A modest amount of red wine reduces the risk of heart disease, possibly because of the polyphenols it contains. Grape seed extract contains the same polyphenols as found in wine and has therefore been widely marketed as a dietary supplement with claims of having a beneficial effect on the human cardiovascular system. The problem here, though, is that the studies that have explored the effects of grape seed extract on human subjects have shown either none or minimal benefits. One study showed a slight increase in the resting diameter of the brachial artery in the arm, a finding of unknown clinical significance.
A meta analysis of nine randomized controlled trials concluded that grape seed extract had no effect on blood cholesterol, inflammation as determined by C-reactive protein levels, or triglycerides. There was a slight decrease of 1.5 mm Hg in systolic blood pressure, which is minimal when compared with what can be achieved with medication.
Overall there does not seem to be much evidence for taking grape seed extract supplements, although given that there is a great variety in supplement composition, it is possible that some specific supplements may be more effective than others. Unfortunately there are no quality control standards, as is clearly demonstrated by a recent analysis of 21 extracts purchased from a variety of outlets. Compared with authentic grape seed extract, there was great variability in chemical composition of the commercial extracts, but on average they all contained significantly less polyphenols than the authentic samples.
That, though, was not the only problem. Six of the samples contained no detectable quantities of grape seed extract, but were instead composed of peanut skin extract. Peanut skin does contain a variety of polyphenols similar to that found in grape seeds but the presence of peanut extract raises the issue of allergenicity. It is certainly possible that people with a peanut allergy may react to the adulterated extract. The motivation for such adulteration is financial, since peanut skin extract is much cheaper than authentic grape seed extract. Adulteration and lack of reliable data about composition is not the only problem. Let’s remember that even with authentic grape seed extract there is no compelling evidence of health benefits. And what about that glass of red wine with dinner? Drink it because you like it, not because of the polyphenols it contains. And we won’t even mention that ethanol is a carcinogen.