CPE: Fiber, Fruits, Vegetables and Risk of Colon Cancer
Colorectal cancer is usually a slowly progressing disease that may be present without symptoms for at least several years. In most the cases, cancer of the colon and rectum arise from polyps (aka adenomas or adenomatous polyps). Ninety-five percent of colorectal cancers are adenocarcinomas. These polyps, or precancerous lesions, are an early warning sign that colorectal cancer may develop. Indeed, screening for polyps and surgically removing them when present dramatically reduces the risk of developing and dying from colorectal cancer. Of course, it is better to prevent these polyps from developing than it is to have them surgically removed every few years. For many years most health authorities have advocated a diet higher in whole grains, fruits and vegetables and lower in fatty foods and especially fatty red meats.[2]
Figure 1 shows that worldwide there is a very large variation in the incidence of colorectal cancer between different countries. Today, Australia, Canada, the Czech Republic and Austria have an even higher incidence of colorectal cancer than seen in the US. The incidence of colorectal cancer in these countries and the U.S. are from 3 to 8 times higher than in countries like China, Columbia, Greece and India. The prevalence of colorectal cancer is also rising in Japan and other low-risk countries that have been switching to a more high-fat, low fiber, Westernized diet from their traditional diets. Migration studies generally show that those who migrate from a low-risk country to a high-risk country experience an increased risk of colorectal cancer after just a few years. Nevertheless the risk never rises as high as that of those who grow up in the high-risk country. The children of parents who had migrated from low-risk to high-risk country for colorectal cancer experience about the same lifetime risk of developing colorectal cancer as the rest of that population. It seems likely that most of the differences seen between populations in terms of the prevalence of colorectal cancer is likely due to some environmental factor(s). Something about a typical high-fat, high-meat, low fiber diet has long been suspected of promoting colorectal cancer.Early epidemiological research suggested that diets high in fat and low in fruit, vegetable and cereal fiber promoted colorectal cancer. Back in 1971, Dean Burkitt suggested that the low incidence of colorectal cancer in black Africans was due to their high dietary fiber intake.[3] White South Africans, who consumed a more typical modern rich diet had a much higher incidence of polyps and colorectal cancer. Many within population studies in high-risk countries also found an inverse correlation between the intake of fiber from fruits, vegetables and whole grains and colorectal cancer and/or polyps. Others have observed that those who eat a greater variety of vegetables are at reduced risk of colorectal cancer.[4]Data from two more recent epidemiological studies suggested that substituting more low-fat foods from the vegetable kingdom for high-fat foods from the animal kingdom is associated with a much lower risk of colorectal cancer. One, a 6 year prospective study found that people who exhibited a high intake of red meat, a low legume intake and a high BMI experienced more than a 3-fold increased risk compared with those who had the opposite pattern. The authors of this study concluded that "The overall findings from this cohort identify both red meat intake and white meat intake as important dietary risk factors for colon cancer and raise the possibility that the risk due to red meat intake reflects a more complex etiology."[5] The other study found a lower risk in people who substituted whole grains for refined grains, poultry for red meat and low-fat dairy products for those high in dairy fat. The authors of this study concluded that their "data support the hypothesis that the overall dietary intake pattern is associated with colon cancer, and the dietary pattern associated with the greatest increase in risk is the one which typifies a Western-style diet."[6] The data from the DASH trials have shown the power of improving many dietary variables at the same time as opposed to looking for just one dietary factor that consistently reduces the risk of disease. The same may be true for colorectal cancer.
Despite the findings of a variety of epidemiological studies, which have shown a reduction in risk associated with greater dietary fiber intake, such findings have not been consistent nor have they been particularly strong when present.[7] It is possible that the protective effect of more dietary fiber requires a much longer time frame than the follow-up period of the studies that have failed to find a strong association. It is also difficult to accurately measure nutrient intake over a prolonged period of time, which makes it more difficult to correlate diet with colorectal cancer. Alternatively, it is possible that something other than dietary fiber from fruits, vegetables and whole grains may be considerably more important in promoting or protecting people from developing ademotamous polyps and ademomas.
