Resveratrol (3,5,4′-trihydroxystilbene) content in red wine is a polyphenolic phytoalexin. It is a stilbenoid, a derivate of stilbene, and is produced in plants with the help of the enzyme stilbene synthase.
It exists as two geometric isomers: cis- (Z) and trans- (E), with the trans-isomer shown in the top image. The trans- form can undergo 
isomerisation to the cis- form when exposed to ultraviolet irradiation. Trans resveratrol in the powder form was found to be stable under “accelerated stability” conditions of 75% humidity and 40 degrees C in the presence of air. Resveratrol content also stayed stable in the skins of grapes and taken after fermentation and stored for a long period.
About 70% of the resveratrol content given orally as a pill is absorbed; nevertheless, oral bioavailability of resveratrol is low because it is rapidly metabolized in intestines and liver into conjugated forms: glucuronate and sulfonate. Only trace amounts (below 5 ng/mL) of unchanged resveratrol could be detected in the blood after 25 mg oral dose. Even when a very large dose of resveratrol (2.5 and 5 g) was given as an uncoated pill, the concentration of resveratrol in blood failed to reach the level necessary for the systemic cancer prevention.
The hypothesis that resveratrol from wine could have higher bioavailability than resveratrol from a pill, has been disproved by experimental data. For example, after five men took 600 mL of red wine with the resveratrol content of 3.2 mg/L (total dose about 2 mg) before breakfast, unchanged resveratrol was detected in the blood of only two of them, and only in trace amounts (below 2.5 ng/mL).
Resveratrol levels appeared to be slightly higher if red wine (600 mL of red wine containing 0.6 mg/mL resveratrol; total dose about 0.5 mg) was taken with meal: trace amounts (1–6 ng/mL) were found in four out of ten subjects. In another study, the pharmacokinetics of resveratrol (25 mg) did not change whether it was taken with vegetable juice, white wine or white grape juice.
The highest level of unchanged resveratrol content in the serum(7–9 ng/mL) was achieved after thirty minutes, and it completely disappeared from blood after four hours. The authors of both studies concluded that the trace amounts of resveratrol reached in the blood are insufficient to explain the French paradox. It appears that the beneficial effects of wine could be explained by the effects of alcohol or the whole complex of substances wine contains, for example, the cardiovascular benefits of wine appear to correlate with the content of procyanidins.
