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Other common
name(s): antioxidants, flavonoids, flavones, isoflavones,
catechins, anthocyanidins, isothiocyanates, carotenoids, allyl
sulfides, polyphenols
Scientific/medical
name(s): various names
Description
The term "phytochemicals" refers to a wide variety of
compounds produced by plants. They are found in fruits, vegetables,
beans, grains, and other plants. Scientists have identified thousands
of phytochemicals, although only a small fraction have been studied
closely. Some of the more commonly known phytochemicals include beta
carotene, ascorbic acid (vitamin C), folic acid, and vitamin E.
Overview
Some phytochemicals have either antioxidant or hormone-like
actions. There is some evidence that a diet rich in fruits, vegetables,
and whole grains reduces the risk of certain types of cancer and other
diseases, and researchers are looking for specific compounds in these
foods that may account for the beneficial effects in humans. Available
scientific evidence does not support claims that taking phytochemical
supplements is as helpful as consuming the fruits, vegetables, beans,
and grains from which they are taken.
How are they promoted for use?
Phytochemicals are promoted for the prevention and treatment
of many health conditions, including cancer, heart disease, diabetes,
and high blood pressure. There is some evidence that certain
phytochemicals may help prevent the formation of potential carcinogens
(substances that cause cancer), block the action of carcinogens on
their target organs or tissue, or act on cells to suppress cancer
development. Many experts suggest that people can reduce their risk of
cancer significantly by eating more fruits, vegetables, and other foods
from plants that contain phytochemicals.
There are several major groups of phytochemicals.
The polyphenols include a large subgroup of chemicals called
flavonoids. Flavonoids are plant chemicals found in a broad range of
fruits, grains, and vegetables. They are being studied to find out
whether they can prevent chronic diseases such as cancer and heart
disease. The isoflavones found in foods and supplements such as soy
products, red clover, garbanzo beans and licorice, and the lignans
found in flaxseed and whole grains may mimic the actions of the female
hormone estrogen (see Licorice
and Soybean).
These estrogen-like substances from these plant sources are called
phytoestrogens. They may play a role in the development of and
protection against some hormone-dependent cancers such as some types of
breast and prostate cancer.
Other polyphenols (including some flavonoids) act as
antioxidants. These are thought to rid the body of harmful molecules
known as free radicals, which can damage a cell's DNA and may trigger
some forms of cancer and other diseases. These compounds are commonly
found in vegetables such as broccoli, brussels sprouts, cabbage, and
cauliflower and in teas. Grapes, eggplant, red cabbage, and radishes
all contain anthocyanidins—flavonoids that act as
antioxidants and may protect against some cancers and heart disease.
Quercetin, another flavonoid with antioxidant properties, is found in
apples, onions, teas, and red wine. Ellagic acid, found in raspberries,
blackberries, cranberries, strawberries, and walnuts, also is said to
have anti-cancer effects (see Ellagic Acid).
Carotenoids, which give carrots, yams, cantaloupe, squash, and
apricots their orange color, are also promoted as anticancer agents.
Tomatoes, red peppers, and pink grapefruit contain lycopene, which
proponents claim is a powerful antioxidant (see Lycopene).
The phytochemicals lutein and zeaxanthin, found in spinach, kale, and
turnip greens, may reduce the risk of some cancers.
Another group of phytochemicals, called allyl sulfides, are
found in garlic and onions (see Garlic).
These compounds may stimulate enzymes that help the body get rid of
harmful chemicals. They may also help strengthen the immune system.
What does it involve?
Phytochemicals are present in virtually all of the fruits,
vegetables, legumes (beans and peas), and grains we eat, so it is quite
easy for most people to include them in their diet. For instance, a
carrot contains more than a hundred phytochemicals. There are thousands
of known phytochemicals, but only a few have been studied in detail.
Many of the better-known phytochemicals are now available as
dietary supplements. However, most available evidence suggests that
these single supplements are not as beneficial as the foods from which
they are derived.
What is the history behind it?
Only a few years ago, the term "phytochemical" was barely
known. But doctors, nutritionists, and other health care practitioners
have long advocated a low-fat diet that includes a variety of fruits,
vegetables, legumes, and whole grains. Historically, cultures that
consume such a diet have lower rates of certain cancers and heart
disease.
Since the passage of the Dietary Supplement Health and
Education Act (DSHEA) in the United States in 1994, a large number of
phytochemicals are being sold as dietary supplements.
What is the evidence?
It has become a widely accepted notion that a diet rich in
fruits, vegetables, legumes, and grains reduces the risk of cancer,
heart disease, and other illnesses. But only recently have researchers
begun to try to learn the effects of specific phytochemicals contained
in those foods.
Much of the evidence so far has come from observations of
cultures in which the diet consists mainly of plant sources, and which
seem to have lower rates of certain types of cancer and heart disease.
