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Being more physically active can have a wide range of positive effects on health. Richard Hamilton Smith/Getty Images
  • Researchers analyzed the link between breast cancer and physical activity as well as time spent sitting.
  • They found that higher levels of physical activity and less sedentary time reduce breast cancer risk.
  • They concluded that a greater focus on promoting active lifestyles would likely reduce breast cancer risk.

Physical activity and less sedentary time have been linked to a lower risk for breast cancer in observational studies.

While research suggests a generally consistent link between breast cancer risk and physical activity, the link between sedentary time and breast cancer risk is less clear and less well-studied.

Most studies investigating the link between breast cancer and physical activity or sedentary time have been observational in nature. This means that rather than providing a causal link, they provide a correlation that may be subject to biases.

Recently, researchers analyzed healthcare data from 76 studies to determine whether there is a causal link between activity levels and breast cancer.

They found that greater levels of physical activity and less sedentary time likely reduce breast cancer risk.

“The findings of this new study lend further support for current recommendations to be physically active for health, including lowering breast cancer risk,” said Dr. I-Min Lee, professor of medicine at Harvard Medical School, who was not involved in the study, in conversation with Medical News Today.

“Even modest increases in physical activity at the population level will decrease the number of new breast cancer diagnoses made each year,” Dr. Brigid Lynch, deputy head of the Cancer Epidemiology Division at Cancer Council Victoria, Australia, one of the study’s authors, told MNT.

The study was published in the British Journal of Sports Medicine.

For the study, the researchers analyzed data from 130,957 women of European ancestry from the Breast Cancer Association Consortium (BCAC).

Altogether 69,838 women had invasive breast cancer tumors—those that had spread locally, 6,667 had in-situ tumors that had not spread, and 54,452 did not have breast cancer and were included in the study as controls, for comparison.

The researchers used a statistical method known as Mendelian randomization to assess the relationship between cancer risk and type, and genetic factors linked to physical activity and sedentary behavior.

The researchers found that higher levels of genetically predicted physical activity were linked to a 41% lower risk of invasive breast cancer independent of menopausal status, tumor type, stage, or grade.

They also found that women with genetically predicted vigorous physical activity on three or more days of the week had a 38% lower risk of pre and perimenopausal breast cancer—but not postmenopausal risk—than those who self-reported no vigorous physical activity.

The researchers also found that genetic variants that predisposed women to greater amounts of sitting time were linked to a 104% higher risk of triple-negative break cancer in hormone-negative tumor types.

They noted, however, that evidence linking sedentary behavior to increased breast cancer risk overall was weak.

When asked how a less sedentary lifestyle may reduce breast cancer risk, Dr. Lee said: “High physical activity and low sedentary time can decrease breast cancer risk by decreasing adiposity [fatty tissue in the body], beneficially influencing sex hormone levels, improving metabolic dysfunction, and decreasing inflammation.”

“The main way that physical activity reduces breast cancer risk is by lowering the levels of sex steroid hormones – lower levels of estrogens and androgens are related to a lower risk of breast cancer.”
— Dr. Brigid Lynch

“There has been less certainty about the role of sitting time—observational studies have produced conflicting findings, but this Mendelian randomization study suggests that long periods of sitting increase breast cancer risk. Our research findings warrant a stronger cancer control focus on increasing physical activity and reducing sedentary behavior,” Dr. Lynch added.

The researchers concluded that multiple evidence types now suggest that greater physical activity and less sedentary time likely reduce breast cancer risk.

While the study points toward a causal link between activity levels and breast cancer, it nevertheless has some limitations.

All research methods, including Mendelian randomization, have limitations,” noted Dr. Lynch, “The genetic instruments we used predicted a small percentage of variation in real physical activity levels; however, we would normally expect this weak-instrument bias to drive results towards showing no effect.”

“Because the results of our Mendelian randomization study are similar (albeit stronger) to those reported by observational studies, we can be confident that there is a causal effect between physical activity and breast cancer,” she added.

Dr. Lee pointed out that the study only included a small number of genes linked to physical activity and sedentary behavior and that, there are likely many more.

“However, this does not negate the findings of the research; if we have better knowledge of the genes, the findings may be even stronger in magnitude,” she noted.

Dr. Zeynep Madak-Erdogan, associate professor of nutrition at the University of Illinois Urbana Champaign, who was not involved in the study, also told MNT:

“What is reported in this study are mathematical associations. Further mechanistic studies are needed to definitively show the molecular underpinnings of the relationship between the [genetic instruments] and breast cancer risk. Further, the [genetic instruments] were originally identified in males; association of these with physical activity in females is warranted.”