household activity did not have a significant decrease in breast cancer risk.
Overall: In general, the authors are finding that regular non-intense physical activity may reduce risk of breast cancer. The authors specifically state that their findings require further confirmation, and only go so far as to say that "increasing physical activity reduces breast cancer risk." They do not suggest that work outside the home is bad for women, but state that moderate types of activities may benefit middle-aged and older women.
The Bottom Line: As often occurs, the hype over this study is not warranted given its actual conclusions. The women in this study were doing an average of 16-17 hours of housework per week, and it is likely to be the regularity of activity and intensity of activity that confers the benefit, rather than the location or purpose of the activity.
Becky J sent me this post from one of the Nature blogs, Spoonful of Medicine, "Housework's suspicious new benefit," which makes the point, "Apparently, the women who showed this benefit did at least 16-17 hours a week of housework. Well, that probably explains it then: the poor things were probably so busy cooking and cleaning that they never got a chance to go out and drink alcohol, eat unhealthy or skip going to the gym because that new Daniel Craig movie opened and they just had to see it. Seems to me that being a hardworking housewife may go along with some other good habits that just might explain the extra benefit."
Tennessee Guerilla Women has a roundup of the interesting and obnoxious comments found on other blogs in response to the study.
Technorati Tags: breast cancer; chores; exercise; housework; risk
MeSH Tags: Breast Neooplasms/Epidemiology; Exercise; Exertion; Risk; Risk Reduction Behavior
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