"In six healthy volunteers, Dr. Walle and colleagues examined the absorption and bioavailability of radiolabeled resveratrol after oral and intravenous administration of the antioxidant in doses roughly matching normal dietary intake.
They found that after oral dosing, most of the resveratrol showed up in urine in the first 12 hours, and it appeared only as sulfate and glucuronic acid conjugates. The free, presumably active form of resveratrol was not present in urine or blood samples after oral dosing. Similarly, after intravenous injection, free resveratrol was only detected in blood samples drawn 30 minutes after injection.
"We found no evidence that any resveratrol will reach into the systemic circulation," Dr. Walle told Reuters Health, "and the reason for that is very extensive metabolism of resveratrol as it is being absorbed into the body. It is completely broken down into metabolites very rapidly."
Dr. Walle knows of one other study involving four individuals that supports his findings that "very low if any" amounts of active resveratrol will reach the circulation in humans."
http://www.oncolink.org/resources/article.cfm?c=3&s=8&ss=23&id=10664&month=04&year=2004
Here is the link to the original study:
http://dmd.aspetjournals.org/cgi/content/abstract/32/12/1377
And the abstract for those who want it:
HIGH ABSORPTION BUT VERY LOW BIOAVAILABILITY OF ORAL RESVERATROL IN HUMANS
Thomas Walle, Faye Hsieh, Mark H. DeLegge, John E. Oatis, Jr., and U. Kristina Walle
Department of Cell and Molecular Pharmacology and Experimental Therapeutics (T.W., J.E.O., U.K.W.) and Digestive Disease Center (M.H.D.), Medical University of South Carolina, Charleston, South Carolina; and Amgen, Inc., Thousand Oaks, California (F.H.)
The dietary polyphenol resveratrol has been shown to have chemopreventive activity against cardiovascular disease and a variety of cancers in model systems, but it is not clear whether the drug reaches the proposed sites of action in vivo after oral ingestion, especially in humans. In this study, we examined the absorption, bioavailability, and metabolism of 14C-resveratrol after oral and i.v. doses in six human volunteers. The absorption of a dietary relevant 25-mg oral dose was at least 70%, with peak plasma levels of resveratrol and metabolites of 491 ± 90 ng/ml (about 2 µM) and a plasma half-life of 9.2 ± 0.6 h. However, only trace amounts of unchanged resveratrol (<5 ng/ml) could be detected in plasma. Most of the oral dose was recovered in urine, and liquid chromatography/mass spectrometry analysis identified three metabolic pathways, i.e., sulfate and glucuronic acid conjugation of the phenolic groups and, interestingly, hydrogenation of the aliphatic double bond, the latter likely produced by the intestinal microflora. Extremely rapid sulfate conjugation by the intestine/liver appears to be the rate-limiting step in resveratrol's bioavailability. Although the systemic bioavailability of resveratrol is very low, accumulation of resveratrol in epithelial cells along the aerodigestive tract and potentially active resveratrol metabolites may still produce cancer-preventive and other effects.