Friedenreich et al. (2001)

Associations between physical-activity and breast cancer has been reported before. Previous studies didn’t examine the risk of breast cancer for all types of physical activity (occupational, household, and recreational) for entire lifetimes, assessing all parameters of physical activity. The purpose of this study was to address the unanswered questions regarding the type and dose of activity and the time periods in life when physical activity may be specifically associated with breast cancer risk.

Evidence supporting an association between physical activity and breast cancer is growing. The present study was designed to address some of the unanswered questions regarding the type and dose of activity and the time periods in life when physical activity may be specifically associated with breast cancer risk. Few of the previous studies have examined the relation between physical activity and breast cancer among subgroups such as post-menopause, which can impact the the risk of breast cancer.

Evidence for the protective effect of physical activity from breast cancer isn’t always conclusive. However, inconsistencies across studies can often be attributed to errors in measurement of physical activity, uncontrolled confounding, and inaccuracies in assessing the effect modification. Thus far, 23 of 35 studies conducted demonstrated a reduction in breast cancer risk for those women who were most active. Studies that had more complete, reliable assessments of physical activity appeared to have more evidence of the effect of physical activity on breast cancer risk.

The study was a population-based case-controlled study conducted in Alberta, Canada between August 1995 and August 1997. Cases were eligible for the study if they were Alberta residents, no older than age 80 years, English speaking, and able to complete an interview. Those that were found ineligible were due to physician refusal, language barrier, having a disconnected telephone number, unavailability during the study period, or death. In the end, 1,233 women with breast cancer were deemed eligible and included in the study. 1,237 controls were included in the study. They were gathered through random digit dialing using the Mitofsky-Waksberg method. The Mitofsky-Waksberg technique is easy to administer and statistically flawless. However, the drawback can be that fewer than 25% of the phone numbers in a sample will be residential.

In-person interviews were conducted; respondents reported their menstrual and reproductive history, hormone use history, mammography history, personal history of breast disease, breast biopsies and cancer, family history of cancer, lifetime physical activity patterns, dietary intake during the reference year, alcohol consumption, smoking habits, demographic characteristics, and past anthropometric measurements. All data collected was current up until diagnosis and controls were assigned a comparable date.

Total life-time activity was a strong predictor of reductions in breast cancer risk for postmenopausal women and the risk was greatest declined by occupational and household activity. Pre-menopausal women did not have the same reductions in breast cancer risk from total life-time activity. Other factors reduced the risk of breast cancer, including post-menopausal women who were nondrinkers, nonsmokers, and nulliparous.

Limitations to the study include selection bias, as controls were selected to match the respondents representing breast cancer. Consistency is also an issue for this study because the findings are both similar and dissimilar to other studies. For example, some studies have found larger risk reductions for premenopausal women or postmenopausal women, while others have observed no differences in the level of risk reductions by menopausal status or by age. This is inconsistent with the findings of this study which showed a greater reduction of risk in post-menopausal women but little to no comparable difference in pre-menopausal women. Therefore, this study seemingly adds to the confusion on the topic and skews the results further. The data on the influence of parity and physical activity on breast cancer is also inconsistent. Weight gain over adulthood did not influence risk for the postmenopausal women participating in Nurse’s Health Study II. This devaluates the hypothesis that physical activity that reduces postmenopausal body weight, controls weight gain over the lifetime, and decreases abdominal adiposity will reduce the risk of breast cancer risk. The consistency of the findings both reject the statement that weight gain or weight maintenance increased risk of breast caner and support it. The study was mixed with several conclusions that were not definite. The four main hypotheses that may be operative regarding the association between physical activity and breast cancer seem to be inconclusive as well. Overall, the study confirms that physical activity reduces the risk of breast cancer. Why this occurs is less obvious, and the conditions that are associated with reduced risk of breast cancer are also hard to pinpoint. Although the risk reduction was notable for occupational and household activity for postmenopausal women, and for nondrinkers and nonsmokers, the study still has conflicting evidence from other studies and within itself.
Further research has to be done on the effects and correlation between these factors and the risk of breast cancer. The causation isn’t strong enough to make definite statements about the possible link between physical activity and breast cancer risk reduction. In the future it is important that more research is conducted on the physiology of exercise and its effects on the risk of breast cancer, a long with the physiology of age in order to make stronger connections about their roles in the prevention of disease.

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