The lure of eternal youth has produced a multibillion-dollar-a-year global marketplace full of potions and pills that, the manufacturers claim, can offer life-extending benefits. Amid the dizzying array, one substance is capturing prime time attention: resveratrol.

Resveratrol advertisements - Reverse your biological clock! A miracle molecule! - are popping up everywhere, from the Internet to local health food stores. The stuff is even showing up in anti-wrinkle creams.

Ever since Harvard University molecular geneticist David Sinclair announced in 2003 that resveratrol appeared to dramatically extend the life span of yeast and fruit flies, the race was on to prove it worked in people.

Six years later, scientists have yet to demonstrate the substance can extend the life of creatures bigger than a mouse. But that hasn't slowed the resveratrol rage or claims by some manufacturers that they have captured the secret to long life in a capsule.

Resveratrol, a compound found in grape skins and red wine, is the most requested type of anti-aging product at the Vitamin Shoppe in Harvard Square.

"We have about seven different company brands," said assistant store manager Koudjo Ala. Most of the requests, he said, come from middle-agers who have "done a lot of research on it and they know exactly what they want."

At Good Health Natural Foods in Hanover, new resveratrol items are "100 times more popular" than the long-sold products with red wine extracts, said manager Tony Latessa.

"A lot of wellness counselors and homeopathic counselors are recommending it," he said. "I even had one doctor recommend it to one of his cardiovascular patients."

After his tantalizing 2003 announcement about resveratrol's apparent impact on yeast and fruit flies, Sinclair cofounded a private company, Sirtris, headquartered in Cambridge, that is researching resveratrol-based drugs designed to target genes called sirtuins. Sirtuins trigger enzymes that help repair and protect against cell damage. Sinclair's early research showed that sirtuins could be switched on by resveratrol.

Scientists have known for decades that rats on severely restricted diets lived longer and suffered fewer maladies of aging, such as heart disease and cancer. The caloric restriction appeared to stimulate sirtuin activity. So scientists have focused on finding a substance, such as resveratrol, that could trigger this same effect in humans - without the famine-like diet.

Yet even Sinclair - who declined through a Harvard spokesman to be interviewed for this story - issued a recent reality check on the resveratrol front. Interviewed on the CBS News program "60 Minutes," Sinclair said he believed it would be "five years, to be conservative" before the public would see a pill that could help people not only live longer, but keep them healthy longer.

"This will happen within our lifetimes," he said. "I'm fairly certain about that."