Introduction
What is a colorectal polyp?
What is the Polyp Prevention Trial?
What were the results of the Polyp Prevention Trial?
What is the Wheat Bran Fiber Study?
What were the results of the Wheat Bran Fiber Study?
What do these results mean in terms of reducing polyp
recurrence via dietary modification?
Why didn't these trials show a protective effect of
dietary changes or fiber supplements on colorectal polyps?
Why look at diet to prevent polyps?
Is there evidence that diet affects colorectal cancer risk?
Are other large trials looking at the role of diet in
cancer prevention?
What other cancer prevention trials are under way for
colorectal cancer?
What are the next steps?
Who is at risk for colorectal cancer?
What tests are used to screen people for colorectal
cancer? Do insurance companies pay for colorectal cancer screening?
Introduction
The Polyp Prevention Trial provided no evidence that adopting a low-fat,
high-fiber, fruit- and vegetable-enriched eating plan reduces the recurrence of
colorectal polyps. The Wheat Bran Fiber Study was a clinical trial to assess
the role of a wheat bran fiber supplement in the prevention of colorectal polyp
recurrence. This article answers some commonly asked questions about
the two studies.
What is a colorectal polyp?
A colorectal polyp is a growth on the inner wall of the colon or rectum.
Precancerous polyps (adenomatous polyps or adenomas) are fairly common in
people over age 50 -- it is estimated that about 40 percent of people over age
50 have them. Because most colorectal cancers develop in polyps, detecting and
removing these growths is a way to prevent colorectal cancer. About 5 percent
to 10 percent of polyps will become cancer if they are not removed, although
physicians cannot yet identify which polyps are likely to lead to cancer.
People who have had a polyp have an increased risk -- perhaps as much as 50
percent -- of developing more polyps within three years of their first
diagnosis.
What is the Polyp Prevention Trial?
The Polyp Prevention Trial was a clinical trial (a research study conducted
with volunteers) to determine the effect of a low-fat (20 percent of calories
from fat), high-fiber (18 grams per 1,000 calories), high fruit/vegetable (3.5
servings per 1,000 calories) eating plan on the recurrence of precancerous
polyps in the colon and rectum.
A group of 2,079 men and women were randomized (divided by chance); half had
intensive counseling to adopt a low-fat, high-fiber, fruit- and
vegetable-enriched eating plan and half were given a standard brochure on
healthy eating. Every participant had been diagnosed with a polyp within the
previous six months, which was removed at colonoscopy. The participants were on
the trial for four years. The trial was headed by the National Cancer Institute
(NCI) and carried out at eight clinical sites known as the Polyp Prevention
Trial Study Group.
What were the results of the Polyp Prevention
Trial?
The Polyp Prevention Trial provided no evidence that adopting a low-fat,
high-fiber, fruit- and vegetable-enriched eating plan reduces the recurrence of
colorectal polyps. Polyp recurrence rates were about the same in the two study
groups.
What is the Wheat Bran Fiber Study?
The Wheat Bran Fiber Study was a clinical trial to assess the role of a wheat
bran fiber supplement in the prevention of colorectal polyp recurrence. A group
of 1,429 men and women who had had one or more polyps removed at colonoscopy
within the previous three months were randomized to a high wheat bran fiber
cereal supplement (13.5 grams of fiber in 2/3 cup cereal per day) or low wheat
bran fiber cereal supplement (2 grams of fiber in 2/3 cup cereal per day). The
participants were on the supplements for at least three years. The study was
funded by NCI, headed by the Arizona Cancer Center, Tucson, Ariz., and carried
out by the Phoenix Colon Cancer Prevention Physician Network.
What were the results of the Wheat Bran Fiber
Study?
The Wheat Bran Fiber Study provided no evidence that adding a wheat bran fiber
cereal supplement to the diet reduces the recurrence of colorectal polyps.
What do these results mean in terms of reducing
polyp recurrence via dietary modification?
The results provided no evidence that the particular dietary interventions
employed (i.e., a low-fat, high-fiber, high-fruit and -vegetable eating plan or
a high-fiber cereal supplement) in the particular population studied
(individuals who had had one or more polyps removed at colonoscopy) were
effective in preventing the recurrence of polyps.
However, overall evidence suggests that a low-fat, high-fruit and -vegetable,
high-fiber diet has benefit in reducing the risk of many chronic diseases --
heart disease, hypertension, obesity, diabetes, and others. This trial
specifically looked at the effect of diet on the growth of new colorectal
polyps in people who had already had a polyp removed. A healthy diet does not
replace the need for people with a history of polyps to have regular checkups.
Why didn't these trials show a protective effect of
dietary changes or fiber supplements on colorectal polyps?
