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||Last Updated: Nov 12th, 2006 - 20:38:00
News released on July 13.
Cancer Experts: Link Between Diet, Colon Cancer
"Stands at a Crossroad"
Is Fiber Over?
WASHINGTON This morning, at a press conference during the International Research Conference on Food, Nutrition and Cancer in Washington, DC, a spokesman for the nationís leading diet-cancer authority pointed to a shift in the focus of research exploring dietís role in colon cancer risk.
"Until recently, researchers studying colon cancer have concentrated on the potential protective effect of diets high in fiber. Today we see more and more investigations into the harmful effect of diets high in meat," said Jeffrey R. Prince, Vice President for Education for the American Institute for Cancer Research (AICR).
He spoke at the end of a press conference in which Dr. Stephen OíKeefe of the University of Pittsburgh described his research associating high incidence of colon cancer among African Americans with that group's meat intake, which is higher than the national average. Dr. O'Keefe's work focuses on gut bacteria that produce carcinogenic by-products when intake of meat is high. AICR's Prince cited O'Keefe's small but suggestive study as one of many exploring the relation of meat to colon cancer.
For years, research on colon cancer had linked high intake of fiber to lower risk of colon cancer. Scientists consistently observed that populations consuming diets high in fiber had lower risk for colon cancer.
Laboratory studies suggested a number of plausible mechanisms to explain the connection, including fiber's tendency to bind to potentially harmful bile acids, and its capacity to increase the bulk of matter moving through the intestine. This increase in bulk helps the body eliminate waste faster, reducing the amount of time the sensitive tissues lining the colon spend in contact with dietary carcinogens.
Clinical Trials Find No Link
But the notion that fiber helps prevent colon cancer was dealt a serious blow in 2000, when three separate clinical trials (two appearing in the New England Journal of Medicine, and one in The Lancet) found no significant association between fiber intake and risk for colon polyps, which are often precursors to colon cancer.
"Clinical trials are ideal for studying how to treat diseases like cancer, not how to prevent them," Prince said. "Those trials are important, but they by no means end the debate."
Nevertheless, those clinical trials and inconsistent results from cohort studies (which track the diets and disease rates of large groups of individuals over several years) have convinced many that fiber intake is not associated with colon cancer risk. With the fiber theory in eclipse, focus has shifted to other potential dietary connections such as fat, lack of folate, sedentary lifestyle, obesity and of the course the intake of red and processed meat.
Meanwhile, Evidence Linking Meat, Colon Cancer Builds
The association between meat and colon cancer has been observed in several large cohort studies. In January 2005, for example, a study published in the Journal of the American Medical Association (JAMA) found that among 148,610 adults aged 50 to 74, those with the highest long-term consumption of processed meat (sausages, ham, bacon, and cold cuts) had a 50 percent higher risk of distal colon cancer (a section of the colon near the rectum) than those who ate the least. This relationship was present even after researchers adjusted for the impact of weight, exercise, fruit/vegetable/whole grain intake & vitamin supplement use on colon cancer risk.
In June 2005 the most comprehensive cohort study ever undertaken, the European Prospective Investigation into Cancer (EPIC), reported in Journal of the National Cancer Institute (JNCI) that among its nearly half-million participants, those subjects who ate the most red meat and processed meats (about 6 ounces a day, on average) had a 35 percent greater risk of developing colon cancer compared to those subjects who ate the least of these foods.
Laboratory research exploring the meat-colon cancer connection at the cellular and molecular level has come up with many plausible reasons for the observed increase in risk. Several components of meat have been shown to cause tumors in the laboratory, such as heme iron (a blood pigment found in red meat), the combination of nitrate, nitrosamines and salt (found in processed meats), as well as heterocyclic amines and polycyclic aromatic hydrocarbons (found in meats cooked over high heat.) Dr. O'Keefe's work suggesting that meat intake has a direct and potentially risk-increasing effect on gut bacteria represents an exciting new addition to these existing theories, Prince said.
Not One or the Other, but the Relationship Between Them
"It is not surprising that the focus of research has moved from insufficient fiber to excessive meat consumption as a cause for colon cancer," Prince noted. "They are two sides of the same coin. A diet that is high in fiber from vegetables and fruit is likely to be low in meat. A diet that's low in fiber is likely to be high in meat and fat."
Prince noted that several presentations and posters related to diet's role in colon cancer are scheduled to be presented at the two-day AICR International Research Conference of Food, Nutrition and Cancer in Washington, DC. Among them are studies exploring the role of meat intake, as well as studies that continue to uncover evidence of a protective role played by fiber.
In fact, since those three clinical trials that failed to find a fiber-colon cancer link were published in 2000, evidence that fiber can indeed help prevent colon cancer has continued to appear steadily in the scientific literature.
In 2003, for example, EPIC researchers reported in The Lancet that subjects who ate the most fiber had 25 percent lower risk of colon cancer than those who ate the least. Just this month, in the Journal of Nutrition, an analysis of data from one of the same clinical trials that failed to find a fiber link in 2000 has found that those subjects who most increased their intake of dry beans (a rich source of dietary fiber) had 65 percent lower risk for polyp recurrence than those who ate the least.
"The crux issue here may be the shape of the overall diet," Prince said. It is likely that a diet in which the proportion of vegetables, fruit and whole grains is significantly higher than what most Americans currently eat, and in which the proportion of meat is significantly lower than the current US average, will prove protective against colon cancer, he noted.
"Watch This Space," Say AICR Experts
"Definitive answers about fiber's role in colon cancer - as well as broader answers about how diet, physical activity and weight management impact cancer in general - are coming soon. In November 2007, AICR and its international affiliate organizations in the World Cancer Research Fund (WCRF) Global Network will release the largest and most comprehensive report on diet, nutrition, physical activity and cancer ever undertaken," Prince said.
"We have convened an international expert panel that has gathered every six months for the past three years to review thousands of studies on diet and cancer. They are weighing the evidence critically and dispassionately. They are ranking this evidence to illustrate how strongly different aspects of how we eat and how we live influence the risk of cancer at specific body cites," Prince said.
When the AICR/WCRF expert report, Food, Nutrition, Physical Activity and Cancer: A Global Perspective is released next year, it will include a list of practical recommendations for the public based on the expert panelís review of the scientific literature. The first AICR/WCRF expert report was released in 1997.
"The study of diet's link to colon cancer and cancer in general stands at a crossroad," said Prince. "When the second AICR/WCRF expert report is published in November 2007, weíll have the roadmap to show us all, individuals, educators, and researchers alike, how to proceed."
The American Institute for Cancer Research (AICR) is the cancer charity that fosters research on diet and cancer and educates the public about the results. It has contributed more than $78 million for innovative research conducted at universities, hospitals and research centers across the country. AICR also provides a wide range of educational programs to help millions of Americans learn to make dietary changes for lower cancer risk. Its award-winning New American Plate program is presented in brochures, seminars and on its website, www.aicr.org. AICR is a member of the World Cancer Research Fund International
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