Background: Despite their powerful biologic activities conducive to protection against atherosclerosis, cancer and inflammatory diseases demonstrated in vitro, there is considerable doubt whether the polyphenolic constituents present in red wine and other dietary components are effective in vivo.
Objective: We have tested the absorptive efficiency of three of these constituents (trans-resveratrol, [+]-catechin and quercetin) when given orally to healthy human subjects in three different media.
Design: Twelve healthy males aged 25 to 45 were randomly assigned to three different groups consuming orally one of the following polyphenols: trans-resveratrol, 25 mg/70 kg; [+]-catechin 25 mg/70 kg; quercetin 10 mg/70 kg. Each polyphenol was randomly administered at 4-week intervals in three different matrices: white wine (11.5% ethanol), grape juice, and vegetable juice/homogenate. Blood was collected at zero time and at four intervals over the first four hours after consumption; urine was collected at zero time and for the following 24-h. The sums of free and conjugated polyphenols were measured in blood serum and urine by a gas-chromatographic method.
Results: All three polyphenols were present in serum and urine predominantly as glucuronide and sulfate conjugates, reaching peak concentrations in the former around 30-min after consumption. The free polyphenols accounted for 1.7 to 1.9% (trans-resveratrol), 1.1 to 6.5% ([+]-catechin) and 17.2 to 26.9% (quercetin) of the peak serum concentrations. The absorption of trans-resveratrol was the most efficient as judged by peak serum concentration, area-under-the curve (4 h) and urinary 24-h excretion (16-17% of dose consumed). [+]-Catechin was the poorest by these criteria (urine 24-h excretion 1.2%-3.0% of dose consumed), with quercetin being intermediate (urine 24-h excretion 2.9%-7.0% of dose consumed). Some significant matrix effects were observed for the serum polyphenol concentrations, but in the case of urine no matrix promoted significantly higher excretion than the other two.
Conclusions: The absorption of these three polyphenols is broadly equivalent in aqueous and alcoholic matrices but, at peak concentrations of 10 to 40 nmol/L, is inadequate to permit circulating concentrations of 5 to 100 micromol/L consistent with in vitro biologic activity. The voluminous literature reporting powerful in vitro anticancer and antiinflammatory effects of the free polyphenols is irrelevant, given that they are absorbed as conjugates.