The scientists offered several possible reasons why these study designs did not
show an effect of diet on polyp recurrences, including:
-
Development of colorectal cancer takes decades, and an intervention of three to
four years may not be long enough to make a difference. Continuing follow-up of
these patients may lead to further understanding of the long-term impact of
dietary interventions.
-
Participants in these studies all had at least one polyp previously removed.
Nutritional factors may influence critical molecular, cellular, or tissue-level
events in colorectal cancer formation well before polyps are formed.
-
In these studies, the recurrent polyps -- the new polyps that develop after the
first ones are removed -- tended to be small. Dietary changes might affect only
the growth of small polyps into large polyps or large polyps into invasive
cancer.
Why look at diet to prevent polyps?
Dietary factors are believed to influence colorectal cancer risk, and polyps
are considered a mid-way point to colorectal cancer. If diet was found to
change the risk of polyp recurrence, that would be strong evidence that diet
could reduce the risk of colorectal cancer. Scientists would also have evidence
of the point at which diet influences colorectal cancer risk.
Is there evidence that diet affects colorectal cancer
risk?
Yes. Colorectal cancer rates vary widely from country to country; studies of
people who emigrate show that as they adopt a diet of the new country, their
colorectal cancer risk becomes more like the country to which they moved. This
shows that while genetic susceptibility (heredity) may play a role in
colorectal cancer, outside (environmental) factors also influence risk.
Studies in laboratory animals have shown that fat increases and fiber decreases
risk of colorectal tumors. Many epidemiologic studies looking at what people
eat (observational studies) have suggested that red meat and dietary fat
increase colorectal cancer risk while vegetables, dietary fiber, and some
nutrients lower the risk. For more detailed information, see
Prevention of Colorectal Cancer.
Are other large trials looking at the role of diet
in cancer prevention?
The Women's Health Initiative, an ongoing 15-year study supported by the
National Institutes of Health and administered by the National Heart, Lung, and
Blood Institute involves 68,000 women in a clinical trial examining the effect
of a diet low in fat and high in fruit, vegetables, and grains in preventing
breast and colorectal cancers and heart disease. The Women's Health Initiative
is also testing the ability of calcium and vitamin D supplements to prevent
fractures and reduce the risk of colorectal cancer. More information on the
study is available at http://www.nhlbi.nih.gov/whi
on the Internet.
What other cancer prevention trials are under way
for colorectal cancer?
The NCI is sponsoring a number of prevention trials in which drugs or
nutritional supplements are being tested to reduce the risk of developing
polyps and/or colorectal cancer. Agents under study include sulindac and
celecoxib (anti-inflammatory agents that inhibit an enzyme known as
cyclo-oxygenase), and the nutrients folic acid and calcium.
What are the next steps?
Polyp Prevention Trial researchers will continue to follow the participants in
the trial to see if the dietary changes they made influence their likelihood of
getting more polyps or colorectal cancer in the future. In addition, they are
using the material from the polyps that were removed to find out whether the
diet affected the molecular genetic characteristics of the polyps.
The Wheat Bran Fiber Study researchers will also continue to follow the
participants in their trial to see if they develop any additional polyps or
colorectal cancer in the future. The researchers will follow study participants
for lifestyle, genetic, and clinical factors that influence polyp recurrence.
Who is at risk for colorectal cancer?
Colorectal cancer results from complex interactions between a person's genetic
make-up and outside factors. Science has shown that these interactions take
decades to develop. Some of the factors that put people at risk include:
-
age (most cases occur in people over age 50)
-
diet
-
personal history of polyps, colorectal cancer, or inflammatory bowel disease
-
family history of polyps or colorectal cancer (in close relatives)
-
smoking, and
-
sedentary lifestyle.
What tests are used to screen people for
colorectal cancer? Do insurance companies pay for colorectal cancer screening?
People who are at risk for colorectal cancer should ask their doctor when to
begin screening, what tests to have, and how often to schedule appointments.
Doctors may suggest one or more of the tests listed below as a part of regular
checkups.
A fecal occult blood test (FOBT)
is a test for hidden blood in the stool. This test has been proven to reduce
the death rate of colorectal cancer.
A sigmoidoscopy
is an examination of the rectum and the lower part of the colon with a lighted
instrument.
A colonoscopy
is an examination of the rectum and entire colon with a lighted instrument.
A double contrast barium enema
is a series of X-rays of the colon and rectum. The X-rays are taken after the
patient is given an enema with a white, chalky solution that contains barium to
outline the colon and rectum on the X-rays.
A digital rectal exam or DRE
is a test in which the doctor inserts a lubricated, gloved finger into the
rectum to feel for abnormal areas.
People should check with their health insurance provider to determine their
colorectal cancer screening benefits. People who are age 50 or older and are
covered by Medicare are eligible for colorectal cancer screening benefits.
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