|
Cancer Prevention Research Program, Fred
Hutchinson Cancer Research Center, Seattle,
Washington, USA.
The practice of medicine-both
past and present-often involves the prescription
of specific foods (almost always plants)
or their potent derivatives, to treat a wide
spectrum of illnesses. Foods that have been
ascribed healing properties include the Cruciferae,
the allium family, celery, cucumber, endive,
parsley, radish and legumes.
Review of the
epidemiological data, including both cohort
and case-control studies, of all cancer sites
strongly suggests that plant foods also have
preventive potential and that consumption
of the following groups and types of vegetables
and fruits is lower in those who subsequently
develop cancer: raw and fresh vegetables,
leafy green vegetables, Cruciferae, carrots,
broccoli, cabbage, lettuce, and raw and fresh
fruit (including tomatoes and citrus fruit).
Other
data suggest that foods high in phytoestrogens,
particularly soy (which contains isoflavones),
or high in precursor compounds that can be
metabolized by gut bacteria into active agents,
particularly some grains and vegetables with
woody stems (which contain precursors to
lignans), are plausibly associated with a
lower risk of sex-hormone-related cancers.
The
human evidence for these latter associations
is not strong. There are many biologically
plausible reasons why consumption of plant
foods might slow or prevent the appearance
of cancer. These include the presence in
plant foods of such potentially anticarcinogenic
substances as carotenoids, vitamin C, vitamin
E, selenium, dietary fibre (and its components),
dithiolthiones, isothiocyanates, indoles,
phenols, protease inhibitors, allium compounds,
plant sterols, and limonene.
Phytoestrogens
are also derived from some vegetables and
berries as well as grains and seeds. Most
of the data for the observations on the anticarcinogenic
potential of all of these compounds have
come from animal and in vitro studies.
At
almost every one of the stages of the cancer
process, identified phytochemicals are known
to be able to alter the likelihood of carcinogenesis-occasionally
in a way that enhances risk but usually in
a favourable direction.
For example, glucosinolates
and indoles, thiocyanates and isothiocyanates,
phenols, and coumarins can induce a multiplicity
of phase II (solubilizing and usually inactivating)
enzymes; ascorbate and phenols block the
formation of carcinogens such as nitrosamines;
flavonoids and carotenoids act as antioxidants,
essentially disabling the carcinogenic potential
of specific compounds; lipid-soluble compounds
such as carotenoids and sterols may alter
membrane structure or integrity; some sulphur-containing
compounds suppress DNA and protein synthesis;
carotenoids can suppress DNA synthesis and
enhance differentiation; and phytoestrogens
compete with estradiol for estrogen receptors
in a way that is generally antiproliferative.
Consumption
of diets low in plant foods results in a
reduced intake of a wide variety of those
substances that can plausibly lower cancer
risk. In the presence of a diet and lifestyle
high in potential carcinogens (whether derived
from fungal contamination, cooking or tobacco)
or high in promoters (such as salt and alcohol),
overall risk of cancer at many epithelial
sites is elevated.
Plant foods appear to
exert a general risk-lowering effect; the
patterns of exposure to cancer initiators
and promoters and of genetic susceptibility
may determine the variations in the site-specific
risks of cancer seen across populations.
PMID: 8923020 [PubMed - indexed for MEDLINE] |
