What Are Flavonoids?

Flavonoids were discovered by Nobel Prize-winning biochemist Albert Szent-Gyorgi, who labeled them "vitamin P". He discovered that they enhanced the function of vitamin C, improving absorption and protecting it from oxidation. Flavonoids that have been shown to have particularly beneficial properties include proanthocyanidins, green tea polyphenols and soy isoflavones. Quercetin and its derivatives; the citrus bioflavonoids, including quercitrin, rutin and hesperidin; have also been fairly well studied.

They are responsible for the colors of many fruits and vegetables, are found also in grains, nuts, leaves and flowers, and studies suggest there value in the treatment of a number of disorders. They are considered one of the most common biological constituents in plants. They have a gentle, beneficial action on numerous physiological processes in the body and may benefit the heart, blood vessels, liver, immune system, connective tissue, adrenal glands, kidneys, musculature and nervous system. Flavonoids may act as anti-oxidants, "anti-allergics", anti-inflammatories, immunostimulating, anti-hepatotoxic, anti-neoplastic and hypoglycemic along with numerous other actions including stabilizing capillary permeability. Important herbs containing flavonoids include milk thistle, hawthorn, echinacea and bilberry. One possible chemical classification consists of PCO (proanthocyanidins), quercetin, citrus bioflavonoids and green tea polyphenols. The following is a description of three popular flavonoids; quercetin, grape seed extract and green tea extract.


Quercetin serves as the backbone for other flavonoids, and is the most active of the flavonoids. Many medicinal plants have a significant quercetin content. It blocks mast cell and basophilic histamine degranulation, inhibits xanthine oxidase (the enzymes producing uric acid) and aldose reductase (the enzyme converting glucose into sorbitol), decreases neutrophil lysosomal enzyme secretion, and normalizes activity if phospholipase A2 and lipoxygenase. Although human studies are limited, its physiological actions explain its use in inflammatory and allergic conditions including asthma, hay fever, rheumatoid arthritis, lupus, diabetes and cancer. Sorbitol is implicated in the development of diabetic complications including cataracts, neuropathy and retinopathy. Studies show decreased cataract formation in the lens of diabetic animals. Its potential influence on diabetes may also relate to enhanced insulin secretion, protection of pancreatic beta cells from free-radical damage, and inhibition of platelet aggregation. Quercitin has anti-viral action against a number of different viruses, and inhibits a number of different cancers in experimental models.

Grape Seed Extract

Proanthocyanidins (Procyanidins) is an important therapeutic class of flavonoids extracted from grape seeds and maritime (Landes) pine. When individual molecules bind together, the result is collectively called procyanidolic oligomers (PCO). They have a broad range of pharmacologic activity through increasing vitamin C levels, decreasing capillary permeability and fragility, scavenging free radicals and inhibiting destruction of collagen. The latter occurs through ability to cross-link collagen fibers, preventing free-radical damage, inhibiting enzymatic cleavage of collagen, and preventing the synthesis and release of inflammatory mediators. PCO has approximately 50 times the anti-oxidant activity of vitamin C or vitamin E. These influences, along with other mechanisms, explain there benefit in venous and capillary disorders, including venous insufficiency, capillary fragility, diabetic retinopathy and macular degeneration. Studies show that PCO also lowers cholesterol levels and can shrink arterial cholesterol deposits.

Green Tea Extract

Green tea is derived from the tea plant, Camellia sinensis, through light steaming of the leaves. Allowing the leaves to oxidize produces black tea. Green tea contains polyphenols, mainly flavonoids. They are potent anti-oxidants as well as anti-cancer substances. They inhibit the formation of carcinogenic substances such as nitrosamines, suppress their activation, and detoxify them.

Consumption of a plant-based diet can prevent the development and progression of chronic diseases associated with extensive neovascularization, including solid malignant tumors. In previous studies, we have shown that the plant-derived isoflavonoid genistein is a potent inhibitor of cell proliferation and in vitro angiogenesis. In the present study, we report that certain structurally related flavonoids are more potent inhibitors than genistein. Indeed, 3-hydroxyflavone, 3',4'-dihydroxyflavone, 2',3'-dihydroxyflavone, fisetin, apigenin, and luteolin inhibited the proliferation of normal and tumor cells, as well as in vitro angiogenesis, at half-maximal concentrations in the low micromolar range. We have previously demonstrated that genistein concentrations in the urine of subjects consuming a plant-based diet is 30-fold higher than in subjects consuming a traditional Western diet. The wider distribution and the more abundant presence of flavonoids in the plant kingdom, together with the present results, suggest that flavonoids may contribute to the preventive effect of a plant-based diet on chronic diseases, including solid tumors.

Researchers have found that drinking at least one cup of tea a day could cut the risk of heart attack by 44 %. The results are probably due to flavonoids, vitamin-like nutrients that make blood cells less prone to clotting. Flavonoids, powerful antioxidants, are found in fruits and vegetables and are connected to the heart-healthy effect of red wine. The study examined 340 men and women who had had heart attacks and investigated their coffee and tea drinking habits over a year. The question of how much tea to drink, and how strong it needs to be brewed to achieve maximum benefits, was not determined.

