Red wine has long been touted as a health elixir. Now wine’s purported health-giving ingredient, resveratrol, is available in daily supplements, beverages and even a new nutritional bar claimed by sellers to help you live longer and help prevent cancer and other diseases. Until recently, the evidence for resveratrol has been animal data, but preliminary human testing has yielded intriguing results.

Resveratrol is a substance found in grapes and other plants. Many of the supplements on the market extract it from the Japanese knotweed plant, which is a plentiful and inexpensive source. Switzerland’s Royal DSM NV sells a synthetic version called resVida, which is found in supplements, a non-alcoholic fruit-flavored beverage and the new Winetime chocolate-fruit bar.

Resveratrol has found been found in animal studies to prevent or slow progression of illnesses from cancer to cardiovascular disease—and even to extend the life span of some organisms. Since 2008, at least five human studies have been presented at scientific meetings showing human benefits, ranging from improved blood flow to the heart to better control of diabetes.

The newest results are exciting, but some scientists say it is too early for the public to begin taking supplements, which contain as much resveratrol in one pill as dozens or hundreds of bottles of wine, depending on the dose. The proper dose for humans isn’t yet known—and more isn’t necessarily better.

Researchers at the University of Connecticut School of Medicine in Farmington found that lower doses of resveratrol protected rats’ hearts from artificially induced heart attacks while high doses actually made the attacks worse.

While some safety studies have been conducted on humans, it is too early to know if there are long-term side effects of high doses, says S. Jay Olshansky, a professor of epidemiology at the University of Illinois at Chicago. “I cannot emphasize enough: Do not experiment on your own body,” he says.

Joseph C. Maroon, a neuroscientist at the University of Pittsburgh Medical Center who has written a book on resveratrol, agrees that more studies are needed but doesn’t think it’s unreasonable to take a moderate dose of resveratrol. “I don’t see any significant downside,” he says.

Dr. Maroon says animal data suggest 50 to 1,000 milligrams a day is an effective dose; he takes 300 milligrams daily—which he says contains the amount of resveratrol in some 150 bottles of wine.

No one knows exactly how resveratrol works, but scientists believe it activates a wide range of genes, creating a cascade-like effect on a variety of body functions. Human data include a 100-person placebo-controlled study by Sirtris Pharmaceuticals Inc. that found lowered blood-glucose levels in diabetics who took the GlaxoSmithKline unit’s proprietary formulation, SRT501, not yet on the market.

A 19-person study presented earlier this month at a British scientific conference, sponsored by Royal DSM, found that “flow mediated dilation,” a measure of cardiovascular health, increased an hour after taking resVida. At an American College of Sports Medicine conference last year, Dr. Maroon and colleagues reported that a three-month study of 51 people found a resveratrol-containing supplement not currently on the market increased endurance on a stationary bicycle compared with a placebo, and also increased verbal memory scores on a standardized test.

And in a report published earlier this month in the journal Optometry, researchers found that five months’ therapy with a Longevinex, a supplement sold by Resveratrol Partners LLC of Las Vegas, resulted in significant improvement of vision of an 80-year-old man who was having difficulty with night driving. The visual measures were subjective but researchers also found a significant decrease in lipofuscin, a granular substance that builds up in aging tissues and is linked to vision decline, says researcher Stuart Richer, chief of optometry at the Veterans Affairs Medical Center in North Chicago. Resveratrol Partners provided the supplements taken by the patient, says Dr. Richer, and he has agreed to conduct a company-funded 24-person follow-up study.

One person isn’t enough to prove resveratrol can help age-related vision decline, says Dr. Richer, but “we want to do this in a controlled situation with many patients.”

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