Polyps unaffected by healthy diet
By
LOIS BAKER
News Services Editor
Adopting
a diet low in fat and high in fruits and vegetables may have many health
benefits, but lowering the risk of developing recurrent colon polyps,
precursors of colon cancer, does not appear to be one of them.
Investigators
from UB and the seven other centers involved in the five-year national
Polyp Prevention Trial report in today's issue of The New England Journal
of Medicine that there was no difference in the recurrence of polyps between
the diet intervention group and a control group that didn't change diets.
The study was sponsored by the National Cancer Institute.
Peter Lance,
associate professor of medicine and physiology, and principal investigator
on UB's portion of the trial, said he was disappointed but not altogether
surprised by the negative results.
"The study
is consistent with the findings of previous smaller trials and of another
large trial (Wheat Bran Fiber Study) published in the same issue. Clearly,
dietary change does not make you less likely to develop new polyps after
you've had all existing polyps removed.
"What the
study does not address," he noted, "and was not designed to address, is
what the environment of the colon should be to reduce the likelihood that
small, innocent polyps will grow to become cancers."
Fifty percent
or more of people over the age of 55 will develop at least one polyp of
the colon during their lifetime, but only a very small number of these
polyps progress to cancer, Lance said. "It remains to be seen whether,
or at what stage, altering the diet keeps people from developing colon
cancer," he said. "We followed these participants only for four years,
and cancer develops over a number of decades. Perhaps intervening earlier
in life would result in a different outcome."
Despite the
negative results on polyp recurrence, Lance pointed out one promising
finding of the study: People can change their diets for the better and
can stick with it.
The PPT trial
involved 1,905 people, all of whom had had benign colon polyps removed
within the previous six months. UB contributed 262 participants, or about
12.5 percent of the total.
Although
few polyps-abnormal growths of the colon lining-progress to cancer, cancer
develops only when polyps are present. The PPT trial was designed to determine
if eating a diet composed of 20 percent or less of total calories from
fat, at least 18 grams of dietary fiber for every 1,000 calories consumed,
and lots of fruits and vegetables could prevent polyps from growing back
after they were removed.
Participants
were assigned randomly to one of two groups: an intervention group, which
received intensive counseling on adopting the new diet; and a control
group, which received a standard brochure on healthy eating. All underwent
colonoscopy at one and four years after randomization, and all completed
food-frequency questionnaires throughout the study.
Based on
these questionnaires, the intervention was remarkably successful in changing
eating habits: Participants in that group cut their fat intake by about
one-third, increased their fiber consumption by nearly 75 percent, and
ate about two-thirds more fruits and vegetables than before the study.
In contrast, participants in the control group made only small changes
in these three diet components.
However,
the percentage of participants who developed at least one recurrent polyp
during the study was nearly identical in both groups: 39.7 percent (intervention)
and 39.5 percent (controls). In addition, there was no difference between
the groups in the mean number of recurrent polyps per person, nor in their
size or degree of progression.
The researchers
noted that despite these findings, the idea that a healthy diet may lower
the risk of colon cancer should not be discarded entirely. Several factors,
including study length and potential reporting errors, could have influenced
this study's findings, Lance said. In addition, such a diet in known to
have a favorable impact on cardiovascular disease and other chronic conditions.
In addition
to UB, the seven clinical sites involved in the study were Veterans Administration
Medical Center, Hines, Ill.; Kaiser Foundation Research Institute, Oakland,
Calif.; Memorial Sloan-Kettering Cancer Center, New York City; University
of Pittsburgh; University of Utah, Salt Lake City; Wake Forest School
of Medicine, Winston-Salem, N.C., and Walter Reed Army Medical Center,
Washington, D.C.
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