Physical Activity Reduces the Risk of Breast Cancer.

Background: Lifestyle has been shown to significantly effect breast cancer incidence evident in the increased risk associated with obesity, diabetes, cigarettes and alcohol. The aim of this meta-analysis was to investigate the effect of two traditionally "beneficial" lifestyle choices: physical activity and weight loss. Specifically, we aimed to investigate the effect of the timing, intensity and duration of exercise on breast cancer incidence in women classified as either normal risk or increased risk. Finally, we aimed to investigate the effect of weight loss on breast cancer incidence. Method: Studies were obtained from a database search of MEDLINE, EMBASE, PubMed, Current Contents Connect and Google Scholar. Databases were searched using subject headings, keyword searches and text word searches wherever possible using the search terms "Exercise" OR "weight loss" AND "breast cancer" OR "breast carcinoma" OR "breast neoplasm". Inclusion criteria required a confirmed diagnosis of breast cancer, reporting of an odds ratio or data to calculate an odds ratio (and 95% confidence interval) and the use of an internal control group as the comparator. Exercise was catagorised as "high intensity" if it was vigorous exercise likely to cause sweating (ie. Running, competitive sport) while lower impact activities such as walking and golf were categorized as "low intensity". The effect of the timing on breast cancer incidence was quantified across three time periods: adolescence, adulthood and post menopause. Collated data was assessed for heterogeneity and a pooled odds ratio calculated. Results: 111 studies were identified in the literature search of which 76 were included in the meta-analysis. Overall, both exercise and weight loss were associated with a significantly reduced risk of breast cancer in women (OR 0.77, 95% CI 0.72-0.81 and OR 0.74, 95% CI 0.60-0.93 respectively). Heterogeneity was high (I²=77.12) and publication bias was significant (p < 0.001). Subgrouping by menopausal status at diagnosis of breast cancer found an increased reduction in the incidence of postmenopausal breast cancer when compared with premenopausal breast cancer and exercise (OR 0.68, 95% CI 0.60-0.78 and OR 0.81, 95% CI 0.71-0.92 respectively). The protection offered by exercise was not effected by the intensity of the exercise evident in a pooled odds ratio of 0.78 (95%CI 0.70-0.86) for low intensity exercise and 0.77 (95%CI 0.69-0.85) for high intensity. A similar consistency in the level of protection was present in regards to the timing of the physical activity with regular exercise during adolescence, adulthood and post menopause associated with odds ratios of 0.79 (95% CI 0.72-0.88), 0.84 (95% CI 0.77-0.92) and 0.82 (95% CI 0.75-0.89) respectively. In regards to the subpopulation at an increased risk of breast cancer, exercise significantly reduced the incidence of breast cancer in overweight individuals (OR 0.80, 95% CI 0.68-0.94) however the reduced risk in women with a positive family history did not reach statistical significance (OR 0.78, 95% CI 0.60-1.00). Conclusion: Breast cancer incidence is significantly reduced by both physical exercise and weight loss, especially in the postmenopausal population. Intensity, timing and nature of the exercise did not significantly alter the protective effect. [ABSTRACT FROM AUTHOR]