This journal article discusses 'Crataegus oxycantha' (Hawthorne) and its use in the treatment of cardiac conditions. It summarizes information about the main constituents of Crataegus, its mechanisms of action, clinical indications, dosage, drug nutrient interactions, and toxicology. Crataegus appears to be an effective and low-risk phytotherapeutic for patients with coronary heart disease, atherosclerosis, hypertension, or hypercholesterolemia. It has been shown to produce subjective and objective benefits in patients with congestive heart failure. Its main constituents are flavonoids, triterpene saponins, and a few cardioactive amines. However, the primary cardiovascular protective activity of the plant is generally attributed to its flavonoid content, particularly the oligomeric proanthocyanadins.

Otto Daniel and colleagues from the Swiss Federal Office of Public Health and the Swiss Federal Institute of Technology, both in Z├╝rich, study the toxic and beneficial human health effects of certain phenolic compounds. These compounds are produced in plants to serve a number of purposes, including repelling herbivores, pigmentation, protection against UV light, and biocidal defense against bacteria and fungi. External stimuli such as chemical stress from heavy metals and pesticides can alter the chemical composition or quantities of phenolic compounds in a plant; depending on its concentration, chemical structure, and any external modulation, a given phenolic compound might be either toxic or beneficial to humans. The scientists examined three such compounds. Resveratrol, which is found in grapes and peanuts, has been found to inhibit the synthesis of substances that cause blood clotting, possibly offering protection against heart disease and thrombosis. Flavonoids, which are found in almost every food or beverage of plant origin, act as antioxidants, inhibit blood coagulation, promote vasodilation, and have anti-inflammatory effects--benefits that appear to outweigh their variable mutagenic properties. Furanocoumarins, which are found in plants such as limes and celery, can cause phototoxic burns but have also been harnessed for use in psoralen UVA therapy, which is used to treat skin conditions such as psoriasis and cutaneous T-cell lymphoma.

Flavonoids from different fruit sources vary in their physical-chemical properties as antioxidants and partition to different extents in biological tissues. After determining which flavonoids inhibited low-density lipoprotein (LDL) oxidation in vitro, investigators validated the hamster as a diet-responsive animal model of atherosclerosis, in which the extent of disease is quantifiably related to dietary antioxidant content. They then showed that supplementation with tocopherol or ingestion of the monomeric flavonoid catechin reduced the extent of atherosclerosis in hamsters on a hypercholesterolemic diet. These studies were supported by fatty-acid analyses in the Fatty Acids Subcore and LDL oxidation measurements in the Antioxidants and Mass Spectrophotometry Core. Related studies showed that polyphenolic flavonoids from grapes exhibit potent vasorelaxing activities in isolated rabbit aortas, further suggesting biologically important targets of action for this class of nutrients.

Flavonoids are among the best candidates for mediating the protective effect of diets rich in fruits and vegetables with respect to colorectal cancer. To gain additional information about their growth effects on colorectal tumors and their cellular mechanisms of action, a series of related flavonoids was added to cultures of colonic tumor cells. Most compounds induced growth inhibition and cell loss at concentrations of 1-100 microM, relative effectivity being quercetin > apigenin > fisetin > robinetin and kaempferol.

Thornton reports on research indicating that nuts may actually improve heart health, regardless of other risk factors such as weight, blood pressure, exercise, or gender. This may spring from the nature of the fat in nuts, which is both monounsaturated and polyunsaturated. These are the healthy forms of fat, found in such foods as olive and canola oils. In addition, nuts contain Omega-3 fatty acids, and are nutritionally dense with such nutrients as vitamin E, folic acid, niacin, copper, potassium, and magnesium. According to Thornton, nuts also contain flavonoids and isoflavones, compounds thought to prevent cancer and cardiovascular disease. Finally, since nuts have a high satiety factor, most people find a small amount filling, which contributes to weight loss.

  • Flavonoids, Farmacopia LLC
  • Fotsis T, Pepper MS, Et al. Flavonoids, dietary-derived inhibitors of cell proliferation and in vitro angiogenesis. Cancer Res; 57(14):2916-21 1997
  • Ross, E. Tea may cut heart-attack risk 44 %. San Jose Mercury News. July 9, 1999.
  • Crataegus Oxycantha. Common Name: Hawthorne. Alternative Medicine Review. 3(2): 138-139. April 1998.
  • Environmental Health Perspectives Volume 107, Number 2, February 1999
  • Scientific Advances/Accomplishments - University of California-Davis Clinical Nutrition Research Unit
  • Richter M; Ebermann R; Marian B. Quercetin-induced apoptosis in colorectal tumor cells: possible role of EGF receptor signaling. Nutr Cancer 1999;34(1):88-99
  • Thornton, J. Health Nuts. Cooking Light. p.96-101,154. March 1999.
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