The heavier a person is and the less exercise he or she does, the greater the likelihood of developing a specific type of colorectal cancer, a new study finds.
Researchers at the Dana-Farber Cancer Institute in Boston analyzed data on weight and physical activity from questionnaires sent every two years to more than 109,046 women who participated in the landmark Nurses' Health Study, an ongoing study about women's health that is following nurses. The questionnaires also went to more than 47,684 men who participated in the Health Professionals Follow-Up Study, an ongoing study about men's health that includes more than 50,000 men who work in health care. Data collection began in 1976 for the women and in 1986 for the men.
When follow-up ended in June 2004, 2,263 cases of colorectal cancer — 842 in men and 1,421 in women — had been diagnosed. The researchers analyzed 861 of the cancers to determine if any contained a molecular biomarker, called CTNNB1, which has been linked to cancer and obesity. Fifty-four percent of the tumors were CTNNB1-negative and 46 percent were CTNNB1-positive.
Researchers next examined how body mass index, or BMI, and physical activity levels affected a person's risk of developing CTNNB1-negative or CTNNB1-positive colorectal cancer. What they found: The higher a person's BMI, the greater the likelihood he or she would develop a CTNNB1-negative cancer. Each 5.0 kg increment in BMI — about 11 pounds — was associated with a 34 percent higher risk for CTNNB1-negative colorectal cancer, said lead study author Shuji Ogino, an associate professor of pathology at Dana-Farber Cancer Institute in Boston.
By contrast, the more physical activity a person did, the lower the risk for CTNNB1-negative colon cancer. Study participants did aerobic activities such as walking (at a usual pace), jogging, running, bicycling, swimming laps, playing racquet sports and lower-intensity activities such as yoga, toning and stretching.
Each exercise was assigned a metabolic equivalent task (MET) score, which is a measure of exercise intensity. The higher the MET score, the more calories an activity burns. For example, sitting quietly is the equivalent of one MET; walking at a pace of 1 to 2 miles per hour is the equivalent of about 2 METS; slow jogging is about 6 METS; cycling at less than 10 mph is about 4 METS; swimming moderately fast to fast laps is about 6 to 10 METS; and running approximately six miles per hour is about 10 METS.
In the study, every 10 METs per hour increase in physical activity was associated with a 7 percent reduction in the risk for CTNNBI-negative colorectal cancer. People who accumulated about 18 MET hours per week in exercise, saw approximately a 20- to 30- percent reduction in risk, said Ogino, who is also an associate professor in the department of epidemiology at the Harvard School of Public Health. That's the equivalent of about 30 minutes of moderate-intensity physical activity on most days of the week.
Neither BMI nor physical activity level was associated with CTNNB1-positive cancer.
Previous research has shown that doing regular physical activity and maintaining a healthy weight are associated with a decreased risk of colorectal cancer, but it has been unclear why. "We now have a biomarker we can subtype," Ogino said "No other prospective study has found this."
Just how BMI and exercise affect the risk of CTNNB1-negative cancer is unclear. One theory is that higher circulating levels of insulin and insulin-like growth factor in people who are heavy or who are physically inactive may promote cancer cell survival and proliferation.
Currently, there is no way to accurately measure risk for CTNNB1-negative colorectal cancer. That said, Ogino recommends doing regular physical activity to reduce the overall risk for colorectal cancer. "Physical activity is more easily controlled than body weight," he said. "Physical activity is easy to incorporate into your life and hopefully it will decrease weight, too."
Not counting skin cancer, colorectal cancer is the third most common cancer diagnosed in American men and women. Overall, people have a 1 in 20 lifetime risk of developing the disease.
According to the American Cancer Society, an estimated 102,480 new cases of colon cancer and 40,340 cases of rectal cancer will be diagnosed in 2013. Some 50, 830 people will die from colorectal cancer.
The study is published today (Feb. 26) in the journal Cancer Research.
Pass it on: Reducing BMI and exercising more may lower risk for a type of colorectal cancer.