This may be sound statistics--although I doubt it--but after reading stuff like this, it's no wonder the lay public gets confused and frustrated. There is a metric ton of studies indicating that omega-3 fatty acids have considerable health benefits. (A brief summary of some of the literature is here.)
But because one study, of a special population, failed to find benefits--for reasons not understood--cranking up the meta-analysis machinery yields the conclusion that there is "no evidence of a clear benefit to health".
When I get some time, I think I'll do a meta-analysis of gravity. Maybe after factoring in studies of self-levitating gurus, it'll turn out that there's "no evidence of a clear gravitational effect".