Resveratrol, the antioxidant touted as the wonderful life-preserver found in chocolate and red wine, sadly isn't all it's cracked up to be.
It cannot explain the "French Paradox" - the low incidence of heart disease suffered by people in France despite a diet laden with cholesterol and saturated fat, they believe.
Other as-yet unidentified plant compounds might be conferring health benefits associated with their diet, according to the study.
Lead researcher Professor Richard Semba, from Johns Hopkins University in Baltimore, US, said: "The story of resveratrol turns out to be another case where you get a lot of hype about health benefits that doesn't stand the test of time.
"The thinking was that certain foods are good for you because they contain resveratrol. We didn't find that at all."
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Belief in the health-giving properties of resveratrol has led to a plethora of supplements containing the compound and the promotion of diets based on boosting its consumption.
Previous research has shown that resveratrol has an anti-inflammatory effect and can improve the health and lifespan of mice.
At the molecular level it mimics the effects of calorie restriction, which is known to lengthen the lives of some animals but not humans.
Some preliminary evidence also suggests that the compound could help prevent cancer and reduce the stiffness of arteries in older women. But there is little real-world data to support links between resveratrol intake and improved human health, the researchers point out.
The new research involved 783 Italians aged 65 and over who were participants in the Ageing in the Chianti Region study from 1998 to 2009.
Regular urine tests were carried out to look for breakdown products of resveratrol and see if their levels were associated with reduced cancer, heart disease and death rates.
None of those taking part were taking resveratrol supplements, so they had to obtain the compound from their diet. The volunteers came from two villages in Tuscany where few people use supplements and the consumption of red wine is a part of life.
During the nine-year follow-up period, 268 (34.3%) of participants died and 27.2% of those free of heart disease at the start of the study developed the condition.
Of the 734 men and women who had no signs of cancer at enrolment, 4.6% were later diagnosed with the disease.
No significant association was seen between urine resveratrol levels and the likelihood of participants developing heart disease or cancer, dying, or bearing markers of chronic inflammation.
Despite the negative result, wine buffs and lovers of dark chocolate should not lose heart, say the scientists whose findings appear in the journal JAMA Internal Medicine.
"It's just that the benefits, if they are there, must come from other polyphenols or substances found in those foodstuffs," Prof Semba said. "These are complex foods, and all we really know from our study is that the benefits are probably not due to resveratrol."