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Institute for Disease Prevention, George
Washington University Medical Center, Washington
DC, USA.
Comparative international epidemiological
data indicate that the difference between
the highest and lowest colon cancer incidence
is approximately 10-fold. This suggests that
the dominant causes of colon cancer are environmental
rather than genetic in origin, with the dominant
environmental cause being the typical diet
of Western industrialized countries.
Many
epidemiological and experimental studies
have suggested an important role for dietary
fiber in the prevention of colon cancer.
Using the Fischer-344 rat as the experimental
model, data clearly demonstrate a strong
protective effect of a diet that is low in
fat, high in fiber and high in calcium (low-risk
diet). Such a diet prevents the development
of both preneoplastic aberrant crypt foci
(ACF) and colon tumors.
Recent experiments
have also demonstrated a direct relationship
between a ras point mutation in ACF at different
stages of rat colon carcinogenesis, and a
ras point mutation that is subsequently present
in colon tumors. Using wheat bran as the
model dietary fiber source, its effects were
compared to the effects of psyllium, phytic
acid, vitamin E, beta-carotene, folic acid,
alone or in combination, for their ability
to prevent colon cancer in rats on high-risk
Western-style diets.
Our studies clearly
demonstrated the ability of wheat bran to
reduce ACF and colon tumors in rats that
consumed high-fat, Western-style diets. Although
phytic acid, which is a constituent of wheat
bran, alone demonstrated strong cancer-preventive
potential, our experiments provided evidence
for the cancer-preventive effect of the crude
fiber fraction that is independent of the
effect of phytic acid.
The synergistic combination
of wheat bran with the soluble fiber psyllium
led to enhanced protection; while the combination
of wheat bran with beta-carotene showed only
an additive effect. Beta-carotene appeared
to show higher protection than wheat bran
at an intake level that is nutritionally
relevant to humans, suggesting the possibility
of using beta-carotene to enhance the effects
of dietary fiber in high-risk Western populations.
Using
ACF as an intermediate endpoint, it was also
shown that vitamin E and beta-carotene appear
to inhibit progression of ACF to colon cancer,
while wheat bran and folic acid appeared
to have weak cancer-preventive potential
at this late stage of carcinogenesis. In
conclusion, wheat bran alone, or in combination
with psyllium, appears to have greater potential
to inhibit earlier phases of carcinogenesis,
while beta-carotene and vitamin E may also
inhibit later stages of carcinogenesis.
Despite
considerable epidemiological and experimental
evidence that increasing the fiber and lowering
the fat content of the Western diet could
substantially reduce the risk of cancer and
heart disease, the real challenge is to find
effective ways to educate and motivate people
to overcome their intrinsic cultural resistance
to such changes in their eating habits.
PMID: 8657180 [PubMed - indexed for MEDLINE] |
