Of the human studies, the results are more varied; of the four studies supplementing with vitamin C alone, three studies reported performance impairment and one reported improvement, although all were nonsignificant. In the untrained humans, while the results are mixed, the magnitude of the performance appears to be greater than in the trained, although the results are insignificant.
Overall, of the seven studies that supplemented with vitamin C alone, three demonstrated significant performance impairments. Those studies demonstrating a performance improvement were all nonsignificant outcomes.
Alternative studies have investigated the effects of vitamins C and E in combination on sport performance, which accounts for 4 of the 12 studies outlined in Table 1. Vitamins C and E are two vitamins that act synergistically, with vitamin E acting as the primary antioxidant and the resulting vitamin E radical then reacting with vitamin C to regenerate vitamin E. The evidence for a change in performance with vitamins C and E is unconvincing, with no studies listed in Table 1 reaching significance. It is common in those studies combining vitamins C and E to adopt a lower supplemental dose of each antioxidant compared with studies investigating vitamin C alone, perhaps explaining why there is little change to performance.
In a rat study, Copp et al. (2) incorporated a mixed antioxidant regimen of tempol and vitamin C given immediately before exercise and found that performance was impaired. Tempol is a membrane-permeable protein that mimics the action of superoxide dismutase, and it has been used for the removal of ROS, thereby acting as a partial antioxidant in its own right. While the effect of tempol on performance is difficult to interpret, the vitamin C supplement regimen adopted by Copp et al. (2) was 2 g·kg−1 of body weight, supporting the notion that small to moderate doses (<0.5 g daily) of vitamin C do not impair performance to the same degree as large doses (≥1 g) may. Interestingly, this is one of the first studies to demonstrate performance changes using an acute antioxidant supplement protocol.
Regarding the dose of vitamin C, of the four studies demonstrating significant performance decrements, three adopted large supplemental doses of vitamin C (≥0.5 g·kg−1 daily) (2,8,9) and two used a long-term supplement protocol (≥8 wk). The interaction of dose, chronic, and optimal intakes will be discussed further below.
Mechanisms for Performance Impairment
Gomez-Cabrera et al. (9) supplemented rats with vitamin C and showed clear reductions in endurance capacity, determined via V˙O2 maximum testing, and markers of mitochondrial growth (PGC1α). Similar consequences of 1 g daily of vitamin C administration were seen on antioxidant enzymes and mitochondrial growth factors in trained males (23). In particular, vitamin C prevented transcription of genes involved in mitochondrial biogenesis (PGC1α) and the training-induced increase in cytochrome c concentration (a marker of mitochondrial content) and messenger RNA (mRNA) expression of the antioxidant enzymes superoxide dismutase and glutathione peroxidase. It is shown that the exercise-induced rise in ROS is necessary for physiological adaptations, including PGC1α, to training. The inhibition of mitochondrial growth is one mechanism by which vitamin C impairs performance, given the important role that the mitochondria play in aerobic energy metabolism.
In addition to the retardation of mitochondrial growth, ROS has been shown to regulate various intermediate activation factors, responsible for optimal muscle cell function (5). ROS increases kinase activity (including ERK, JNK, and p38) while reducing phosphatase activity (including PTEN and calcineurin), resulting in the activation of transcription factors (p53, NK-κB, and ATF2) (3,21). Physiological changes that occur following moderate ROS production include increases in mitochondrial growth factors, cell survival proteins (B-cell lymphoma 2), reduction in muscle atrophy and proteins involved in cell death signaling pathways (Bcl-2), and amplification of immune function (3,5,21). It is likely that supplementing with vitamin C may blunt numerous transcription factors responsible for adaptation to training not just PGC1α.
The trigger to changes in transcription factors following exercise is the accumulation of ROS, as discussed. It is likely that a moderate concentration of ROS is required for optimal training adaptation and muscle function. However, a dose of vitamin C in excess of 1 g·d−1 will attenuate the exercise-derived rise in ROS. As supporting evidence, young, healthy males were given an acute dose of 1 g of vitamin C with α-lipoic acid and vitamin E, which reduced free radical concentration (measured using electron paramagnetic resonance spectroscopy) by 98% at rest and by 85% following cycling exercise (22). Therefore, a 1-g daily dose of vitamin C has been shown to effectively eliminate or reduce the exercise-derived free radicals.
