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Pol. J. Food Nutr. Sci. 2004;54(Special issue 1s):101–114
Flavonoids are secondary metabolites widely distributed in the plant kingdom. They belong to a large group of compounds with highly diversified structure-referred to as polyphenols. This diversity determines a variety of life functions they play in plants. As indispensable components of plant-originated food products, these widespread bioactive compounds are consumed by humans in amounts significant from the physiological point of view. Species- and varietal variability, vegetation season, light, climatic conditions, technological processing, and the way of preparing meals are the main factors determining their content in food and consequently their intake. Flavonoids get to the human’s alimentary tract and then into the general blood circulation. Thus, while estimating their physiological functions, consideration should be given to transformations they undergo in metabolic processes. Although knowledge on the absorption and metabolism of flavonoids is still incomplete, it substantiates the statement that - after absorption – these compounds occur in the blood circulation as glucuronised, and/or sulfated or methylated conjugates. Therefore, most of the effects shown in in vitro experiments with aglycones cannot be directly extrapolated to in vivo systems. Despite the fact that conjugation is one of the stages of detoxification process and elimination of xenobiotics from the organism, the metabolites formed may still affect consumer organism. This concerns also the antioxidant properties of flavonoids which may be significantly reduced or even lost. However, it does not mean that flavonoids simultaneously lose their positive impact on consumer health. Even after being metabolised they may act locally or systemically indirectly influencing redox balance by inhibition of oxidative enzymes, inducing antioxidative and detoxifying enzymes or compounds which may be involved in sustaining homeostasis. This, at least in part, may explain their beneficial physiological function resulting from epidemiological studies, especially in the prevention of atherosclerotic lesions development. Nevertheless, it is unclear whether metabolised flavonoids are capable of coming into contact with cellular membranes and penetrate the cell’s interior. It is known, however, that a considerable part of absorbed flavonoids is rapidly excreted back to the alimentary tract with bile. Thus, the not-absorbed flavonoids and these excreted with bile may act as agents protecting the surface of the epithelial cells of the alimentary tract and prevent their degenerative changes.