In theory, dietary fat may promote colorectal cancer by increasing the release of bile acids. Bile acids are degraded by bacteria in the colon and the secondary bile acids have been shown to stimulate the growth of colorectal cancer. Most countries that have been found to have a low prevalence of colorectal cancer also consume a low-fat diet.However, the fairly low incidence of colorectal cancer in Greece compared to more northern European countries suggests that a diet high in olive oil does not promote colorectal cancer. This certainly conflicts with the theory that a high-fat diet necessarily promotes colorectal cancer by increasing the release of potentially cancer promoting bile acids. Colorectal cancer is also much less common in Finland than in Germany and other central European countries. The Fins eat a lot more whole grain bread than do Germans and Americans but the relationship between increased cereal fiber and a reduced risk of colorectal cancer is even weaker than for fiber from fruits and vegetables and the Fins do not eat more fruits and vegetables than other Northern Europeans who experience a higher risk of colorectal cancer.This does not mean that dietary fat plays no role in promoting the development of colorectal cancers. Obesity has been strongly linked to an increased risk of developing colon cancer in men. However, in women the association between obesity and colon cancer is much weaker and inconsistent.[8] Some researchers suspect that the stronger association between obesity and colon cancer in men relative to women may be due to the higher prevalence of intraabdominal or central adiposity in men compared to women.. A prospective study found a strong positive association between waist-to-hip ratio and the risk of both large adenomatous polyps and colon cancer in men.[9] Diets high in fat, sugars and refined grains may be promoting colorectal cancer largely by promoting increased abdominal fat stores which usually lead to hormonal changes that may over stimulate colonic endothelial cells and encourage the growth of polyps and adenomas. More research is needed in this area to elucidate the impact of excessive energy intake on the development of colorectal and other types of cancer. Even if being overweight is shown not to promote colorectal cancer there is unequivocal research linking increased BMI with a greater risk of diabetes, cardiovascular disease, gallstones and at least some types of cancer. Reducing dietary fat can reduce calorie density and make it easier to lose excess body fat. It therefore seems prudent to counsel all overweight patients to eat less fat and more fruits, vegetables, and whole grains.
Publications of several large studies, which question the benefits of counseling patients at high risk for colorectal cancer to simply eat more fruits, vegetables and whole grains. In 1999, a study that was highly publicized in the national press failed to find any significant inverse correlation with dietary fiber and the development of polyps or colorectal cancer.[10]Two studies looked at the impact of dietary changes in people at high-risk of developing colorectal cancer due to the presence of adenomas. The Polyp Prevention Trial followed 2079 patients whose colon had been cleared of polyps. Half were provided with intensive counseling and placed on a low-fat (20% fat calories), high-fiber (18g/1000kcal) diet that also had more fruits and vegetables 3 1/2 servings per 1000 kcal). The control group was simply given standard brochures on healthy eating. All patients then underwent colonoscopy after between 1 and 4 years. The incidence of polyps was virtually identical in the two groups.[11] In the second study, a similar group of 1429 patients were randomly assigned to either a low intake of wheat fiber (2g/day) or a high wheat fiber (13.5g/day) diets. Both groups were then re-examined 3 years later. The recurrence of polyps was 51% and 47% in the two groups, which was not significantly different.[12] The results of these two studies suggest that encouraging middle-aged or older people to eat more fruits, vegetables, whole grains and less fat is not likely to have much of an impact on their risk of developing adenomas and colorectal cancer over the next several years.
So why is the incidence of colorectal cancer only 1/8 as common in India as the U.S.? In India, much of the population is Hindu and cattle are considered sacred. As a result, India has one of the lowest intake of red meat of any country. Indeed, all countries where the intake of colorectal cancer is low also have a low intake of red meats. It is unlikely that it is the saturated fat content of the meat that is promoting colorectal cancer because the Fins eat more saturated fat than all of the highest risk countries (due to a heavy intake of dairy products). Meat contains a substance called creatine. At high temperatures, creatine reacts with amino acids to form heterocyclic aromatic amines, which are potent carcinogens. Sausages and other cured meats are commonly consumed in Germany, Austria, Poland and the Czech Republic. These contain nitrites, which reacts with amino acids to form nitrosamines, another class of cancer-causing chemicals. Indeed, a large epidemiological study found an association between red meat intake and the incidence of colorectal cancer.The research linking an increased intake of red meat to colorectal cancer is stronger and more consistent than research suggesting that a diet with less fat or more fiber from fruits, vegetables and whole grains reduces the risk of colorectal cancer. It may be that there are carcinogens and/or cancer promoting substances in fatty red meats.Of course, even if there was conclusive evidence that an increased intake of fruits, vegetables and whole grains does not reduce the risk of colorectal cancer, this does not mean these foods should not be recommended to people at risk for colorectal cancer. An increased intake of fruits and vegetables does help prevent cardiovascular disease.[13] And even if cutting back on red meats and processed and cured meats is proven to not promote colorectal cancer such foods still may promote cardiovascular and other diseases. There is no doubt that a more vegetarian diet with less red meat, saturated fat, salt, cholesterol and refined sugar and grains reduces the risk of cardiovascular disease and may help prevent obesity, diabetes, and several other types of cancer. Even if cutting back on red meats is never proven to reduce the risk of colorectal cancer there is already more than even scientific evidence to recommend most people cut back on red meats and particularly processed meats like sausages, hot dogs, bacon and bologna.