For instance, the relatively low rates of breast and endometrial
cancers in some Asian cultures are credited at least in part to dietary
habits. These cancers are much more common in the United States,
possibly because the typical American diet is higher in fat and lower
in fruits, vegetables, legumes, and grains.
Because of the number of phytochemicals and the complexity of
the chemical processes in which they are involved, it is difficult for
researchers to determine which phytochemicals in foods may fight cancer
and other diseases, which may have no effect, and which may even be
harmful.
Many studies have looked at the relationship between cancer
risk and eating fruits and vegetables, legumes, and whole grains. Most
of the evidence indicates that eating a diet high in these foods seems
to lower the risk of some cancers and other illnesses.
Some of the links between individual phytochemicals and cancer
risk found in laboratory studies are compelling and make a strong case
for further research. So far, however, none of the findings are
conclusive. It is still uncertain which of the many phytochemicals in
fruits and vegetables actively helps the body fight disease.
Researchers have also shown much interest in phytochemical
supplements. Some laboratory studies in cell cultures and animals have
shown that certain phytochemicals have some activity against cancer
cells or tumors. But at this time there have been no strong studies in
humans showing that any phytochemical supplement can prevent or treat
cancer.
Until conclusive research findings emerge, health care
professionals advise a balanced diet with an emphasis on fruits,
vegetables, legumes, and whole grains. The interaction between certain
phytochemicals and the other compounds in foods is not well understood,
but it is unlikely that any single compound offers the best protection
against cancer. A balanced diet that includes 5 or more servings a day
of fruits and vegetables, along with foods from a variety of other
plant sources such as nuts, seeds, whole grain cereals, and beans is
likely to be more effective in reducing cancer risk than consuming one
particular phytochemical in large amounts.
Are there any possible problems or
complications?
These products are sold as
dietary supplements in the United States. Unlike companies that produce
drugs (which must provide the FDA with results of detailed testing
showing their product is safe and effective before the drug is approved
for sale), the companies that make supplements do not have to show
evidence of safety or health benefits to the FDA before selling their
products. Supplement products without any reliable scientific evidence
of health benefits may still be sold as long as the companies selling
them do not claim the supplements can prevent, treat, or cure any
specific disease. Some such products may not contain the amount of the
herb or substance that is written on the label, and some may include
other substances (contaminants). Though the FDA has written new rules
to improve the quality of manufacturing processes for dietary
supplements and the accurate listing of supplement ingredients, these
rules do not take full effect until 2010. And, the new rules do not
address the safety of supplement ingredients or their effects on health
when proper manufacturing techniques are used.
Most such
supplements have not been tested to find out if they interact with
medicines, foods, or other herbs and supplements. Even though some
reports of interactions and harmful effects may be published, full
studies of interactions and effects are not often available. Because of
these limitations, any information on ill effects and interactions
below should be considered incomplete.
Phytochemicals, in the amounts consumed in a healthy diet, are
likely to be helpful and are unlikely to cause any major problems. Some
people assume that because phytochemical supplements come from
"natural" sources, they must be safe and free from side effects, but
this is not always true. Many phytochemical supplements, especially
when taken in large amounts, have side effects and may interact with
some drugs. Some of these interactions may be dangerous. Before taking
a phytochemical in supplement form, consider talking to your doctor and
pharmacist to be sure it will not interact with other medicines or
herbs you may be taking. Relying on the use of phytochemicals alone and
avoiding or delaying conventional medical care for cancer may have
serious health consequences.
Additional Resources
More information from your American Cancer
Society
The following information on complementary and alternative
therapies may also be helpful to you. These materials may be found on
our Web site (www.cancer.org)
or ordered from our toll-free number (1-800-ACS-2345).
References
Craig WJ. Health-promoting properties of common herbs. Am J Clin Nutr.
1999;70:491S-499S.
Gruenwald J. PDR
for Herbal Medicines. 3rd ed. Montvale, NJ: Thomson PDR;
2004.
Kushi LH, Byers T, Doyle C, Bandera EV, McCullough M,
McTiernan A, Gansler T, Andrews KS, Thun MJ; American Cancer Society
2006 Nutrition and Physical Activity Guidelines Advisory Committee.
American Cancer Society guidelines on Nutrition and Physical Activity
for cancer prevention: reducing the risk of cancer with healthy food
choices and physical activity. CA
Cancer J Clin. 2006;56:254-281.
Setchell KD, Cassidy A. Dietary isoflavones: biological
effects and relevance to human health. J Nutr.
1999;129:758S-767S.
Wang YH, Chao PD, Hsiu SL, Wen KC, Hou YC. Lethal
quercetin-digoxin interaction in pigs. Life Sci.
2004;74:1191-1197.
Note: This information may not cover
all possible claims, uses, actions, precautions, side effects or
interactions. It is not intended as medical advice, and should not be
relied upon as a substitute for consultation with your doctor, who is
familiar with your medical situation.
Last Medical Review: 11/01/2008
Last Revised: 11/01/2008
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