A number of studies have found that taking antioxidant vitamins, namely vitamins C and E, hampers or blocks the physiological adaptation to exercise. However, other studies have not, so we have a state of conflicting information in this area. (You can read some of the background and other studies here.) Three of the most important exercise adaptations are increased VO2 max (better utilization of oxygen), increased mitochondria (the powerhouses of the cell), and better insulin sensitivity. Obviously you want these adaptations; these are the main reasons why exercise makes one healthy.
The adaptations to exercise depend upon the generation of reactive oxygen species (ROS) during exercise. ROS act as cellular signals, upon which the cell’s machinery to generate the adaptations is turned on or up. Since these signals are oxidants, it follows that antioxidants, such as vitamins C and E, might very well block them to the extent that their signalling power is rendered null, or at least diminished.
As mentioned, there are conflicting studies on this. Some of the studies that found that adaptations were blocked by C and E have been criticized on methodological grounds.
Along comes a new study to try to clarify our knowledge on antioxidants and exercise: Vitamin C and E supplementation hampers cellular adaptation to endurance training in humans: a double‐blind, randomised, controlled trial.
Recent studies have indicated that antioxidant supplementation may blunt adaptations to exercise, such as mitochondrial biogenesis induced by endurance training. However, studies in humans are sparse and results are conflicting.
Isolated vitamin C and E supplements are widely used, and unravelling the interference of these vitamins in cellular and physiological adaptations to exercise is of interest to those who exercise for health purposes and to athletes.
Our results show that vitamin C and E supplements blunted the endurance training‐induced increase of mitochondrial proteins (COX4), which is important for improving muscular endurance.
Training‐induced increases in and running performance were not detectably affected by the supplementation.
The present study contributes to understanding of how antioxidants may interfere with adaptations to exercise in humans, and the results indicate that high dosages of vitamins C and E should be used with caution.
So, add this study to the list of positive results: vitamins C and E diminish the physiological adaptations to exercise.
As a practical matter, what does this mean for one who exercises? A hint can be found in the discussion section of the article:
Our observations conflict with findings in a recent human study by Yfanti et al. (2010), who reported that supplementation with vitamin C and E did not alter training adaptations… A plausible explanation for this discrepancy could be that Yfanti et al. (2010) used a vitamin C supplement of 500 mg day−1, rather than the 1000 mg day−1 used in the present study. Furthermore, our participants were instructed to take the supplements in two doses (half dosage: 500 mg vitamin C and 117.5 mg vitamin E) 1–3 h before and within 1 h after each exercise session. By contrast, participants in the study by Yfanti et al. (2010) consumed their vitamin supplement only at breakfast. Given the pharmacokinetics of vitamin C in plasma [which decrease within a few hours], this might have caused a different cellular response to the supplementation.
It looks to me that the key here is not to take vitamin C in the few hours around exercise, and that may very well be enough to negate its effects on exercise.
I intend to continue to take vitamin C in under 1 gram amounts, but not around workouts. However, it certainly does appear that the common practice of athletes of taking large amounts of antioxidant vitamins is counterproductive.