How safe is Peyton Manning's stem cell therapy?

Indianapolis Colts star quarterback Peyton Manning said over the summer that he would do "everything I can to get my health back," after a bulging disc in his neck benched him for the foreseeable future. Apparently, the future Hall of Famer really meant it. According to Fox's Jay Glazer, Manning flew to Europe on a private jet in early September for an experimental stem-cell treatment that's not approved in the U.S. Here's what you should know:

What did Manning have done to his neck?

Doctors "took some fat cells, probably out of his belly," put them in a culture, and injected them into Manning's neck, says Fox's Glazer. The hope is "that these cells are going to regenerate the area" and fix the damaged tissue. The stem cells were probably mesenchymal cells, which can develop into bone, cartilage, fat, or muscle, says Dr. Joshua Hare, a University of Miami stem cell expert.

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Is that safe?

"When you leave a well regulated environment, all bets are off," says Dr. Lawrence Goldstein at U.C. San Diego's stem cell program. There's a real risk of injury and "no evidence of benefit to be gained" from stem-cell injections. As such, it's a problem "when a highly visible celebrity athlete chooses to undergo an untested/unproven therapy," Goldstein adds. Someone like Manning might "encourage many other people to ignore scientific evidence and to substitute hope and blind trust for proof."

Did it work for Manning?

Apparently not. After his "medical Hail Mary" in Europe, Manning had his third neck surgery in 19 months. We're probably up to a decade away from being able to heal an injury like Manning's with stem cells, says Dr. Hyun Bae at Saint John's Health Center in Santa Monica, Calif. For now, he needs about six months off the football field. But on the upside, Bae says, Manning's treatment "most likely doesn't create much harm."

Why isn't this type of stem-cell treatment approved in the U.S.?

Blame — or thank — the Food and Drug Administration, says Rebecca Taylor at Life News. Unlike many European government agencies, the FDA has classified stem cell transplants as drugs, meaning they "must go through the same rigorous phase trials that a new drug would." U.S. researchers need to "hurry up with some of the clinical research," says the University of Miami's Hare. "The medical tourism makes it even more urgent."

How common is this type of "medical tourism"?

"I read nearly everyday about patients going to China, India, and Germany to get treated for anything from spinal cord injuries to autism," says Taylor. People either intuitively believe that their own body has the power to heal itself, if only given the chance, or they've run out of the more conventional treatments available in the U.S., says Hare. Besides Manning, other recent high-profile stem cell "tourists" include Texas Gov. Rick Perry (R), who traveled to Japan in July to have fat-derived stem cells injected into his back, and NFL star Terrell Owens, who started stem-cell therapy in South Korea on Monday to treat his torn anterior cruciate ligament.

Does stem-cell tourism violate NFL policy?

No, says NFL spokesman Greg Aiello. Stem-cell therapy may not be legal in the U.S., but "in and of itself is not prohibited under the steroids policy." That tells me the NFL is having trouble walking the "vanishingly thin line between [performance] enhancement and simple maintenance," says Tommy Craggs at Slate. Don't worry, says Mike Florio at NBC Sports. "With the league constantly revising the list of things players can and can’t do, don't be surprised if non-U.S.-approved stem-cell therapy lands on the roster of NFL 'thou shalt nots'" soon enough.

Sources: ABC News, LifeNews, Los Angeles Times, MSNBC/Health News, NBC Sports, NFL, Slate, Washington Post