The researchers -- from the University of Pennsylvania and the University of Minnesota -- report in the December issue of Cancer Epidemiology Biomarkers & Prevention that a high level of physical activity in women who smoked reduced their relative risk of developing lung cancer by 72 percent. Moderate activity among smokers was associated with a 65 percent risk reduction, and lower relative risks were also seen in former smokers who had moderate or high activity levels.
While this may sound like welcome news to female smokers who don't want to quit, the investigators emphasize that the absolute risk of developing lung cancer is still much greater in current and former smokers regardless of activity level.
"The most important thing a smoker can do to reduce risk is to quit smoking. That said, exercising and being active can offer a marginal change in risk," according to the study's lead author, Kathryn Schmitz, Ph.D., an assistant professor at Penn.
In other words, she said that a physically active smoker has a 35 percent lower risk of lung cancer than a sedentary smoker, but if both smokers quit, they would both reduce their risk by as much as 10- or 11-fold.
It Pays to Exercise
"The helpful message from this study is that if a smoker is having trouble quitting, exercise can be a first step toward better health," said Schmitz.
The findings were derived from the Iowa Women's Health Study, which in 1986 began to follow almost 42,000 women between the ages of 55 and 69. Over the years, five questionnaires were sent to the participants, who recorded their smoking status and physical activity, among other variables.
This analysis, which began in 2002, included 36,410 participants, including 777 women diagnosed with the cancer. Among this group, 125 were nonsmokers, 177 were former smokers, and 475 were current smokers.
Compared to women who were physically active, women with a low physical activity level at the baseline analysis were more likely to smoke, less likely to have a high-school education and more likely to be obese.
Among smokers, the most number of cancer cases (324) were seen among women who currently smoked and had low activity, and the lowest number (40) was in the group of women who formerly smoked and were highly active. Compared to never smokers, current and former smokers had proportionally more squamous cell and small-cell lung cancer, which can be harder to treat than other subtypes.
Researchers don't know why activity could lower lung cancer risk, but suggest that improved pulmonary function may reduce both the concentration of carcinogenic particles in the smoker's airway and the extent to which they are deposited in the lungs. They also theorize that exercise training improves immune function and reduces the inflammatory responses that can impact cancer development.
"This may be useful information for smokers who are not currently willing to attempt smoking cessation or have recently failed a quit attempt," explained Schmitz. "But even if there was a significant risk reduction, quitting smoking is unarguably the most important action a person can take for reduction of lung-cancer risk."