The most efficient way of administering resveratrol in humans appears to be buccal delivery, that is without swallowing, by direct absorption through the inside of the mouth. For example, with resveratrol lozenges. When one mg of resveratrol in 50 mL solution was retained in the mouth for one min before swallowing, 37 ng/ml of free resveratrol were measured in plasma two minutes later. This level of unchanged resveratrol in blood can only be achieved with 250 mg of resveratrol taken in a pill form.
About 70% of the resveratrol dose 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.
In humans and rats, less than 5% of the oral dose is being observed as free resveratrol in blood plasma. The most abundant resveratrol metabolites in humans, rats, and mice are trans-resveratrol-3-O-glucuronide and trans-resveratrol-3-sulfate. Walle suggests sulfate conjugates are the primary source of activity, and Boocock et al. also emphasized the need for further study of the effects of the metabolites, including the possibility of deconjugation to free resveratrol inside cells. Goldberd, who studied the pharmacokinetics of resveratrol, catechin and quercetin in humans, concluded "it seems that the potential health benefits of these compounds based upon the in vitro activities of the unconjugated compounds are unrealistic and have been greatly exaggerated. Indeed, the profusion of papers describing such activities can legitimately be described as irrelevant and misleading. Henceforth, investigations of this nature should focus upon the potential health benefits of their glucuronide and sulfate conjugates." 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.
Adverse effects and unknowns
Long term effects of using resveratrol are as of yet unknown. While the health benefits of resveratrol seem promising, one study has theorized that it may stimulate the growth of human breast cancer cells, possibly because of resveratrol's chemical structure, which is similar to a phytoestrogen. However, other studies have found that resveratrol intake is inversely associated with breast cancer risk and acts to slow the progression of breast cancer that has been transplanted into mice. Some studies suggest that resveratrol slows the development of blood vessels, which suppresses tumors, but also slows healing. Citing the evidence that resveratrol is estrogenic, some retailers of resveratrol advise that the compound may interfere with oral contraceptives and that women who are pregnant or intending to become pregnant should not use the product, while others advise that resveratrol should not be taken by children or young adults under eighteen, as no studies have shown how it affects their natural development. A small study found that a single dose of up to 5 g of ''trans''-resveratrol caused no serious adverse effects in healthy volunteers.
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