Both Japanese knotweed and giant knotweed are important concentrated sources of resveratrol, replacing grape byproducts. Many large supplement sources of resveratrol now use Japanese knotweed and use its scientific name in the supplement labels. The plant is useful because of its year-round growth and robustness in different climates.
Resveratrol is found in the skin of red grapes and in other fruits. However, red wine contains very little of it, in the order of one milligram per glass. Resveratrol has also been produced by chemical synthesis and by biotechnological synthesis (metabolic engineered microorganisms) and is sold as a nutritional supplement derived primarily from Japanese knotweed.
A Korean study showed that trans-resveratrol supplementation increased testosterone levels in mice in vivo, which has led to its marketing as a bodybuilding supplement. A Spanish study has also shown the antioxidant to increase sperm production in rats.
Abstract Polyhydroxystilbenes including resveratrol were reported to competitively inhibit monoamine oxidase-A without structural relation with substrates and synthetic inhibitors for the enzyme. We attempt to explore a plausible mechanism for their inhibitory activity on MAO-A. All the polyhydroxystilbenes tested showed the antioxidant activity on liver homogenate. Furthermore, the antioxidant activity turned out to closely correlate with the MAO-A inhibitory activity.
Resveratrol is not a tremendously potent MAOI. It has an IC50 of 2uM with the rat enzyme. To obtain that level in plasma would require a very large dose of resveratrol, in the neighborhood of 5 grams for ordinary resveratrol, and that level would not be maintained for long. We don't know the IC50 of the human enzyme, nor do we have solid information about the intracellular concentration of resveratrol aglycone, so there's nothing wrong with being cautious here