What we do and don’t know about physical activity and cancer
This is the second in a series of four blog posts that explore key research topics from our recent conference: Obesity, physical activity and cancer.
Kate Wolin, ScD, FACSM is an Associate Professor of Public Health Sciences at Loyola University Chicago. Her research focuses on the role of physical activity and obesity in cancer prevention and post diagnosis outcomes.
Hundreds of studies have shown exercise reduces risk of several common cancers, but we still have much to learn about this important prevention tool.
Data consistently shows individuals who are more active are less likely to be diagnosed with, and die from, many forms of cancer including colon, postmenopausal breast and endometrial. Most of this research has focused on aerobic activity, but evidence is emerging for the benefits of resistance training and the risk of sedentary time, independent of aerobic physical activity participation.
What does the evidence show?
Over 50 studies of physical activity and colon cancer have been published and most found a 20-25% lower risk among the most versus least active. Convincing evidence also shows that physical activity reduces the risk of breast cancer by about 25%. Physical activity also decreases endometrial cancer risk and may lower the risk of advanced prostate cancer. Physical activity has also been associated with decreased risk of cancer mortality in colon, breast and prostate cancers.
30 minutes – a magic number?
I’m often asked when talking to the non-scientific community what we really mean when we tell people to get 30 minutes of physical activity. What people are asking is a good question. If I only get 20 minutes does it matter? What if I do 60? Do I need to run or does my gardening count? I don’t think we have done enough as a research community to answer these questions.
We DO know that where physical activity reduces cancer risk, we have a dose response effect. That means that as the amount of exercise goes up, the risk goes down.
However, in different studies and for different cancers, the amount of activity where that risk reduction becomes statistically significant changes. Some studies, including one that I led, indicate that walking is sufficient to reduce risk of colon cancer, but not all studies say that.
On average, more is better and risk seems to go down with about 30 minutes of moderate intensity activity, like walking. But that number isn’t magic. You may get some risk reduction with less, but you also get greater prevention benefit from more.
Research is increasingly focused on teasing out some of the nuances of these associations. One area of interest for numerous cancer risk factors is the timing of exposure. Regular participation in activity over the life course is associated with a reduction in the risk of breast cancer. Because we know there are several key hormone associated events (menarche, menopause) connected with breast cancer risk, research over recent years has examined whether activity at key time points is differentially important. The evidence indicates that activity at ALL time points matters, but that consistent activity across the life course confers the greatest risk reduction.
Combined, this timing-centered data has important policy implications, particularly in light of the declining activity rates of US children and the cuts to school based physical education classes and recess time. Cancer prevention is not just for adults, but matters across our entire lifetime.
Do we all benefit equally?
Healthy lifestyle behaviors do cluster together, but physical activity is not merely a marker of healthier lifestyle, it exerts an independent protective effect.
Many of the studies that report an association between physical activity and cancer have statistically controlled for other lifestyle factors (e.g., body weight, alcohol intake, diet) that may be associated with physical activity.
In addition to knowing physical activity has effects independent of weight, it is important to distinguish whether the effects of physical activity are different for population subgroups. The effect of physical activity on colon cancer risk is robust, persisting for men and women and regardless of weight. In contrast, physical activity appears to be more protective of cancer risk in leaner women than obese. But physical activity helps all women, regardless of weight, reduce their risk of breast cancer related death.
The bottom line
Being unclear about what we don’t know doesn’t undermine our ability to deliver the message of what we do.
We do know that physical activity has many health benefits, including reducing the risk of some cancers. But we still have much to learn and improve, including our ability to measure what people are doing in our studies, both in their active and sedentary time. We are also working to better understand the mechanisms behind these associations, which may help us with our questions about the amount or type of activity that confers the most benefit or when in life activity is most important for each cancer.
As this research develops, we shouldn’t sit around waiting. We do know enough to argue strongly for being active!