Aspirin inhibits the formation of the pro-inflammatory COX-2 enzyme. This enzyme is over expressed in adenomas and some cancers. Inhibition of this enzyme has led to the regression of tumors. People who take aspirin regularly have a reduced risk of colorectal cancer. Bile and stomach acids have been shown to stimulate COX-2 and may promote polyps and cancers.[14] Dietary calcium may help to neutralize these acids and there is some evidence that increased calcium reduces the risk of colorectal cancers. Omega-3 fatty acids may also reduce COX-2 enzyme and inflammation and so may inhibit the growth of some types of cancer. People with inflammatory bowel diseases (e.g. ulcerative colitis and Crohn's disease) have a much greater risk of developing colorectal cancer.
Recently, an increased intake of garlic was associated with a reduced risk of colorectal cancer.[15] There are many other potential anticarcinogen phytochemicals found in a variety of fruits and vegetables.[16] There is also some evidence suggesting that a lack of selenium, folic acid and/or vitamin D may contribute to the development of colorectal cancer. However, there is a need for more prospective studies and perhaps clinical trials with these nutrients and phytochemicals before any firm conclusions should be drawn. Keep in mind that prospective studies with supplements of beta-carotene, and vitamins C and E have shown no reduction in the risk of polyp formation despite some preliminary evidence that they may reduce the risk of polyps and colorectal cancer.
People concerned about the development of colorectal cancer should be advised to cut back on red meats, processed meats and cured meats. These should be replaced in the diet by beans, soy products and some seafoods. High fat dairy products should be replaced with non-fat dairy products. Replacing refined fats, oils, sugars and grains with more fruits, vegetables and whole grains should aid weight loss and may very well reduce the risk of developing colorectal cancer over a lifetime. These dietary changes probably will reduce the risk of developing colorectal cancer in the long run but even if they do not they have already been shown to have many other well established health benefits.
[1] Colon and Rectum Cancer Resource Center. www3.cancer.org[2] US Dept. of Health And Human Services, US Dept. of Agriculture. Dietary Guidelines for Americans. 4th ed. Washington, DC: US Government Printing Office. 1995 and Proposed Dietary Guidelines 2000 generate comments, controversy. J Am Dietet Assoc 2000;100:518[3] Burkitt D. Epidemiology of cancer of the colon and rectum. Cancer.1971;28:3-13[4] Fernandez E, et al. Preventative Medicine 2000; 31:11-4[5] Singh PN, Fraser GE. Am J Epidemiol 1998;148: 761-74[6] Slattery ML, et al. Am J Epidemiol 1998;148:4-16[7] Newbauer S, Bebo P. Fiber and colon cancer. Vegetarian Nutrition Update. Fall 2000[8] Potter JD, et al Colon cancer: a review of the epidemiology. Epidemiol Rev 1993;15:499-545[9] Giovannucci E, et al. Physical activity, obesity and risk of colon cancer and adenomas in men. Ann Intern Med 1995;122:327-34[10] Fuchs CS, et al. Dietary fiber and the risk of colorectal cancer and adenoma in women. N Engl J Med 1999;340:169-76[11] Schatzkin A, et al. Lack of effect of a low-fat, high-fiber diet on the recurrence of colorectal adenomas. N Engl J Med 2000;342:1149-55[12] Alberta DS, et al. Lack of effect of a high-fiber cereal supplement on the recurrence of colorectal adenomas. N Engl J Med 2000;342:1156-62[13] Liu S, et al. Am J Clin Nutr 2000;72:922-8[14] Shrvani VN, et al. Cycloxygenase 2 expression in Barrett's esophagus and adenocarcinomas: Ex-vivo induction by bile acids and acid exposure. Gastroenterology 2000;118:487-96[15] Flieschauer AJ, et al. Am J Clin Nutr 2000;72: 1047-52[16] Steinmetz KA, Potter JD. Vegetables, fruit and cancer prevention: a review. J Am Diet Assoc 1996;96:1027-39Please note: It is your responsibility to report your CPE hours to ADA and your state licensing department. Proof of completion will be mailed to you as soon as we grade your test. Please retain it for reporting and your records.TAKE THE TEST