Which Antioxidants Help and Which Hurt?

Everyone knows that you should eat a healthy diet with plenty of vitamins in it. But what about pills with concentrated doses of antioxidants? The evidence there is much shakier, even though many athletes continue to turn to supplements in search of an edge.

In the upcoming issue of the journal Sports Medicine, Andrea Braakhius of the University of Auckland and Will Hopkins of Victoria University in Australia take a comprehensive look at the effect of antioxidants on sport performance. Crucially, they’re not looking at intermediate endpoints like antioxidant levels or markers of inflammation in the body – they’re looking only at studies where actual increases or decreases in athletic performance are measured. In all, they pool the results of 71 different studies.

The overall take? “Chronic consumption of dietary antioxidants is likely harmful.” In other words, if you faithfully take your 1,000 milligram vitamin C pill every morning, you’re likely doing more harm than good to your athletic performance. Of course, the details are a little more complex.

There’s no doubt that intense exercise leads to the production of “reactive oxygen species” that can damage cells and muscle fibers, disrupt immune function, and contribute to fatigue. The tricky part is that reactive oxygen species can also have positive effects, triggering glycogen resynthesis, reducing infection risk, and initiating the body’s all-important adaptive responses to training. That last point is the most concerning one for athletes: use antioxidants to suppress the reactive oxygen species generated by training, and you may also suppress the fitness gains that are supposed to follow training. This effect has been demonstrated in numerous studies.

As a result of these mixed effects, context matters. Antioxidants may be useful in some cases, and harmful in others. A general trend that emerges in the review is that acute use of some antioxidants may have immediate benefits for performance, but if you keep taking them on a regular basis, they end up having negative effects.

But even the acute benefits are very difficult to harness, when you take a closer look. For example, N-acetylcysteine (better known as NAC) has some of the most positive research suggesting that it can help performance acutely (though it, too, has negative effects if taken chronically). But it’s extremely difficult to absorb when taken orally (only 6-10 percent is absorbed), so most of the studies use intravenous injections – which, as the authors acknowledge (and the current controversy over Alberto Salazar’s use of injected supplements emphasises) raises some significant ethical questions. If you do take it orally, there are serious side effects including “conjunctival irritation; dysphoria, vomiting, diarrhoea, nausea and loss of coordination.” All in all, it seems like a pretty iffy proposition.

Other seemingly promising supplements run into similar tradeoffs. As the authors conclude: “In summary of this review, chronic vitamin E intake appears to enhance performance at altitude but potentially impairs performance at sea level. Quercetin has a small benefit on endurance performance but only in untrained subjects. Resveratrol appears to benefit performance in fit, healthy rats but is potentially detrimental to inactive rodents and humans…” And so on.

One antioxidant the authors didn’t cover in detail is vitamin C, because one of them had already published a detailed review of its effects on sport performance in 2012. Of the 12 studies in that review, four showed significant negative results on performance and another four showed negative results that weren’t statistically significant – a fairly damning overall picture. The conclusion from that study is one that I think can be applied more generally to the antioxidants in the new review:

“Doses of ∼0.2 g·d(-1) of vitamin C consumed through five or more servings of fruit and vegetables may be sufficient to reduce oxidative stress and provide other health benefits without impairing training adaptations.”


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