Diets Low in Fruits, Vegetables and Fiber May Increase Colon Cancer Risk

Diets Low in Fruits, Vegetables and Fiber May Increase Colon Cancer Risk

A recent study published in the journal Cancer Causes and Control, indicates that diets, that include a very low intake of fruits, vegetables and whole grains may increase the risk of colon cancer.

Colorectal cancer is the second leading cause of cancer related deaths in the United States. Colorectal cancer is a malignancy that involves both the large intestines (colon) and a distal portion of the colon known as the rectum. Although there is no single known cause for colon cancer, certain risk factors have been found to increase the risk of colon cancer. Risk factors for colorectal cancer include colorectal polyps, a family history of colorectal cancer, and a history of ulcerative colitis. Diet has long been suspected as a contributing factor to the risk of developing colorectal cancer; however, a direct link has so far been unclear.

This recent study was conducted between 1992 and 1993 and included 62,609 men and 70,554 women who completed questionnaires on medical history, diet and lifestyle behaviors. In 1997 when the results were examined, 298 cases of colon cancer had been diagnosed among the men, and 210 cases had been diagnosed among the women. Dietary factors were determined to have no effect on the incidence of colon cancer. However, a 30% reduction in the risk of developing colon cancer was observed in the men with the highest vegetable intake. Women were found to have an increased risk of colorectal cancer when their diets included very small amounts of fruit.

Researchers concluded that although diets high in fruits, vegetable, and fiber were not related to lower risks of colon cancer, diets with very low intakes of fruits, vegetables and fiber might increase the risk of colon cancer.

Reference: Cullough M.. A Prospective Study of Whole Grains, Fruits, Vegetables and Colon Cancer Risk. Cancer Causes and Control. 2004; 14: 959-970.

A High-Fiber Diet Does Not Reduce Risk for Colorectal Cancer

Eating a diet high in fiber does not reduce the risk for developing colorectal cancer? This is the surprising finding of a recent study conducted at Harvard University.

Cancers of the colon and rectum, sometimes referred to together as colorectal cancer, are characterized by the presence of cancerous tumors in the colon or rectum, each part of the body’s digestive system. Non-cancerous tumors, called adenomatous polyps, may also grow in the colon or rectum; however, it is thought that these polyps eventually become cancerous in some persons, developing into colorectal cancer.

Researchers continue to work toward determining what factors may contribute to the development of colorectal cancer so that better treatments—and better strategies for prevention—can be produced and implemented. A person who has 1 or more characteristics, or risk factors, for a type of cancer has a higher chance to develop that type of cancer than a person who does not have these risk factors. Risk factors can be determined by studying the differences between persons who have and persons who do not have a type of cancer.

Several risk factors have been suggested in association with colorectal cancer: age older than 40 years; a diet high in fat, protein, calories, alcohol, and meat while low in calcium and folate; a diet high in saturated fat in combination with a sedentary lifestyle; and smoking tobacco. Some have speculated that the low incidence of colorectal cancer in parts of Africa may be linked with a high-fiber diet. However, despite much focus on the possible benefits of a high-fiber diet, several studies have failed to associate such a diet with a decrease in the incidence of colorectal cancer. Researchers at Harvard University investigated the relationship, if any, between a high-fiber diet and a number of diseases, including colorectal cancer.

Harvard University researchers studied 88,757 women, evaluating the correlation between their diet and the development of certain diseases, such as colorectal cancer, symptomatic diverticular disease of the colon, coronary artery disease, hypertension, and non-insulin-dependent diabetes. After a followup period of 16 years, the researchers found that a high-fiber diet lowered the incidence of symptomatic diverticular disease of the colon, coronary artery disease, hypertension, and non-insulin-dependent diabetes—but not of colorectal cancer. There was no evidence to suggest that increasing the dietary intake of total, cereal, fruit, or vegetable fiber is associated with a reduction in risk for colorectal adenomatous polyps or colorectal cancer. In fact, one unexpected finding was a significant increase in the incidence of colorectal cancer in those women who had a high intake of vegetable fiber. Although this association was surprising, 1 previous study did note an increased risk for colorectal cancer in persons who ate more potatoes and cruciferous vegetables.*

These researchers concluded that there are many health reasons to eat a diet high in fiber, particularly to help reduce the risk for coronary artery disease; however, such a diet does not appear to help prevent the development of colorectal polyps or colorectal cancer.**

*(American Journal of Epidemiology, Vol 139, No 1, pp 1-15, 1994) January

**(New England Journal of Medicine, Vol 340, No 3, pp 169-176, 1999) January

No Clear Link Between Dietary Fiber and Colorectal Cancer

While high fiber intake has been hypothesized to reduce the risk of colorectal cancer, previous studies have produced mixed results. In a large pooled analysis of 13 studies, published in the Journal of the American Medical Association (JAMA), researchers found that dietary fiber was not associated with a reduced risk of colorectal cancer.

