Washington, May 20 (IANS): The leading theory as to why the blood of younger mice rejuvenates the muscles of older mice has been called into question as new research has found that a protein known as the "Fountain of Youth Hormone" appears to do just the opposite.

The vampiric exchange of young blood and old blood has long been reported to have anti-aging effects, but it was in 2013 when the Harvard University researchers first linked GDF-11, a protein that circulates in the blood, to this effect.

In 2014, the Harvard researchers demonstrated that GDF-11, short for Growth Differentiation Factor-11, can be used to rejuvenate the muscles and brains of older mice, a discovery later named by the respected US journal Science as one of the top 10 breakthroughs of 2014.

Now, in a study published in the US journal Cell Metabolism, a new analysis found the methods that were previously used to detect GDF-11 were not specific for the protein, but also measured another molecule it closely resembles, called myostatin, which is well known to inhibit muscle growth.

The new study, led by David Glass at the Novartis Institutes for BioMedical Research, in collaboration with Massachusetts General Hospital and the University of California, San Francisco, also found that GDF-11 showed hints of increasing with age, and clearly did not decrease with age, as previously suggested, Xinhua news agency reported on Tuesday.

In addition, regularly injecting mice with pure GDF-11 causes muscle repair to worsen, resembling effects seen in older age, they said.

"I think that these new results definitely raise questions as to whether GDF-11 was really being exclusively detected in the prior paper," Se-Jin Lee, an expert on growth/differentiation factors and molecular biologist at Johns Hopkins University who did not participate in the research, said in a statement.

"Clearly, these discrepancies will need to be resolved with additional studies, especially given the enormous effort being undertaken in the pharmaceutical community to target the myostatin pathway to treat muscle loss."

Caroline Brun and Michael Rudnicki of the Ottawa Hospital Research Institute have similar opinions.

"Clearly, like the mythical fountain of youth, GDF-11 is not the long sought rejuvenation factor," they wrote in a preview to the Cell Metabolism paper.

Given these new findings, "the suggested 'rejuvenating' activity of GDF-11 in the heart and brain should also be re-examined -- since the underlying premise of those other two manuscripts, that GDF-11 decreases with age, is contradicted by (the new) manuscript," they added.

Harvard stem cell biologist Amy Wagers, who discovered the role of GDF-11 in rejuvenated aging tissue, however, said the Novartis data on GDF-11 levels are not persuasive.

Although they "appear to conflict with" her group's, there could be multiple forms of GDF-11 and "we remain convinced that at least one form of GDF-11 declines in blood with age," Wagers told the Science magazine.