Other common name(s): quercetine, sophretin, meletin
Scientific/medical name(s): 3,3',4',5,7-pentahydroxyflavone
Quercetin is a type of plant-based chemical, or phytochemical, known as a flavonoid (see also Phytochemicals). Good sources include apples, onions, teas, red wines, and many other foods. Quercetin is also available as a dietary supplement.
Quercetin appears to have anti-inflammatory and antioxidant properties. It has been promoted as being effective against a wide variety of diseases, including cancer. While some early lab results appear promising, as of yet there is no reliable clinical evidence that quercetin can prevent or treat cancer in humans.
How is it promoted for use?
Quercetin is said to have a number of uses, but most of these are based on early findings from laboratory studies. Some early studies have suggested quercetin has antihistamine properties, and it is often promoted to help control allergies and asthma. Some proponents claim it can help stabilize small blood vessels and may help protect against heart attacks and strokes.
Quercetin is sometimes promoted to help prevent or treat different types of cancer. It has also been promoted to help with the symptoms of chronic prostatitis (swelling of the prostate gland) and to relieve some of the neurologic complications of diabetes.
What does it involve?
Quercetin is a common chemical pigment in the rinds and barks of a wide variety of plants. It is one of the main flavonoids in the diet, and is found in large amounts in apple skins, onions, tea, and red wine. It is also found in leafy green vegetables, berries, and in herbs such as ginkgo and St. John's wort.
Quercetin is available in higher amounts in dietary supplements than would typically be found in food sources. Supplements are sold as capsules or tablets ranging in doses from 50 milligrams (mg) to 500 mg. There is no recommended standard dose for quercetin.
What is the history behind it?
Plants containing flavonoids have a long history of use in traditional medicines in many cultures, but flavonoids themselves were not discovered until the 1930s. Quercetin first gained attention several decades ago when it was found to cause DNA mutations in bacteria, a sign that it might actually contribute to causing cancer. Animal research done since that time has been inconclusive, and what little evidence there is in humans does not seem to support this idea. Research in recent years has focused on several possible helpful effects of quercetin, including its potential role in preventing cancer.
What is the evidence?
Most of the research on quercetin and cancer has been done in cell culture or animal studies. These types of studies can suggest possible helpful effects, but they do not provide proof that such effects can be achieved in humans. It is still unclear how well quercetin is absorbed by the human body when taken by mouth. Controlled clinical trials are needed to show whether quercetin has helpful properties in humans.
Studies done in cell cultures have shown that quercetin has activity against some types of cancer cells. This may be due to its antioxidant or anti-inflammatory properties, or it may be due to other mechanisms. Recent studies suggest that quercetin can slow the growth of cancer cells and can help foster apoptosis, a form of natural cell death that doesn't happen in most cancer cells. Some studies in animals have shown that quercetin may help protect against certain types of cancer, particularly colon cancer.
Studies in humans have mainly been population-based and have focused on the role of flavonoids in the diet as a group as opposed to quercetin in particular. These types of studies are not as conclusive as clinical trials. They cannot prove cause and effect but often suggest links that can then be tested in clinical trials. While some of these population-based studies have found that people with diets high in flavonoids may have lower risk of breast, lung, pancreatic, and other types of cancer, it is not clear what role quercetin played in their findings. One clinical study of people with a strong inherited tendency to develop colorectal cancer found that the combination of quercetin and curcumin supplements decreased the number and size of precancerous rectal tumors. No other clinical trials testing quercetin's ability to prevent or treat cancer have been reported in the medical literature. Clinical trials are needed to further clarify quercetin's possible benefits.
In addition to cancer prevention and treatment, preliminary studies have also suggested potential value for quercetin in prostatitis (inflamed prostate) and heart disease. Further studies are needed before any recommendations can be made.
Until conclusive clinical research findings emerge, it is reasonable to include foods that contain quercetin as part of 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 eating one particular phytochemical in large amounts.
Are there any possible problems or complications?
This product is sold as a dietary supplement in the United States. Unlike drugs (which must be tested before being allowed to be sold), the companies that make supplements are not required to prove to the Food and Drug Administration that their supplements are safe or effective, as long as they don't 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). Actual amounts per dose may vary between brands or even between different batches of the same brand.
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.
Quercetin in the amounts consumed in a healthy diet is unlikely to cause any major problems. There have been some occasional reports of nausea when supplements are taken in high doses. Quercetin supplements have not been studied for safety in women who are pregnant or breast-feeding. Relying on this type of treatment alone and avoiding or delaying conventional medical care for cancer may have serious health consequences.
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).
The ACS Operational Statement on Complementary and Alternative Methods of Cancer Management
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Cruz-Correa M, Shoskes DA, Sanchez P, et al. Combination treatment with curcumin and quercetin of adenomas in familial adenomatous polyposis. Clinical Gastroenterology & Hepatology.2006;4:1035-1038.
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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 Revised: 11/01/2008