Distinct from the effects of vitamin C on mitochondrial growth, one study has demonstrated reductions in vascular function with mixed antioxidants. Twenty-five healthy males taking an acute dose of vitamins C and E demonstrated reduced normal brachial artery vasodilatation during a submaximal forearm handgrip test, compared with placebo (22). Reductions in exercise-induced redistribution of blood flow to skeletal muscles have been shown to reduce exercise capacity (18). Thus, vitamin C may reduce exercise-induced blood flow, reducing exercise capacity and performance. Eskurza et al. (6) conducted a similar experiment investigating the impact of 500 mg of vitamin C daily for 30 d on exercise-induced vasodilatation and reported little difference between placebo and vitamin C. Presumably the 500-mg dose of vitamin C was insufficient to reduce the accumulation in free radicals with exercise.
Optimal Vitamin C Intake for Athletes
The U.S.-recommended intake of vitamin C for healthy individuals of 60 mg·d−1 is based on the needs of nonactive healthy individuals, therefore not necessarily appropriate for athletes (27). Given the evidence presented above, athletes should obviously consume <1 g·d−1, but is 60 mg too low? According to various reviewers (4,33), vitamin C supplementation with 0.2 to 1 g daily will reduce oxidative stress. While modest doses of vitamin C (<0.5 g·d−1) may reduce oxidative stress by a modest margin, there are likely other health benefits conferred by vitamin C that may assist an athlete.
Levine et al. (15) suggested an adequate intake of vitamin C to be 0.2 g daily from five servings of fruits and vegetables to maintain immune cell concentrations of vitamin C and prevent chronic diseases such as cancer. While athletes may not be considered prime candidates for cancer, certainly mortality rates in athletes with high training loads have greater mortality than those in sedentary population. Five servings of fruit and vegetables appear to be protective. However, consumption of vitamin C as a supplement in experimental trials did not decrease the incidence of colorectal and stomach cancers (10). Fruit and vegetable intake may be associated with lower cancer risk not because of vitamin C alone but perhaps because of the interactions between the vitamin and bioactive compounds and phytochemicals naturally present in plant foods. In one study, a higher plasma level of vitamin C was associated with lower mortality only for those with higher-dietary-plus-supplemental intakes of vitamin C, suggesting that vitamin C in isolation is not enough (13).
For athletes with low vitamin C concentrations at baseline, will supplementing alter performance outcomes? While it is natural to presume that athletes with low baseline levels will benefit, this issue has not been well answered. Certainly in the general population, patients with ulcers and low vitamin C concentrations who supplemented with 500 mg daily demonstrated improved healing compared with controls (28).
At present, it is safe to advise athletes to aim at the higher end of the recommended fruit and vegetable servings, in an attempt to consume adequate vitamin C, folate, magnesium, potassium, fiber, vitamins A, and other as-yet unidentified phytochemicals.
Chronically megadosing with vitamin C is not recommended, although a moderate intake of 0.2 g daily consumed in the form of fruit and vegetables can be justified for health reasons. The impact vitamin C has on sport performance will depend on the exercise duration, dosage period, and background diet of the athlete. As far as the ideal vitamin C dose is concerned, it could be speculated that higher intakes taken for short period of time, such as onset of illness or during training camps, may provide benefit. However, further research is obviously required to substantiate these theories.
Antioxidant supplements are widely used by athletes to avoid elevated oxidative stress, the consequences of which include muscle damage, immune dysfunction, and fatigue. It appears to be an erroneous assumption to provide athletes with megadoses of antioxidant supplements to avoid oxidative stress and subsequently performance. Unfortunately, very few studies have investigated the efficacy of antioxidant supplements making recommendations very difficult.
Vitamin C decreases oxidative stress taken in doses of 0.2 to 1 g·d−1. Vitamin C in larger doses appears to reduce training-induced adaptations by reducing mitochondrial biogenesis or by possibly altering vascular function (>1 g·d−1). A small dose of vitamin C (0.2 g·d−1), provided by five servings of fruit and vegetables daily, may be sufficient to reduce oxidative stress but not past a threshold that will impair optimal training adaptations. Short-term intakes (1 to 2 wk) of >0.2 g daily may benefit athletes during times of increased stress. Further research is required to clarify a dose-response and nutrient timing protocols on vitamin C.
The author declares no conflict of interest and does not have any financial disclosures.
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