Bioavailability and Metabolism of Flavonoids: A Review

Akhlaghi, Foshati Int J Nutr Sci 2017;2(4):180-184 International Journal of Nutrition Sciences Journal Home Page: ijns.sums.ac.ir Review Article Bioavailability and Metabolism of Flavonoids: A Review Masoumeh Akhlaghi1*, Sahar Foshati2 1. Nutrition Research Center, School of Nutrition and Food Sciences, Shiraz University of Medical Sciences, Shiraz, Iran 2. Department of Clinical Nutrition, School of Nutrition and Food Sciences, Shiraz University of Medical Sciences, Shiraz, Iran ARTICLE INFO A BSTRAC T Keywords: Flavonoids are a group of plant antioxidants that are widely distributed in plants from the root and stem to the leaves, flowers, and fruits. They are generally present as glycosides, conjugated to sugars, although their aglycone forms may also exist. Flavonoid glycosides are hydrolysed from sugar moieties in the intestine. After release from sugars, the flavonoid aglycones undergo conjugation with other molecules to facilitate the excretion and shorten the half-life of flavonoids. Conjugation also reduces the antioxidant and potential pro-oxidant effects of flavonoids. Flavonoids have low intestinal bioavailability and rapid urinary and biliary excretion. However, the bioavailability of flavonoids varies between different kinds of flavonoids. The best rate of absorption among flavonoids is seen in gallic acid and isoflavones, followed by catechins, flavanones, quercetin, proanthocyanidins, and anthocyanins. Quercetin metabolites are excreted very slowly, whereas anthocyanins and catechins are excreted very rapidly. Information on bioavailability and metabolism of flavonoids can help designing the best intervention strategies. In conclusion, flavonoids have low intestinal bioavailability and rapid urinary and biliary excretion. The bioavailability of them varies between different kinds of flavonoids. The most rapid excretion rates belong to anthocyanins and flavanols while the slowest rates are for flavonols. Flavonoids Bioavailability Metabolism *Corresponding author: Masoumeh Akhlaghi, Nutrition Research Center, School of Nutrition and Food Sciences, Shiraz University of Medical Sciences, Shiraz, Iran Tel: +98-71-37251001 Fax: +98-71-37257288 Email: ; Received: March 12, 2017 Revised: September 11, 2017 Accepted: October 13, 2017 Please cite this article as: Akhlaghi M, Foshati S. Bioavailability and Metabolism of Flavonoids: A Review. Int J Nutr Sci 2017;2(4):180-184. Introduction Flavonoids are a subgroup of the more extended family of polyphenols. Polyphenols constitute a widespread group of plant compounds implicated in plants’ wellbeing, growth and reproduction, pigmentation, and protection against microorganisms and enemies (1). They are widely distributed in plants from the root and stem to the leaves, flowers, and fruits. Their quantities in fruit and vegetables are influenced by numerous 180 factors including light, environmental conditions, plant species, degree of ripeness, germination, processing, and storage. As an example, cherry tomatoes possess six times more quercetin per gram fresh weight than normal size varieties of tomatoes, probably because polyphenols are generally synthesized and stored in the skin, and therefore smaller varieties have a higher skin to volume ratio (2). Total dietary intake of polyphenols is firmly Int J Nutr Sci December 2017;2(4) Bioavailability and flavonoids metabolism dependent on the food culture and the individual food preferences, but has been estimated to be about 1 g/ day (3) and that of flavonoids between 2 and 70 mg/ day (4). Moreover, it has been reported that 10% of the population and 30-70% of patients with specific diseases consume herbal medicines, which naturally contain considerable amounts of polyphenols (5). There are more than 8000 polyphenolic compounds identified, each with a structure containing at least one phenol which is a hexagon ring (benzene) with a hydroxyl group (6). Based on their chemical structure, polyphenols can be divided into at least 10 different classes, one of the major groups of which is flavonoids (1, 7). Classification Flavonoids were discovered in the 1930’s when a factor extracted from lemon juice could attenuate vessel permeability and bleeding in scorbutic Guinea pigs where vitamin C was not effective, leading to their nomination as vitamin P, although this terminology was later dismissed (8). More than 5000 flavonoids have been identified (9). The basic structure of flavonoids consists of two benzene rings (A and B) with a pyran ring (C) in the middle (Figure 1). Flavonoids are divided into several subclasses including flavonols, flavanols (including proanthocyanidins), flavanones, flavones, isoflavones, and anthocyanins (1, 7). Flavonols are represented by quercetin, kampferol, and myricetin (7, 10). They are wide-spread in fruit and vegetables, and may contribute largely to our daily flavonoid consumption. Quercetin, one of the most studied flavonoids, is ingested especially through consumption of tea, onions, red wine, and apples. Flavanols include catechins, which are largely found in green tea (Figure 1). Black tea has far less catechin than green tea due to oxidation of catechins during fermentation. Other major sources of flavanols are chocolate, apples, pears, grapes, and red wine (11). Proanthocyanidins or condensed tannins (Figure 2) are high-molecular weight oligomers and polymers of catechins, and are the most ubiquitous polyphenols in nature after lignans (12). The major sources of proanthocyanidins in the diet are chocolate, grapes, and apples. Isoflavones, such as genistein and daidzein, are phytoestrogens and may be beneficial in prevention of breast and prostate cancer, menopausal symptoms, cognitive disabilities, osteoporosis, and heart diseases. Soy bean is a rich source of isoflavones (13). Anthocyanins (Figure 2), such as cyanidin, malvidine, and delphinidin, provide red and purple pigments for fruits. They are abundant in red and black cherries, berries, grapes, and legumes. Consumption of 200 g of black grapes and berries can afford about 1 to 1.5 g anthocyanins (14). Bioavailability and Metabolism In nature, flavonoids are generally present as glycosides, conjugated to sugars, although their aglycone forms may also exist (9). The aglycones have stronger antioxidant activity than glycoside forms. The weakening of the antioxidant activity of flavonoids after glycosylation may be due to removing hydroxyl groups by conjugated glycosides, and thereby inhibiting them from scavenging reactive oxygen species (ROS) or chelating transition metals. Furthermore, as glycosylation enlarges the molecule, the passage through membranes may decrease upon glycosylation, leading ultimately to less antioxidant activity. Nonetheless, glycosylation enhances water solubility of the compound and subsequently improves its absorption from the gastrointestinal tract (9). Flavonoid glycosides can be hydrolysed from sugar moieties by hydrolases at the int (...truncated)