You may remember a study that suggested that antioxidant supplement actually negated the effects of exercise in muscle tissue. (The reactive oxygen species generated are apparently being used by the cells as a signaling mechanism, one that you don't necessarily want to turn off). That was followed by another paper that showed that cells that should be undergoing apoptosis (programmed cell death) could be kept alive by antioxidant treatment. Some might read that and not realize what a bad idea that is - having cells that ignore apoptosis signals is believed to be a common feature in carcinogenesis, and it's not something that you want to promote lightly.
Here are two recent publications that back up these conclusions. The BBC reports on this paper from the Journal of Physiology. It looks like a well-run trial demonstrating that antioxidant therapy (Vitamin C and Vitamin E) does indeed keep muscles from showing adaptation to endurance training. The vitamin-supplemented group reached the same performance levels as the placebo group over the 11-week program, but on a cellular level, they did not show the (beneficial) changes in mitochondria, etc. The authors conclude:
Consequently, vitamin C and E supplementation hampered cellular adaptions in the exercised muscles, and although this was not translated to the performance tests applied in this study, we advocate caution when considering antioxidant supplementation combined with endurance exercise.
Then there's this report in The Scientist, covering this paper in Science Translational Medicine. The title says it all: "Antioxidants Accelerate Lung Cancer Progression in Mice". In this case, it looks like reactive oxygen species should normally be activating p53, but taking antioxidants disrupts this signaling and allows early-stage tumor cells (before their p53 mutates) to grow much more quickly.
So in short, James Watson appears to be right when he says that reactive oxygen species are your friends. This is all rather frustrating when you consider the nonstop advertising for antioxidant supplements and foods, especially for any role in preventing cancer. It looks more and more as if high levels of extra antioxidants can actually give people cancer, or at the very least, help along any cancerous cells that might arise on their own. Evidence for this has been piling up for years now from multiple sources, but if you wander through a grocery or drug store, you'd never have the faintest idea that there could be anything wrong with scarfing up all the antioxidants you possibly can.
The supplement industry pounces on far less compelling data to sell its products. But here are clear indications that a large part of their business is actually harmful, and nothing is heard except the distant sound of crickets. Or maybe those are cash registers. Even the wildly credulous Dr. Oz reversed course and did a program last year on the possibility that antioxidant supplements might be doing more harm than good, although he still seems to be pitching "good" ones versus "bad". Every other pronouncement from that show is immediately bannered all over the health food aisles - what happened to this one?
This shouldn't be taken as a recommendation to go out of the way to avoid taking in antioxidants from food. But going out of your way to add lots of extra Vitamin C, Vitamin E, N-acetylcysteine, etc., to your diet? More and more, that really looks like a bad idea.
Update: from the comments, here's a look at human mortality data, strongly suggesting no benefit whatsoever from antioxidant supplementation (and quite possibly harm from beta-carotene, Vitamin A, and Vitamin E),