by J. W. Chiao, Ph.D, Jyothi Parapurath, M.D. and Hongyan Wu, M.D.
Cruciferous Vegetables
and Cancer Preventive Potential
What are cruciferous vegetables?
It is the classification
name for vegetables in the
cabbage or mustard family.
The family Cruciferae consists of about
3,200 species in 375 genera. Cruciferae is
derived from the Latin word “crucifer,”
meaning branch-like. They include the varieties
of cabbage (Shetland, red, savoy, pakchoi,
Portugal, wild, etc.), kale (curly,
Scotch, Siberian, etc.), broccoli (sprouting,
cape, nine-star perennial, etc.), mustard
(brown, black, Indian, Chinese, etc.),
turnips, rutabagas, and also radish, rap, cauliflower,
kohlrabi, watercress, Brussels
sprouts, and others. In Europe, cabbage,
Brussels sprouts, cauliflower and broccoli are
consumed, mostly in that order.
Cruciferous vegetables were used for
medicinal purposes in ancient times.
As early as the 1st century, Roman botanist
Pliny wrote about a vegetable used in Asia
Minor, which might be broccoli or cauliflower,
with numerous heads and a stalk.
Sea-going traders carried the plant to Italy
and propagated it. One of the medicinal
uses of the vegetables, as interpreted in
modern days, is the prevention of diseases
related to cancer. The preventive effects
have been suggested to be partly due to the
high content of sulfur containing compounds
like glucosinolates and their
hydrolyzed products which distinguishes
them from other vegetables. Cutting and
chewing of the vegetable releases a plant
enzyme that hydrolyzes and breaks down
glucosinolates, eventually yielding a
number of small chemicals called thiocyanates,
nitriles, and isothiocyanates.
There are a variety of isothiocyanates associated
with different cruciferous vegetables.
These chemicals are largely responsible for
the characteristic aroma and smells released
during cooking of these vegetables (broccoli,
cauliflower and cabbage).
Population studies.
There is epidemiological evidence that the
incidence of cancer varies in different regions
of the world. As populations move to a highrisk
area, they assume the risk of that geographic
region. This points to the potential
relationship of diets with the cancer risk.
Evidence concerning the cancer-preventive
effects of cruciferous vegetables has come
from the results of more than 70 case-control
studies of populations worldwide during the
past three decades. Such studies compared
the past diet patterns between hundreds of
patients and disease-free controls. About
64 percent of the studies showed a relationship
between cruciferous vegetable consumption
with a lower risk of cancer of the
colon, oral cavity, pharynx, lung and rectum.
Consumption of one or more kinds of
cruciferous vegetables showed these benefits.
Raw forms of the vegetables were generally
more consistent with the cancer preventive
effects. A recent population-based case-control study of men under 65 years of age
showed that three or more servings of cruciferous
vegetables per week significantly
reduced prostate cancer cases as compared to
that of one or less serving per week.
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Figure 1 legend:
The targets of cruciferous vegetables for cancer chemoprevention. The constituents of cruciferous vegetables are able to decrease the phase 1 enzymes and increase the phase 2 enzymes. These processes block the effects of the cancer-causing carcinogens and facilitate their excretion. |
Mechanism of cancer chemoprevention.
How do the vegetables prevent cancers? One
of the known mechanisms is that the vegetable
constituents facilitate the excretion of cancercausing
materials, called carcinogens, after we get
exposed to them in our
environment. Thus the
initiation of the cancer
process by the carcinogens
is avoided. As described in
Figure 1, dietary and environmental
carcinogens
require an enzymatic
transformation to exert
their carcinogenic effects.
This process is commonly
catalyzed by a set of phase
1 enzymes which insert an
atom of molecular oxygen.
The electrophiles formed
during this reaction can
bind to sites on DNA, RNA
and proteins which may then become permanent damaged. However,
these toxic electrophiles can be readily excreted
before the damage is done, with the aid of a
group of phase 2 detoxifying enzymes. It has
been shown that the cruciferous vegetable constituents
can prompt cells to manufacture less
of the phase 1 enzymes, and more of the phase
2 detoxifying enzymes, thereby removing the
harmful substances more effectively.
Much evidence about the cancer-preventive
effects of the constituents, i.e. hydrolyzed
products of glucosinolates such as isothiocyanates,
has come from studies in animals.
When provided as part of the daily feed to
experimental animals, the constituents
reduced cancers induced in the lung, mammary
gland, esophagus, liver, pancreas,
stomach, small intestine, colon and bladder of
mice, rats and other rodents. Recently it was
demonstrated, in mice, that a major isothiocyanate
(sulforaphane) present in broccoli was
able to inhibit the bacteria that causes
stomach ulcers (H. pylori) and thereby also
prevent stomach cancer. It is expected that
multiple processes are involved with the constituents
of cruciferous vegetables in preventing
a complicated disease like cancer.
Scientists have also described how certain
isothiocyanates are capable of reducing the growth of some human cancer cells, implying
that cruciferous vegetables might even be beneficial to controlling the development of cancers.
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| . . . cruciferous vegetables (like broccoli)
have been eaten since ancient times . . . |
Sizable quantities of the natural constituents
can be obtained from eating regular
portions of cruciferous vegetables. For example,
an individual who ingests about one quarter of
a head of broccoli would intake about 50–200
µmol of sulforaphane, an isothiocyanate
known to be active. After consuming two ounces of watercress, an
average portion, 12 milligrams
of a major isothiocyanate
is released.
Cruciferous vegetables are
also known to have good
nutrient contents. For
example, one cup of
cooked broccoli (240 ml)
yields twice the amount of
vitamin C as two oranges.
It also contains only 50
calories (less than one-third
the calories from a cup of
cooked rice) and 5 grams of
protein (equivalent to a
cup of rice or corn).
“Prevention is better
than cure” is indeed true
with the dietary effects of cruciferous vegetables. Preventive interventions
allow more opportunities than the treatment
modalities to control diseases. As
preventive measures can be applied regularly,
it will cover the whole course of disease development
from initiation to high-risk stages.
While treatments can only be applied after the
diseases are discovered, by then many cases are
already beyond the low-risk stage. This
strategy is especially practical for cancer prevention,
since cancer usually takes many years
or even decades to develop and is often unnoticed
at the early stages. Therefore, the application
of strategies for the prevention of
disease with diets is a very practical goal,
reducing the risk factors and pushing the
occurrence of disease until much later in life.
As cruciferous vegetables have been eaten
since ancient times without any known
harmful effects and the benefits have been
shown to be significant, they should be
included as a component of a healthy diet.
J. W. Chiao, Ph.D., Jyothi Parapurath,
M.D. and Hongyan Wu, M.D. have been
involved in cancer chemopreventive
studies for a number of years. They are
affiliated with New York Medical College
in Valhalla, New York
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