The case against antioxidant vitamins

The December issue of Sports Medicine has an enormous, detailed review of research on the effect of antioxidant (i.e. vitamin C, vitamin E, coenzymeQ10, etc.) supplements on training. To most people, this seems like a no-brainer: what could be smarter than popping a multivitamin as “insurance” in case your diet isn’t giving you all the vitamins you need? But (as I’ve blogged about before) there’s an emerging school of thought arguing that taking antioxidants can actually block some of the gains you’d otherwise get from training. Here’s how I explained the debate back in April:

The traditional theory goes like this: strenuous exercise produces “reactive oxygen species” (ROS), which cause damage to cells and DNA in the body. Taking antioxidant supplements like vitamins C and E helps to neutralize the ROS, allowing the body to recover more quickly from workouts.

The new theory, in contrast, goes like this: strenuous exercise produces ROS, which signal to the body that it needs to adapt to this new training stress by becoming stronger and more efficient. Taking antioxidant supplements neutralizes the ROS, which means the body doesn’t receive the same signals telling it to adapt, so you make smaller gains in strength and endurance from your training.

The new paper comes down firmly on the side of the latter view:

The aim of this review is to present and discuss 23 studies that have shown that antioxidant supplementation interferes with exercise training-induced adaptations. The main findings of these studies are that, in certain situations, loading the cell with high doses of antioxidants leads to a blunting of the positive effects of exercise training and interferes with important [reactive oxygen species]-mediated physiological processes, such as vasodilation and insulin signalling.

So is this definitive? Far from it. As the review notes, there have been a few studies that found beneficial effects of antioxidant supplements on exercise performance, tons that have found no effect, and a few (23, to be exact) that have found negative effects. What most of the studies have in common:

As commonly found in sports nutrition research, the vast majority do not adhere to all the accepted features of a high-quality trial (e.g. placebo-controlled, double-blind, randomized design with an intent-to-treat analysis). Indeed, most studies fail to provide sufficient detail regarding inclusion and exclusion criteria, justification of sample size, adverse events, data gathering and reporting, randomization, allocation and concealment methods, and an assessment of blinding success. The poor quality of the majority of studies in this field increases the possibility for bias and needs to be always considered when evaluating the findings.

This is a really important point to bear in mind, and not just when it comes to sports nutrition. Whatever the supplement, training method, or piece of equipment you’re talking about, there’s nearly always a crappy, poorly executed study that seems to “prove” that it works. So where does that leave us? On this topic, I’m in agreement with the authors:

We recommend that an adequate intake of vitamins and minerals through a varied and balanced diet remains the best approach to maintain the optimal antioxidant status in exercising individuals.