Sitting on the couch or behind a desk could be increasing your risk of breast cancer, a new genetics-driven study suggests.
People more likely to engage in physical activity based on their DNA had a 41% lower risk of invasive breast cancer, researchers report.
Previous research also has shown a link between exercise and reduced cancer risk, but "our study suggests that the strength of the relationship may be even stronger than suggested by observational studies," said senior researcher Brigid Lynch, deputy head of cancer epidemiology for Cancer Council Victoria, in Melbourne, Australia.
"Our study also suggests that sedentary behavior may increase the risk of breast cancer," Lynch continued. "The risk increase is greater for receptor-negative tumors, including triple-negative breast cancer — a more aggressive type of breast cancer with a poorer prognosis than other types."
For this study, the Australian researchers performed a sophisticated genetic analysis of nearly 131,000 women from around the world, including nearly 70,000 who had been diagnosed with invasive breast cancer.
Previous research has identified genetic variants that are linked to a person's overall predisposition to work out at all, engage in vigorous exercise or sit around all day, the study authors said.
The researchers applied these known variants to their international sample of women, to see if a genetic inclination for either physical activity or sedentary behavior would influence cancer risk.
Younger women whose genes would typically drive them to work out three or more days a week appear to have a 38% lower risk of breast cancer, the investigators found.
On the other hand, women genetically predisposed to be sedentary were 77% more likely to develop hormone receptor-negative breast cancers.
"The results of our study suggest that reducing the overall duration of sitting time is key," Lynch said. "For women with desk jobs, try taking walking breaks throughout the day – don't eat lunch at your desk, go out for a half hour walk instead."
The findings were published Sept. 6 in the British Journal of Sports Medicine.
Using genetics to judge a person's expected physical activity levels is "a little controversial," but these results jibe with previous studies that have tied exercise to cancer risk using self-reported behavior or wearable trackers that monitored how much people moved, said Dr. Jennifer Ligibel, an expert with the American Society of Clinical Oncology.
"Whether or not this provides a higher level of evidence than actually looking at what people do in terms of their activity and how that's related to cancer, I think is the source of maybe a little debate," said Ligibel, an oncologist with Dana-Farber Cancer Institute in Boston. "We already have a fair amount of research that has shown sedentary behavior is a cancer risk factor, and this verifies that using a different way of looking at the relationship."
However, a genetics-driven study like this "raises interesting scientific questions for next steps," said Karen Knudsen, chief executive officer at the American Cancer Society.
"What is it about those genetic alterations that are associated with changes in physical activity and reduced cancer risk?" Knudsen said "What are these variations that were identified? How do they affect metabolic programming of the individual? I think these are important next-step questions."
There are many different theoretical means by which exercise could help ward off cancer, Lynch and Ligibel said.
For example, physical activity decreases the level of circulating sex hormones like estrogen, which "increase the risk of developing breast cancer, particularly in postmenopausal women," Lynch said.
Exercise also suppresses inflammation, enhances the immune system, and lowers insulin levels and other growth factors associated with cancer, Ligibel said.
The American Cancer Society recommends that adults get 150 to 300 minutes of moderate intensity physical activity or 75 to 150 minutes of vigorous exercise each week.
Not only does exercise protect against many different types of cancer, but "emerging data suggests that physical activity will reduce the risk of development of aggressive disease," Knudsen noted.
This study showed some cancer risk benefit with just 50 minutes of moderate activity each week, Lynch said.
"We also found benefits for engaging in vigorous activity more than 10 minutes at a time, at least three times per week," Lynch said.