Risk Communication: Information for the Clinician about Colorectal Cancer Risk

Modifiable Risk Factors

A number of lifestyle factors are associated with colorectal cancer, either as risk factors or protective factors. It is important to understand that this information derives from observational studies, which have well-known limitations.  For example, there may be something different about people who choose to adopt an intervention versus those who are randomly assigned to it.  Nevertheless, studies strongly suggest that lifestyle factors play an important role in modifying the risk of colorectal cancer.  

Many of these behaviors tend to cluster in the same individual.  For example, physical activity appears to be protective for CRC, whereas obesity appears to be a risk factor.  But since those who are physically active tend to be leaner, these two associations may point to a single underlying causal factor.  Similarly, people who consume more fruits and vegetables and less meat (two dietary factors which may reduce CRC risk) are also likely to be less obese.

Physical Activity.  Both leisure-time physical activity and job-related physical activity are associated with reduced risk for CRC.  Several studies show a 40-50 percent reduction in risk for people who engage in regular exercise, with the largest reduction for people who are the most active. (20)  Evidence for this benefit is stronger for men than for women.

The American Cancer Society recommends engaging in moderate physical activity (e.g., brisk walking, bicycling, vacuuming, gardening) for 30 minutes or more at least 5 times per week.  Forty-five minutes or more of moderate to vigorous activity (e.g., running, aerobics, heavy yard work) is believed to augment the risk reduction. (37, 38)

Red Meat.  Several prospective studies and meta-analyses have reported an increased risk for CRC with increasing consumption of red meat, but other studies have failed to demonstrate a connection.  Two recent cohort studies, appearing in the Journal of the American Medical Association and the Journal of the National Cancer Institute, do support an increased risk in people with a high vs. low intake of red meat. (20, 21, 22)  The American Cancer Society continues to recommend reducing red meat consumption.

Multivitamin use.  Two large cohort studies have found a possible connection between decreased CRC risk and long-term intake of folate supplements or multivitamins with folate.  Vitamin B6 may also help reduce risk. (23, 24)  On the other hand, a 2004 meta-analysis of antioxidant vitamin use (beta-carotene and vitamins A, C, and E) found no decreased risk. (28)  Recent studies suggest that magnesium may have a protective effect.

Calcium and Vitamin D.  Epidemiologic studies suggest that calcium may have a protective effect against CRC. A Cochrane Review article in 2005 found some evidence in populations already at increased risk due to previous adenomas. However, the evidence was not deemed strong enough to recommend the general use of calcium as a preventive measure. (29)  More recently, Women’s Health Initiative centers carried out a large, double-blind, placebo controlled study of calcium plus vitamin D intake and CRC risk in postmenopausal women. This study failed to find a reduction in risk, but the study period was only 7 years; the researchers note that the long latency of CRC could have contributed to the apparent null effect. (30) 

Obesity.  Obesity is associated with increased risk of CRC. In one study, obesity in premenopausal women was associated with a 50% increase in risk.(20)  Obese men have been found to be at 40% higher risk. (32)  Other studies have shown similar connections.

Fruits and Vegetables.  A diet rich in fruits and vegetables may decrease the risk of CRC.  In one large Swedish study, people consuming less than 1.5 servings of fruits and vegetables per day had a relative risk of CRC of 1.65, compared to people who ate more than 2.5 servings.  However, an analysis of data from two large prospective studies, the Nurses' Health Study and the Health Professionals Follow-up Study, did not show a connection. (20)  Neither did a 2006 Women’s Health Initiative report of a randomized controlled trial which included increased fruit and vegetable intake as part of a low-fat diet, with 8 years of follow-up. (33)  Further study will be needed to clarify any association. Meanwhile, the American Cancer Society recommends a diet rich in fruits and vegetables as a general preventive measure.

Fiber.  Evidence for fiber as a protective factor is mixed.  One meta-analysis of case-control studies showed a decrease in risk with consumption of fiber-rich foods.  Other studies have supported this finding.  However, the large, prospective Nurses' Health Study failed to show a decrease in risk, at least for women. (20)

Alcohol.  Alcohol, particularly beer, is associated with increased risk of CRC.  Both meta-analyses and later case-control studies support this finding. (20)

Tobacco.  A connection between cigarette smoking and CRC is supported by multiple case studies as well as large cohort studies.  Based on data from the Cancer Prevention II cohort study, 12% of colorectal cancers in the U.S. may be attributable to smoking. (20)

According to the Behavior Risk Factor Surveillance System data from 2005, 20.6% of all American adults, 20.4% of White adults, 20.7% of African American adults, and 19.5% of Hispanic adults currently smoke. (40)

Aspirin.  Aspirin use is associated with decreased risk of CRC.  For example, in the Health Professionals Follow-up Study of 47,000 men, those who took aspirin at least twice a week had a 30% reduction in risk compared to those who never took aspirin.  The Nurses' Health Study showed a similar trend, although risk reduction did not become significant until after 10 years of use.  There appears to be a dose-response trend, so that higher intake is associated with lower risk.  However, this must be balanced against the risk of gastrointestinal bleeding. (20)  The U.S. Preventive Services Task Force does not recommend aspirin for CRC chemoprevention, at least until benefits and harms are evaluated further. (34)

Exogenous hormones.  Postmenopausal hormone therapy reduces the risk of colon cancer, but it is not recommended for prevention.  A risk reduction of 20% was shown in a meta-analysis among women who had ever taken hormone replacement, compared to those who had never taken hormones.  The largest reduction, 44%, was seen among current users. (26)  It is not clear whether a longer duration of use confers greater protection.  There was also a reduction in colorectal cancer reported among women taking combined estrogen and progestin in the Women’s Health Initiative’s randomized double-blinded clinical trial (hazard ratio 0.63). (27)  Data from this study did show that cancers in the women taking HRT were more advanced at the time of discovery, however. (20)  Because hormone therapy carries increased risk of other morbidities, such as heart disease, stroke, and thromboembolism, it is not recommended for the prevention of colorectal cancer.