Study questions value of diet restrictions prior to colon cancer screening test

August 15, 2001

CHAPEL HILL - Avoidance of certain foods before taking a commonly recommended screening test for colon cancer may not be necessary, according to a report from the University of North Carolina at Chapel Hill.

The report appears Sept. 15 in Effective Clinical Practice, a publication of the American College of Physicians. The findings are based on an analysis of five randomized clinical trials published in the medical literature. The report points out that the proportion of people who test positive for fecal occult blood (traces of blood in the stool) is about the same whether or not they are advised to restrict their diet before obtaining the necessary stool samples.

Moreover, the report notes that dietary restrictions calling for avoidance of red meat, dark fish, and certain vegetables for several days before and during fecal occult blood testing (FOBT) might pose a barrier to screening.

This barrier, if lifted, may result in more people completing the test, the report suggests. "Our systematic review of randomized trials has shown that [test] completion rates do not differ when modest dietary restrictions are recommended, but more severe restrictions may reduce them," state the authors headed by Michael P. Pignone, MD, MPH, assistant professor of medicine at the UNC-CH School of Medicine.

Dietary restrictions for FOBT are meant to avoid getting erroneous test results, either false negative or false positive. Along with avoiding certain foods, current FOBT guidelines to doctors also recommend advising patients to abstain from aspirin and related anti-inflammatory drugs (NSAIDS) because of concern that they may also cause false positive results. Avoidance of vitamin C in amounts over 250 milligrams is recommended because it can cause false negative results.

Pignone, a member of UNC Lineberger Comprehensive Cancer Center, points out that screening for colorectal cancer with FOBT, despite its demonstrated effectiveness, remains underutilized. "Only 20 to 25 percent of U.S. adults older than 50 have had the test in the past year. In some studies, patients have reported difficulty in preparing for or performing the test," he said.

Pignone and his co-authors note that in one clinical trial in which an especially restrictive diet was used, 22 percent fewer people completed FOBT compared to those who did not receive dietary restrictions (51.3% versus 72.7%). In an accompanying editorial, Harvard Medical School professor Robert H. Fletcher, MD, MSC agreed with the report's conclusions. "I believe it is time to simplify dietary advice for FOBT testing, when possible," he wrote.

Fletcher, a former co-division chief of internal medicine at UNC-CH, added: "As for the diet itself, I recommend advising something similar to the following: 'Do not take any pain medications or more than a single multivitamin per day.'" He says such advice could make it easier for the occasional patient who finds FOBT dietary advice burdensome - at the cost of few, if any, erroneous test results.
Pignone's study co-authors were Marci Kramish Campbell, PhD, MPH, RD, associate professor of nutrition, School of Public Health and a member of the Lineberger Cancer Center; Carol Carr, MA, also with the center's Cancer Prevention and Control Project; and Christopher Phillips, MD, MPH, U.S. Force Medical Operations Agency, San Antonio, Texas.

Pignone's research is supported by an American Cancer Society Career Development Award.

Media note: Contact Dr. Pignone at 919-966-2276, School of Medicine contact: Leslie Lang, 919-843-9687, Lineberger Cancer Center contact, Diane Shaw, 919-966-5905,

University of North Carolina Health Care

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