A new study from the National Cancer Institute (NCI) shows that physical activity is associated with lower breast and colon cancer mortality rates.
The study, led by Rachel Ballard-Barbash, MD, Applied Research Program at the Division of Cancer Control and Population Sciences at the NCI, analyzed published observational and randomized controlled studies on the link between mortality, cancer biomarkers, and physical activity among cancer survivors.
Overall, all evidence points to the benefit of exercise in terms of quality of life as well as actual mortality outcomes. Still, the evidence thus far is suggestive but not proven.
The 45 studies analyzed were published between 1950 and 2011, with the majority from 2005 and later. Of those analyzed, 27 observational studies showed evidence that physical activity is linked to a reduced all-cause, breast cancer, and colon cancer mortality; and 11 randomized studies analyzing specific biomarkers suggest that exercise can benefit the insulin levels of survivors, reduce inflammation, and could even improve immune function.
Breast Cancer Studies
There were 17 observational studies that tracked breast cancer survivors, generally women who were diagnosed with invasive nonmetastatic disease. Leisurely physical activity was the primary focus of most of the research. Four studies showed no link of physical activity with breast cancer mortality while seven showed non–statistically significant decreased risks, ranging from 13% to 51% in relative risk reduction—six studies reported risk reduction between 41% and 51%.
Seven of the observational studies reported statistically significant reduction in all-cause death and five showed a non–statistically significant reduction. “There is some evidence suggesting a dose–response effect of increasing risk reduction with increasing activity levels,” the authors report, however there is still a general inconsistency in controlling for confounding factors such as stage of breast cancer, body mass index (BMI), and co-morbidities.
The Health, Eating, Activity, and Lifestyle (HEAL) observational study—the only observational study to analyze biomarkers—followed 933 women with breast cancer for a median of 6 years and found an inverse relationship between exercise and circulating levels of inflammatory markers and insulin-like growth factor 1 (IGF-1).
Almost all of the breast cancer studies showed a benefit of physical activity on disease and all-cause mortality, and a dose-dependent decrease in mortality risk was seen in almost half of the studies.
“The majority of the data out there is on breast cancer and colorectal cancer because these cancers are two of the four most common cancers in the United States. In part the amount of funding reflects the prevalence of a particular cancer,” explained Dr. Ballard-Barbash.
Physical Activity Interventions—Controlled, Randomized Trial Results
Five randomized trials studied breast cancer survivors, three studied prostate cancer survivors, two studied colorectal cancer survivors, and one analyzed gastric cancer survivors. None analyzed more than 155 patients and most were short-term studies. One breast cancer study suggested that the effect physical activity had on insulin levels was more pronounced for obese and sedentary women, than for active women.
Evidence for the benefit of exercise for colorectal cancer survivors has led to the ongoing Colon Health and Life Long Exercise Change (CHALLENGE) Trial that is enrolling 973 survivors of stage II and III colon cancer. The trial consists of 3 years of supervised physical activity and has disease-free survival as the primary end point.
There are many more studies looking at both breast and colon cancer survivorship and currently, there is insufficient evidence for physical activity and mortality links for other types of cancer, according to the study authors.
“This is a very rapidly growing area of research,” said Dr. Ballard-Barbash. “We analyzed thirteen randomized trials and one observational trial that actually looked at biological mechanisms. These studies suggested that biomarker changes such as insulin, and inflammatory markers are beneficial in terms of cancer survival.” Ballard-Barbash highlighted that because this is still a young field, much more work needs to be done—the cause-and-effect relationship of physical activity and improvement in cancer survival is necessary before clinicians recommend an exercise plan based on prospective study results. Future randomized trials should examine different exercise types as well as how BMI, weight loss, and cancer treatments affect biomarker readouts.
“One area that may be important is the effect of angiogenesis and how physical activity can modulate blood supply to tumors. This should be explored further,” said Dr. Ballard-Barbash.
As cancer treatments and techniques for catching cancer earlier continue to improve, the estimated 13.8 million cancer survivors in the United States as of 2010 is projected to be 18.1 million by the year 2020. More studies to better understand how to impact the quality of life of these survivors and prevent disease recurrence are needed. “We need larger studies with bigger sample sizes,” said Colleen Doyle, dietitian and director of nutrition and physical activity at the American Cancer Society, who was not involved with the current study. “Much of the research that we have right now includes relatively small sample sizes. We also need more research in other types of cancer survivors and to determine what the optimal intensity, doses, and type of exercise is, to be able to give more specific recommendations to survivors.”
All evidence thus far does point to the safety and physical and mental benefits of exercise for survivors. The American College of Sports Medicine (ACSM) published a review in 2010 on the evidence for the benefits of exercise for cancer survivors, but with limited prospective cohort data. This older review indicated that exercise is safe during and after cancer treatment with benefit for physical functioning, quality of life, and cancer-related fatigue—consistent with the newer review published today.
Edward L. Giovannucci, MD, ScD, of the department of nutrition at the Harvard School of Public Health, commenting in an editorial that accompanies the publication of the review, stated that although the direct effects of physical activity is not yet “definitively proven,” because physical activity is generally safe and improves quality of life and has extensive overall health benefits, “adequate physical activity should be a standard part of cancer care.”
“Part of the mechanism by which physical activity is believed to play a role in both prevention and survivorship is related to its impact on hormones—both directly, and indirectly, by impacting weight,” said Doyle.
“I am not inclined to wait to encourage all cancer survivors to be as active as possible. We do know that being active helps reduce the risk of a variety of other types of primary cancers, and we also know it’s good to reduce the risk of heart disease, diabetes, and other chronic conditions that many survivors may also experience,” said Doyle who also pointed out that exercise helps to reduce stress, anxiety, and boosts confidence.