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FEATURE ARTICLE - Phytochemicals and Cancer Prevention
MEDICAL NEWS ALERT - Dietary Supplements containing KAVA, linked to liver toxicity
FEATURE ARTICLE - Phytochemicals and Cancer Prevention - Back to Top
It is unreasonable and impractical to perform large-scale double blind studies on diet and its
relationship to cancer. Therefore, much of our knowledge about what constitutes the optimal
cancer prevention diet comes from population (epidemiological) studies or animal studies.
That is if you look at a culture with a high intake of fruits and vegetables and they have a
low incidence of cancer and this continues to be the case among different cultures as well,
then a statement can be made about that trend. Cohort studies take a group of individuals
(the cohort) and follow their food intake over time and see if there is an association between
aspects of the diet and cancer. Other studies look at individuals with a particular cancer and
then go back over their diets to see what features they had in common that might have
resulted in cancer. Examining data from numerous epidemiological studies, the World Cancer
Research Fund concluded that the evidence that fruits and vegetables may reduce the risk
of oral, esophageal, lung, stomach, and colon cancer was convincing and that the evidence
for an association between intake of fruits and vegetables with breast, pancreatic, and
bladder cancer was probable. It is unclear whether any single substance in a plant based
diet accounts for this relationship. Most current research, however, has focused on the
protective role of individual phytochemicals (plant-based chemicals). The
National Cancer Institute recommends eating 5 serving of fruits and vegetables a day.
However more recent studies suggest that more is in fact better. Although 5-a-day
is recommended by the National Cancer Institute, higher levels of vegetable
intake may be more beneficial for reducing cancer risk. Furthermore, current
evidence suggests that vegetables are more effective than fruits in reducing
cancer.
Vegetables, fruits, and whole grains contain a wide variety of phytochemicals that have the
potential to interfere with the development of cancer. Let’s look at some of these
classes of phytochemicals and what important foods contain them.
Cruciferous vegetables
Carotenoid containing vegetables
The major sources of lycopene are tomatoes and tomato products. Studies show
that populations with the highest tomato intake or blood lycopene levels have the lowest risk
of cancer. This association is highest for prostate, lung and stomach cancers. Lycopene may
account for or contribute to these benefits, but this possibility is not yet proven. The availability
of lycopene from different food items varies considerably. Lycopene concentrations
are highest in cooked tomatoes.
Polyphenol containing fruits
Here then is my summary of recommendations for vegetable and fruit
consumption as part of a cancer prevention diet:
Vegetables
Fruits
Medical News Alert -
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Dietary supplements containing kava (also known as kava kava or Piper methysticum) are
promoted for a variety of uses, including relaxation (e.g., to relieve stress, anxiety, and tension) and insomnia.
Products containing herbal extracts of kava have been implicated in cases of serious liver
toxicity in Germany and Switzerland. Approximately 25 reports of liver toxicity associated
with the use of products containing kava extracts have been reported in these countries.
Serious adverse effects include hepatitis, cirrhosis, and liver failure. At least one patient required a
liver transplant. Based on their assessment of the adverse events reported to them, the regulatory authority in Switzerland has prohibited the sale of products containing the kava extract associated
with the adverse effects. Last month, the German authorities issued a proposal to remove all
kava extract-containing products from the market. The French Agency for the Safety of
Health Products suspended the sale of products and preparations containing kava in France.
The FDA is investigating whether the use of kava-containing dietary supplements in the
United States poses similar public health concerns.
It must be emphasized that these cases involved relatively high doses of kava taken for several months. In my practice, I have prescribed kava extracts for many years for anxiety and insomnia and have never seen evidence of liver toxicity. The apparent toxicity may be an issue of product contamination or individual variation in the ability to metabolize the herb. If you have found kava helpful, I see no reason to discontinue use as long as it is occasional and not continuous and the product is from a reputable manufacturer. The suggested dose of kava is 75-100 mg three times daily of a standardized extract containing 30-50% kavalactones. I will keep you updated on this issue.
Cruciferous vegetables (i.e., broccoli, brussels sprouts, cauliflower, and cabbage)
contain isothiocyanates, which may provide protection against
environmental carcinogen exposure by inducing detoxification pathways thereby
neutralizing potential carcinogens. Additionally, these vegetables contain
indole-3-carbinol (I3C). I3C has been shown to reduce the risk of
breast cancer by decreasing estrogen activity.
Population studies show an association between high dietary intake of carotenoid-containing
fruits and vegetables and reduced risk of prostate, breast, and head and neck cancers. A high
dietary intake of fruits and vegetables provides a spectrum of carotenoids including
alpha-carotene, gamma-carotene, beta-cryptoxanthin, zeaxanthin, lutein, and
lycopene. Vegetable juices (carrot, tomato, spinach, and other greens) represent a particularly potent form of carotenoids. Scientists have demonstrated that drinking
carrot juice significantly reduces free radical damage to genes. Tomato juice also reduces
the oxidation of the "bad" LDL cholesterol in healthy men which can lead to atherosclerosis. Carotenoid-rich extracts of carrots and tomatoes have been shown to substantially inhibit
the early stages of liver cancer in animals.
Fruits contain various key phytochemicals. Resveratrol, present in grapes and
wines, has been reported to exert a variety of anticancer effects. Studies have demonstrated
resveratrol causes growth inhibition of human colon cancer cells. Other studies suggest
resveratrol may have an inhibitory effect on breast cancer cell growth. Ellagic acid found
in strawberries and grapes, blueberries, raspberries and blackberries, is another polyphenol
that has demonstrated anticarcinogenic potential in animal studies.
Other sources of phytochemicals
Garlic contains two substances, glutathione-S-transferase and diallyl sulphide,
which appear to increase the elimination of environmental carcinogens by the liver. Other
substances in garlic act to improve immune function.
A number of studies have suggested that the regular consumption of tea, particularly
non-fermented green tea, moderately decreases the risk of cancer, especially
cancers of the upper digestive tract. However a recent a cohort study conducted in Japan,
reported no association between the consumption of green tea and the risk of gastric cancer.
With respect to anticarcinogenic activity, the substance in green tea that has generated the
most interest is a potent, naturally occurring antioxidant, epigallocatechin
gallate (EGCG). The mechanism of action of EGCG and other similar substances is
uncertain, but they may function in several ways: by acting as an antioxidant, and by
inhibiting enzymes involved in cell division.
From a practical standpoint, the research suggests that to derive any cancer
preventive effect from green tea intake would require drinking 10-12 cups per
day. Note that milk added to green tea will reduce its antioxidant activity.
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