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Flavonoids and Health

Plants vary within and among species in the types and concentrations of phytochemicals due to variables in plant growth, soil, weather conditions and the age of the plant. Phenolic phytochemicals are the largest category of phytochemicals and the most widely distributed in the plant kingdom. Flavonoids, the largest group of plant phenols and the most studied, include more than 4000 different compounds.

The basic flavonoid structure allows a multitude of variations in chemical structure, giving rise to flavonols (quercetin, kaempherol, myricetin), flavones (apigenin, luteolin), flavanones (catechin, epicatechin), anthocyanidins and isoflavonoids (genistein, daidzein). The flavonoids occur in fruits, vegetables, nuts, seeds, flowers, leaves, bark, tea and wine. The function of polyphenols in plants are antioxidants (protection from UV light), protection form insects, fungi, viruses and bacteria, visual attention- pollinator attraction, feed repellent and plant hormone controllers. Due to their activity as antioxidants, flavonoids found in botanical dietary supplements may play a role in the prevention of heart disease and cancer.

Studies of the flavonoids' health benefits started with Szent-Gyorgi in 1936. He showed that 2 flavonoids derived from citrus fruit decreased capillary fragility and permeability in humans and proposed to call the flavonoids vitamin P. In more recent years it was shown that flavonoids have potent antioxidant, anti-inflammatory and antiviral capacity. They alter enzyme activities affecting cell division, proliferation, platelet aggregation and immune response. Flavonoids have also been investigated for their anticarcinogenic activities. Various flavonoids, most notably the isoflavonoids, are able to bind non-trivially to estrogen receptors and possess estrogenic or antiestrogenic activities.

There is overwhelming epidemiological evidence for a protective effect of vegetable and fruits against cancer and heart disease. It is not known which dietary constituents are responsible for this protective effect, but it is often assumed that antioxidants such as carotenoids, vitamin E and C are responsible for to the protection. Several large chemoprevention trials involving interventions with these antioxidant micronutrients have been carried out with conflicting results. Since many of the flavonoids have stronger antioxidant activities compared to carotenoids, vitamin E and C, and are available in larger amounts in fruits and vegetables, they are excellent candidates to be considered responsible for the beneficial health effects of fruits and vegetables.

Studies supporting the hypothesis for coronary heart disease are well reviewed. For example the study by Hertog et al. found a significant inverse relationship between flavonoid intake and mortality from coronary heart disease and a borderline significance with the incidence of a first fatal or nonfatal myocardial infarction. There is also increasing evidence for the cancer preventive effect of flavonoids for tea catechins, citrus flavonoids and soy isoflavones.

At least two of the most commonly used botanical dietary supplements, Echinacea (E.Purpurea, E. Angustifolia ) and St. John's wort (Hypericum perforatum) contain flavonoids. The quercetin content of E.Purpurea species has been estimated at 0.48%, while E. Angustifolia contains 0.38%. About 12% of the dried weight of the aerial portions of Hypericum perforatum include flavonoids. These are proanthocyanidins, consisting of dimers, trimers, and higher polymers of catechin. Flavonols and other flavonoids have also been reported.

The extent of absorption of dietary flavonoids is an important unsolved problem in judging their potential health effects. Hydrolysis of the glycosides occurs only in the colon by microorganisms, which at the same time degrade flavonoids, and it has been often stated that flavonoids present in foods cannot be absorbed from the intestine if they are bound to sugars as glycosides. Most of the information on subsequent metabolism is derived from animal studies, but little data in humans is available The limited source of knowledge on absorption and metabolism of the flavonoids has been generated by studying isolated flavonoids or single foods. More studies are needed investigating the absorption and metabolism of various flavonoids in individual foods and combinations of foods.







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