51成人动漫

Can a single meta-analysis of three retrospective studies really close the case on a controversial topic? Two researchers believe they have done just that for the risk of concussion for athletes in contact sports at higher altitude.

, of Georgia State University in Atlanta, and , of High Point University in High Point, N.C., said their review provides conclusive evidence that playing football at a higher elevation isn't protective against concussion -- and that other researchers shouldn't waste any more time on the question.

"We firmly believe epidemiologic data are already sufficient to indicate that this is an issue that should not be examined further," they wrote in a in .

They noted that any additional research "will simply divert resources from more clinically effective research aimed at identifying modifiable risk factors for concussion, developing scientifically sound technologies that improve athlete safety, and improving acute and long-term management of sport-related head injuries."

The potential biological mechanism? If higher altitude increases cerebral blood flow, it may lead to a slight brain swelling that creates a tighter fit between the brain and the skull, which prevents the brain from sloshing around, and potentially reducing concussions.

But Zavorsky and Smoliga say that mechanism "is not scientifically sound."

In their meta-analysis, they found similar ranges in concussion rates at sea level (0.07% to 0.45%) and at higher altitudes of about 650 ft and above (0.06% to 0.50%), leading to no difference in relative risk of concussion in a random effects model.

However, not everyone is convinced that these findings are the last word. Four concussion experts from the University of Florida in Gainesville -- , , , and -- reviewed the research letter, and said that while the statistics were solid, there were several issues that limit the conclusions that can be drawn.

Only three studies met their inclusion criteria, which is too small a number "to 'close the book' on any scientific question, the large number of hits [included] notwithstanding," DeKosky said, speaking on behalf of the foursome, none of whom were involved in the meta-analysis.

And all three were retrospective studies, which carry several caveats that prevent researchers from drawing firm conclusions. In this case, for instance, that includes variables such as "differences in acclimatization to the altitude by the visiting team and if it had an effect, and effects on autoregulation that might be different in low-altitude versus high-altitude players," DeKosky said.

"The idea that three retrospective studies answers all the questions to the point where there is not more work to be done is, I think, a risky conclusion to an otherwise methodologically well-performed study," he told 51成人动漫. "Prospective studies, especially noting differences in the high-altitude acclimated versus the low-altitude dwelling visiting team, would be helpful in trying to determine actual risks."