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Polyphenols: food sources and bioavailability

Bioavailability and bioefficacy of polyphenols in humans. I. Review of 97 bioavailability studies

The main dietary sources of polyphenols are reviewed, and the daily intake is calculated for a given diet containing some common fruits, vegetables and beverages. Phenolic acids account for about one third of the total intake and flavonoids account for the remaining two thirds. The most abundant flavonoids in the diet are flavanols (catechins plus proanthocyanidins), anthocyanins and their oxidation products. The main polyphenol dietary sources are fruit and beverages (fruit juice, wine, tea, coffee, chocolate and beer) and, to a lesser extent vegetables, dry legumes and cereals. The total intake is approximately 1 g/d. Large uncertainties remain due to the lack of comprehensive data on the content of some of the main polyphenol classes in food. Bioavailability studies in humans are discussed. The maximum concentration in plasma rarely exceeds 1 microM after the consumption of 10-100 mg of a single phenolic compound. However, the total plasma phenol concentration is probably higher due to the presence of metabolites formed in the body's tissues or by the colonic microflora. These metabolites are still largely unknown and not accounted for. Both chemical and biochemical factors that affect the absorption and metabolism of polyphenols are reviewed, with particular emphasis on flavonoid glycosides. A better understanding of these factors is essential to explain the large variations in bioavailability observed among polyphenols and among individuals.

Dietary Intake and Bioavailability of Polyphenols

The ferritins: molecular properties, iron storage function and cellular regulation☆

Inhibition of oxidation of human low-density lipoprotein by phenolic substances in red wine

Antioxidant nutrients from fruits and vegetables are believed to be a class of compounds that exert their effects in humans by preventing oxidative processes which contribute to the onset of several degenerative diseases. This study found a new class of dietary cationized antioxidants in red beets (Beta vulgaris L.). These antioxidants are betalains, and the major one, betanin, is a betanidin 5-O-beta-glucoside. Linoleate peroxidation by cytochrome c was inhibited by betanin, betanidin, catechin, and alpha-tocopherol with IC(50) values of 0.4, 0.8, 1.2, and 5 microM, respectively. In addition, a relatively low concentration of betanin was found to inhibit lipid peroxidation of membranes or linoleate emulsion catalyzed by the "free iron" redox cycle, H(2)O(2)-activated metmyoglobin, or lipoxygenase. The IC(50) inhibition of H(2)O(2)-activated metmyoglobin catalysis of low-density lipoprotein oxidation by betanin was <2.5 microM and better than that of catechin. Betanin and betanidin at very small concentrations were found to inhibit lipid peroxidation and heme decomposition. During this reaction, betanidin was bleached completely, but betanin remained unchanged in its absorption. This difference seems to derive from differing mechanisms of protection by these two compounds. The high affinity of betanin and betanidin for membranes was demonstrated by determining the rate of migration of the compounds through a dialysis tube. Betanin bioavailability in humans was demonstrated with four volunteers who consumed 300 mL of red beet juice, containing 120 mg of the antioxidant. The betacyanins were absorbed from the gut and identified in urine after 2-4 h. The calculated amount of betacyanins found in the urine was 0.5-0.9% of that ingested. Red beet products used regularly in the diet may provide protection against certain oxidative stress-related disorders in humans.

Betalains--a new class of dietary cationized antioxidants.

Nitric oxide as an antioxidant

Initiation of membranal lipid peroxidation by activated metmyoglobin and methemoglobin.

The present study demonstrated that nitric oxide, which is an important mammalian metabolite, can inhibit oxidation by lipoxygenase, cyclooxygenase and hemoglobin. The inhibition is manifested as a lag-phase that is reversible. The inhibitory effect of nitric oxide on lipoxygenase and cyclooxygenase seems to derive from i) the capability of ·NO to reduce the ferric enzyme to the ferrous form, which is inactive; ii) competition for the iron site available for exogenous ligands; and iii) the radical scavenging ability of the nitroxide radical. Nitric oxide may act as a modulator of the arachidonic acid cascade and in the generation of oxygen-active species.

Nitric oxide, an inhibitor of lipid oxidation by lipoxygenase, cyclooxygenase and hemoglobin.

Anthocyanins are one of the main classes of flavonoids in red wines, and they appear to contribute significantly to the powerful antioxidant properties of the flavonoids. In grapes and wines the anthocyanins are in the flavylium form. However, during digestion they may reach higher pH values, forming the carbinol pseudo-base, quinoidal-base, or the chalcone, and these compounds appear to be absorbed from the gut into the blood system. The antioxidant activity of these compounds, in several metal-catalyzed lipid oxidation model systems, was evaluated in comparison with other antioxidants. The pseudo-base and quinoidal-base malvidin 3-glucoside significantly inhibited the peroxidation of linoleate by myoglobin. Both compounds were found to work better than catechin, a well-known antioxidant. In a membrane lipid peroxidation system, the effectiveness of the antioxidant was dependent on the catalyst: In the presence of H(2)O(2)-activated myoglobin, the inhibition efficiency of the antioxidant was malvidin 3-glucoside > catechin > malvidin > resveratrol. However, in the presence of an iron redox cycle catalyzer, the order of effectiveness was resveratrol > malvidin 3-glucoside = malvidin > catechin. The pH-transformed forms of the anthocyanins remained effective antioxidants in these systems, and their I(50) values were between 0.5 and 6.2 microM.

Stela Harel

Papers

Betalains--a new class of dietary cationized antioxidants.

Nitric oxide as an antioxidant

Initiation of membranal lipid peroxidation by activated metmyoglobin and methemoglobin.

Nitric oxide, an inhibitor of lipid oxidation by lipoxygenase, cyclooxygenase and hemoglobin.

PH-dependent forms of red wine anthocyanins as antioxidants.