Thanks to Alex at Sweat Science for this one, which I have also posted on drdavemd.com.
When no good evidence exists to guide us (doctors, scientists, athletes, etc.), we frequently rely on theory or what "sounds good" to guide our actions. For example, we know that compounds called free radicals, or oxidized particles, in the body can damage things like our DNA. Our white blood cells use oxidants and free radicals to kill invading pathogens. The assumption, therefore, might be that getting rid of oxidants, by taking antioxidant supplements, would be a good thing, and prevent damage to the body. Examples include vitamin C, vitamin E, or coenzyme Q10.
A new review in Sports Medicine contradicts that notion by reviewing evidence, in sports medicine literature, and comes to the conclusion that taking antioxidant supplements might actually be harmful. While they admit that the vast majority of sports med studies are poor quality, they found evidence that oxidants may be functioning as signals to the body as part of repair mechanisms. Thus, counter to our intuition, blocking them may be bad and not good. This is why the ideal thing to do is not rely on theory or basic science, but to actually do a trial where you have some people take a supplement while others do not, and see what the differences are (ie: a randomized trial).
This conclusion parallels some recent and landmark studies in medicine (HOPE-TOO, Physician's Health Study II) which have found no significant benefit and the possibility of harm with vitamin supplementation, when given to people without deficiencies in those compounds. As an aside, my quick Google search for those two papers also turned up an essay refuting the results of the HOPE-TOO trial. While the essay is hosted on a website that sells vitamins and supplements, the author is quick to blame the "pharmaceutical-medical-industrial complex" for "attacking" the "vitamin/alternative medicine movement" by being so bold as to, you know, do research. The essay has many references and makes many arguments against the studies, but failed to refute the ultimate findings that the supplements did not have any benefit.