For people diagnosed with cancer, the risk of cancer death falls as physical activity rises, according to a new analysis of more than 70 existing studies.
Researchers found the same holds true for everyone - supporting the current World Health Organization recommendation of moderate physical activity to combat the risk of chronic disease, they write in the British Journal of Sports Medicine.
The WHO recommends two and a half hours of moderate exercise per week for some health benefit and five hours of moderate exercise per week for additional benefit. Half as much time per week of vigorous physical activity, like running, may confer the same benefits.
There are no specific recommendations for physical activity levels to combat cancer risk, although more activity has been tied to lower risk of death from breast, colorectal and prostate cancers, the authors note.
"Our results might help to update the recommendation concerning the advisable amount of physical activity to reduce cancer mortality," said senior author Dr. Li Liu of Huazhong University of Science and Technology in China.
Doctors could start to incorporate physical activity into cancer treatments, Liu told Reuters Health by email.
The researchers included 71 studies of physical activity and cancer death risk in the general population or among cancer survivors.
When they pooled these results, people in the general population who got at least two and half hours of moderate activity like brisk walking, per week, were 13 percent less likely to die from cancer than those with the lowest activity levels.
They also looked at data in terms of MET-hours, a measure of the relative amounts of energy expended in given activities and time spent doing them. Resting represents 1 MET, while a 4-MET activity like brisk walking uses four times as much energy, according to the U.S. Office of Disease Prevention and Health Promotion. Doing a 4-MET activity for 30 minutes equals 2 MET-hours.
Cancer survivors who completed at least 15 MET hours per week of physical activity were 27 percent less likely to die from cancer.
Exercise after cancer diagnosis reduced cancer death risk more than prediagnosis exercise, the study team notes.
Exercise may change the body's response to cancer, and those who exercise more may live healthier lifestyles in other ways as well, Liu said.
But many of the high-quality studies included in this analysis accounted for other healthy-lifestyle factors that may have played a role, Liu noted.
"Physical activity, mostly before diagnosis, and breast cancer mortality has been studied for decades, but only in the last 10 years or so have we been studying physical activity after diagnosis," said Patrick T. Bradshaw of the University of California, Berkeley, who was not part of the new study.
"Other cancers (e.g. colorectal, ovarian) have been studied much less than breast cancer, but some researchers there have also found a reduction in mortality associated with increasing physical activity levels," Bradshaw told Reuters Health by email.
So far, most studies have not been able to address which types of physical activity are most beneficial, he said.
Leisure time physical activity or recreational physical activity, but not occupational activity, is protective against cancer according to most research, Liu said.
"The take-home message here is encouraging - exercise may be beneficial even if started after diagnosis," Bradshaw said.
"Based on huge evidence of the inverse association between physical activity and cancer mortality, there is no doubt that cancer patients should be physically active," Liu said. "We suggest that cancer patients to consult their doctors about a personalized physical activity plan, including exercise time, exercise frequency, exercise mode and so on, which may help to promote the survival of patients without bringing too much physical burden."