Blood, not water, might be what flows from the fountain of youth, say scientists who found that blood from young mice introduced into elderly ones has reversed some effects of aging.

Significant improvements in the brains and muscles of the older mice were confirmed after two mice were conjoined, one older and one younger, allowing their blood circulations to combine, they say.

The same kinds of advantages yielded by the shared blood supply were also discovered when a special protein abundant in young blood was injected directly into the older mice.

In studies at Harvard University's Stem Cell Institute, and one conducted by Stanford University along with the University of California at San Francisco, researchers found stem cells in both muscles and brains of the older mice had their activity boosted, producing muscle tissue and neurons more efficiently when the young blood was introduced.

"All of the studies are very consistent -- the data are complementary and support one another," says Harvard biologist Amy Wagers.

The Harvard work was published in the journal Science, while the study of the California team has been published in Nature Medicine.

While the older mice in all the studies seemed to benefit by the blood exchanges, the same can't be said for the younger mice, the researchers found; young mice conjoined with an older one experienced a slowing of new cell production, and the old blood created apparent signs of premature aging.

The obvious question becomes, will such a process have any benefit in humans?

Researchers say the protein seemingly at the center of the younger blood's beneficial effect, known as GDF11, is also present in human blood but seems to decline with advancing age.

Tony Wyss-Coray, an author of the Nature Medicine study, say he has hopes of beginning at least a limited trial in humans almost immediately, with a focus on age-related degenerative diseases including Alzheimer's and dementia.

At a clinical trial set for later this year at Stanford, Alzheimer's patients will be treated with young blood and researchers will measure the cognitive abilities before they receive the blood and again afterward, he says.

"Right now, we can't do anything for Alzheimer's patients, and this seems so easy and simple," Wyss-Coray says.

Some scientists say any further research should proceed with caution, as ramping up the production of stem cells in humans might see them beginning to multiply uncontrollably -- the very definition of cancer.

"It is quite possible that it will dramatically increase the incidence of cancer," says Irina M. Conboy a, a bioengineering Professor at the University of California, Berkeley. "You have to be careful about overselling it."