Primary prevention of colon cancer, time to act is now!
In this short update we draw attention to the strength of evidence that colon cancer is largely preventable with what we already know. While we have provided more extensive summaries of the overall evidence in the past 1, and have reported in detail on specific lifestyle habits and colon cancer 2,3, our goal here is to provide a quick update to help readers see just how strong the evidence is. For each lifestyle factor we provide a short summary of the evidence.
Data from US based cohorts show that more than 90% of adults have one or more lifestyle factors that they could change to reduce their risk of colon cancer 4 and other chronic diseases like diabetes 5 and heart disease.6 We conclude that if Americans modify the behavioral factors that we summarize here, with changes at an early enough age to reverse risk, then more than 80% of colon cancers could be prevented in the long term 4. While risk reduction strategies can be evaluated either individually or in combination showing substantial benefits for the population 7, we do not yet have community wide studies showing this benefit in real time – some of the challenge is the time lag from change in a health behavior to subsequent development of genetic changes, growth of tumor, and diagnosis of cancer.8
Table Summary of relative risk, as well as the prevalence, of each of the modifiable and no-modifiable factors related to colon cancer.
| Lifestyle factor | Relative risk | Reference | Population percentage who can change behavior | |
| Modifiable | | | Women | Men |
| Physical activity (>3 hours per week) | 0.75 | Wolin 2 | 80% | 80% |
| Meat >7 servings per week | 1.5 | WCRF 9 | 25% | 25% |
| Obesity Per 5kg/m2 | 1.24 | Renehan 10 | 40% | 35% |
| Alcohol >4 drinks per day vs. never | 1.5 | Fedirko 11 | 5% | 10% |
| Cigarette smoking | 1.4 | Tsoi 12 Liang 13 | 20% | 20% |
| Aspirin use Daily for 5 years | 0.5 | Flossmann 14 Rothwell 15 | 80% | 70% |
| Calcium 1200 mg/day | 0.80 | Baron 16 Cho 17 | 20% | 25% |
| Estrogen use | 0.80 | Grodstein 18 | ~ | n/a |
| Oral contraceptive use | 0.80 | Martinez 19 Bosetti 20 | ~ | n/a |
| Non modifiable | | | | |
| Family history Parent or sibling | 1.8 | Fuchs 21 | 5% | 5% |
| Height Per 6 inches | 1.2 | Wei 7 | ~ | ~ |
| Screening | | | | |
| Colonoscopy | 0.5 | Frazier 22 | Approx 50% total population up to date & screened | |
| Sigmoidoscopy | 0.5 | Atkin 23 | ||
The factors are listed based on the strength of the scientific evidence that a particular factor affects colon cancer risk. We also note that the majority of modifiable risk factors also contribute to other health benefits if changes are made to reduce colon cancer risk.
For example, the observational data on physical activity and colon cancer are very consistent 2, the benefits of physical activity for cardiovascular and bone health are well-established 24, and the adverse consequences of physical activity are minimal if it is done sensibly. Furthermore the level of inactivity in the population suggests that the vast majority of Americans could gain health benefits form increasing their level of activity.
Notably, the majority of the population is not engaging in the behaviors known to be most protective against colon cancer: 80% of adults are active less than 3 hours a week 25 and 25% consume more than 7 servings of meat a week. The observational data on red meat consumption and colon cancer are only a little less consistent 9, there may be cardiovascular benefits to restricting red meat consumption, and the adverse consequences are almost entirely cultural and economic.
The data on obesity and colon cancer are rigorously combined by Renehan and show consistent direct relation between increasing Body Mass Index (BMI) and risk of colon cancer. 10
Calcium supplementation reduces risk of colorectal polyps and colon cancer. A randomized trial shows the does of 1200 mg per day reduces risk of polyps by 20% 16 and that this benefit persists for many years after stopping therapy. 26 In addition, combined data from prospective cohort studies shows this level of calcium intake (1200mg per day) is sufficient for protection against colon cancer and that there is little added benefit from higher intakes. 17 Importantly, the lack of benefit in the randomized trial component of the Women’s Health Initiative that evaluated calcium and vitamin D in relation to colon cancer risk had the mean intake at randomization already at the 1200 mg per day for women in the trial. Thus there was likely little room for benefit in terms of reduction in risk with even high intakes during the trial. 27
Alcohol is a known carcinogen causing cancer of the mouth and throat as well as breast and colon. Data on colon cancer have been combined from 27 cohort studies and 34 case-control studies.11 In the combined analysis risk for colorectal cancer increased with the amount of alcohol consumed. Compared to non-drinkers, those consuming 50 grams per day (4 drinks) had a relative risk of 1.38 (95% confidence interval 1.28 to 1.50). 11 Risk was present in men and women.
For aspirin, it is now clear that eicosanoids and the COX pathway play a role in neoplasia. However, there is no certain knowledge about dose and regimen, and the side effects of gastrointestinal and cerebral bleeding are well known. Evidence from several randomized trials suggests that a daily does of 75 mg is sufficient to obtain the benefit of reduce colon cancer with no added benefit form higher doses. 15 Importantly, data show that the benefit accrues some years after starting daily aspirin. 15 Furthermore, when dose and duration are taken into account the data from randomized trials and the prospective cohort studies show equivalent benefits from use of aspirin. 14 In sum, the data from randomized trials for prevention of cardiovascular disease agree with observational data when dose and duration are considered together 14,15,28. Five years of use gives approximately 50 percent reduction in risk of death from colon cancer through 20 years of follow-up.
Cigarette smoking is a cause of many cancers. Colon cancer has been added to the list of sites where smoking now is directly related to increased risk of cancer. Combining data from 28 prospective cohort studies Tsoi and colleagues reported that current smokers had an increased risk of colorectal cancer (RR 1.20) and that the risk was stronger among men (RR=1.38). Longer duration of smoking and number of cigarettes smoked per day also increased risk of colorectal cancer. 12
Among women use of oral contraceptives is related to reduced risk of colorectal cancer. 19,20 In addition, among postmenopausal women, those who currently use hormone therapy have reduced risk of colon cancer. 18
Family History
Strong evidence shows that this common malignancy has an inherited component. Those with family history gain added benefit from changing lifestyle factors and from screening. Recommendations for screening now indicate that hose with a family history should begin screening at a younger age. Obviously you need to let your health care providers know about your family history if they are to order screening tests at the appropriate age.
Given that most Americans are not engaging in behaviors known to prevent development of malignancy, early detection of polyps and colon cancer must become routine and commonplace.
Screening
For most diseases, screening is considered ‘secondary prevention’ because it detects early forms of cancer, but does not prevent the actual development of disease.
However, colon cancer screening can be considered either primary prevention or secondary prevention because the tests have the ability to detect, and often remove, both precancerous polyps and carcinomas. Approaches to colon cancer screening are cost-effective22 and are now widely integrated into primary care. Primary prevention via screening involves the removal of precancerous polyps that may have progressed to carcinoma if left undetected. Evidence suggests that removal of polyps in a population does lead to a significant reduction in the incidence of colon cancer. 23 A randomized trial of flexible sigmoidoscopy included 113,195 people assigned to the control group and 57,237 assigned to flexible sigmoidoscopy. 23 During follow-up of 11 years colon cancer incidence was significantly reduced in the screened group (23 percent reduction compared to that in the unscreened group). Mortality from colon cancer was reduced by 31 percent. Both reductions were statistically significant providing further support for recommendations that screening reduced incidence and mortality form this cancer.
Related CNiC posts
Colon Cancer Screening - Just a (great) first step
Literature cited
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