Greater physical activity, less sedentary time may protect against breast cancer

Being more active and spending less time sedentary reduced women’s risk for breast cancer, according to data published in the British Journal of Sports Medicine.

“Increasing physical activity and reducing sedentary time are already recommended for cancer prevention,” Suzanne C. Dixon-Suen, PhD, an epidemiologist at Deakin University and Cancer Council Victoria in Australia, and colleagues wrote. “Our study adds further evidence that such behavioral changes are likely to lower the incidence of future breast cancer rates.”

According to Brigid M. Lynch, PhD, deputy head of cancer epidemiology at Cancer Council Victoria and joint senior author of the study, the connection between physical activity and breast cancer lies in sex steroid hormones.

“Physical activity decreases the levels of circulating estrogens and androgens,” Lynch told Healio. “These sex hormones increase the risk of developing breast cancer, particularly in postmenopausal women.”

“Inflammatory pathways may also be involved,” Lynch added. “We know that long-term exercise reduces circulating inflammatory markers, and at least one of these — C-reactive protein — is inversely related to breast cancer risk.”

Data analysis

Dixon-Suen, Lynch and colleagues performed a Mendelian randomization using data from the Breast Cancer Association Consortium (BCAC) and the U.K. Biobank. They used the BCAC to identify 130,957 women in 76 studies, 69,838 of whom had invasive breast cancers, 6,667 of whom had in situ breast cancers and 54,452 of whom did not have breast cancer.

Data from the U.K. Biobank — comprising 91 genome-wide association studies — consisted of overall physical activity, vigorous physical activity and sedentary time measured by wrist-worn trackers. These data were associated with certain single nucleotide polymorphisms (SNPs) that genetically predicted physical activity and sedentary behavior.

The researchers predicted BCAC participants’ activity levels using their SNP data, and the SNP data were then used to evaluate the risk for breast cancer.

Correlation between activity, breast cancer

Results showed that greater overall physical activity reduced the risk for all invasive breast cancers (OR = 0.48; 95% CI, 0.3-0.78). Correcting for outliers, the researchers determined the odds ratio for all invasive breast cancers was 0.59 (95% CI, 0.42-0.83).

Vigorous physical activity was associated with a lower risk for pre- and peri-menopausal breast cancer (OR = 0.62; 95% CI, 0.45-0.87). However, there was little protective effect of vigorous activity against postmenopausal breast cancer risk (OR = 0.95; 95% CI, 0.75-1.19).

On the other hand, spending more time sedentary was associated with a greater risk for hormone-receptor-negative breast cancer (OR = 1.77; 95% CI, 1.07-2.92) — although “there was no strong evidence of differences in association by subtypes and weak evidence of an increased risk overall,” the researchers explained — as well as a greater risk for triple-negative cancers (OR = 2.04; 95% CI, 1.06-3.93). More sedentary time was also associated with greater risk for in situ cancers (OR = 1.75; 95% CI, 1-3.07).

Additionally, the researchers acknowledged that the findings could have arisen by chance — given the study design — but the results “are consistent with observational studies, which have suggested a 20% to 25% breast cancer risk reduction for the most versus least active women, with evidence of dose-response.”

Dixon-Suen and colleagues emphasized that “a stronger cancer-control focus on physical activity and sedentary time as modifiable cancer risk factors is warranted, given the heavy burden of disease attributed to the most common cancer in women.”