Current and former heavy smokers who increase their physical activity levels may lower their risk of getting cancer or dying from the disease according to a new study by researchers in the Public Health Sciences Division.
Dr. Catherine Alfano, a postdoctoral research fellow working in the Cancer Prevention Program, led the analysis of 7,045 current and former smokers. Results of the study were published in the December 2004 issue of the journal Cancer Epidemiology, Biomarkers & Prevention.
The cohort was drawn from a subset of the NCI-funded Carotene and Retinol Efficacy Trial (CARET), a large study of smokers and asbestos-exposed workers. The CARET study was halted early in 1996 after the intervention — dietary supplements beta-carotene and retinol (vitamin A) — was associated with increased lung-cancer risk. Fred Hutchinson was one of the CARET study sites.
The participants in the current study, who had smoked for an average of 49 pack-years (a tally of the number of cigarettes smoked and number of years spent smoking), were 54 to 78 years old. They reported averaging more than 35 hours per week in moderate to vigorous activities and more than eight hours per week in vigorous activities. Moderate activities included housework, yard work, regular walking, light repair work or carpentry and light sports. Strenuous sports, brisk walking, jogging, chopping wood and digging in the garden were among the activities classified as vigorous.
Alfano and colleagues looked at the effect of physical activity on both cancer incidence (risk) and mortality (death). They found that age influenced the impact of physical activity on the participants' risk of getting or dying of any cancer and specifically, lung cancer. There was a 25 percent reduction in cancer deaths in the 54- to 62-year-old smokers. There was also a 16 percent reduction in the risk of getting lung cancer for this age group. For participants over age 63, activity levels did not offset cancer risk or mortality.
The researchers also detected gender differences in the findings. Male smokers who increased their physical activity levels reduced their risk of getting any cancer by 14 percent, but there was no such effect for women. However, active female smokers lowered their risk of death from any cancer and from lung cancer by 32 percent and 31 percent, respectively. Activity levels did not lower the cancer mortality rate for men.
Alfano provided several explanations for the incongruous results for different ages and genders in the study. "The reason we didn't find an effect on risk for women could be because we didn't have enough women who actually developed cancer," she said. "You need a very large number of people to find an effect on risk."
"It's not clear why we found the reduction in cancer mortality only among women. It could be due to the types of cancer women commonly get or how cancer, smoking and physical activity act in the body for women versus men. For the age-related results, this study suggests that physical activity may not be enough to counteract the extremely detrimental effects of many years of smoking."
The study adds to growing evidence that exercise is helpful in reducing risk and mortality from cancer, even among heavy smokers. The findings are consistent with the results of a larger cohort study that found total physical activity to be significantly associated with incidence of any cancer among men, but not among women.
Aside from lung cancer, the most common type of cancer diagnosis in women in the study was breast cancer. For men, prostate cancer was most common. There is much clearer evidence for the protective role of physical activity in breast cancer than for prostate cancer.
While physical activity may confer modest protection against all-site and lung-cancer incidence and mortality in people with lengthy smoking histories, Alfano pointed out that it's still important for smokers to kick the habit. "I don't want people to think they can engage in physical activity and not quit smoking," she said. "Even though we found that smokers may lower their risk of cancer if they exercise, the point is that they should still do both."
Alfano's main research focus is physical activity and cancer, both as a preventive measure and as a method to manage symptoms and prevent secondary cancers in cancer patients and survivors. She is currently conducting a pilot study of physical activity with breast-cancer survivors through the Seattle Cancer Care Alliance.
Co-authors included Drs. Anne McTiernan, Deborah Bowen, and Mark Thornquist and Matt Barnett, of PHS; Drs. Robert Klesges, Mark Vander Weg, David Murray and Leslie Robinson, University of Memphis; Dr. Brenda Cartmel, Yale University; Dr. Gary Goodman, Swedish Medical Center Cancer Institute; and Dr. Gilbert Omenn, University of Michigan.