Colorectal cancer is the second leading cause of cancer-related deaths in the U.S. The disease develops in the large intestine, which includes the colon (the longest part of the large intestine) and the rectum (the last several inches).

Dietary fiber (found in cereals, fruits, and vegetables) could potentially reduce the risk of colorectal cancer though a variety of mechanisms: Fiber could dilute cancer-causing agents that pass through the colon and rectum in the feces; it could speed stool passage through the colon and rectum, minimizing exposure to cancer-causing agents; and it could reduce exposure to bile acids.

In spite of the potential benefits of dietary fiber on colorectal cancer risk, previous studies have produced conflicting results. In an attempt to provide more definitive findings, researchers combined information from 13 prospective cohort studies of diet and cancer. In cohort studies, information about exposures such as diet is collected before the study subjects develop disease. Subjects are then followed forward in time to determine who develops disease. This approach minimizes some types of bias.

The 13 combined studies included a total of 725,628 men and women. During six to 20 years of follow-up, colorectal cancer developed in 8081 of these individuals. After accounting for age, subjects with the highest intake of dietary fiber had a 16% reduction in risk of colorectal cancer compared to subjects with the lowest intake of dietary fiber. After accounting for other dietary factors, however, such as red meat, milk, folate, and alcohol, the effect of dietary fiber on colorectal cancer risk was greatly reduced. The final results indicated a nonsignificant 6% decrease in risk of colorectal cancer among subjects with the highest fiber intake.

In spite of the apparent lack of effect on colorectal cancer risk, the researchers note that there are other benefits of a high fiber diet. They conclude, “Although high dietary fiber intake may not have a major effect on risk of colorectal cancer, a diet high in dietary fiber from whole plant foods can be advised because this has been related to lower risks of other chronic conditions such as heart disease and diabetes.”

Reference: Park Y, Hunter DJ, Spiegelman D et al. Dietary Fiber Intake and Risk of Colorectal Cancer: A Pooled Analysis of Prospective Cohort Studies. JAMA. 2005;294:2849-2857.

Low-Fat Diet Does Not Reduce Colorectal Cancer Risk

According to the results of the Women’s Health Initiative Dietary Modification Trial, risk of colorectal cancer was not significantly reduced among postmenopausal women assigned to a diet low in fat and high in fruits, vegetables, and grains. These results were published in the Journal of the American Medical Association.

Studies of cancer rates across countries have suggested that a low-fat diet may be linked with a lower risk of colorectal cancer. Countries with lower average fat intake than the U.S. tend to have lower rates of colorectal cancer. Furthermore, individuals who move from a country with low fat intake to a country with higher fat intake tend to experience the higher colorectal cancer rates of their new country.

In order to provide more definitive information about the role of diet in the development (or avoidance) of colorectal cancer and breast cancer, researchers conducted a large clinical trial known as the Women’s Health Initiative (WHI) Dietary Modification Trial.

The study enrolled close to 50,000 women between the ages of 50 and 79 years. Roughly 20,000 of the women were randomly assigned to the dietary modification group, with the remaining 30,000 women forming the comparison group.

Women in the dietary modification group were asked to make dietary changes that would reduce fat intake to 20% of total energy intake, increase fruit and vegetable intake to five or more servings a day, and increase grain intake to at least six servings a day. Women in the comparison group were not asked to change their diet.

Analysis of food intake in the two study groups indicated that fat intake was indeed lower among women in the dietary intervention group. The difference in fat intake between women in the intervention group and women in the comparison group was greatest during the early part of the study, but was still apparent by year six. However, although fat intake was lower among women in the intervention group, many of the women in the intervention group continued to consume more than the recommended amount of fat.

After an average of eight years of follow-up, colorectal cancer had developed in 201 of the subjects in the intervention group (0.13% per year) and 279 of the comparison subjects (0.12% per year).

The researchers conclude that “an intervention aimed toward a low-fat eating pattern did not reduce colorectal cancer risk in postmenopausal women.” They note that it’s still unclear whether initiation of dietary changes at a younger age or for a longer duration or greater adherence to the dietary recommendations would influence colorectal cancer risk.

Reference: Beresford SAA, Johnson KC, Ritenbaugh C et al. Low-Fat Dietary Pattern and Risk of Colorectal Cancer: The Women’s Health Initiative Randomized Controlled Dietary Modification Trial. JAMA. 2006;295:643-654.

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