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Micronutrients, including vitamin supplements, are widely used in the general population and by athletes. A variety of claims or areas of interest have promoted the use of vitamin supplements, with antioxidant claims becoming a popular interest over the past two decades. While the antioxidant issue has typically targeted the prevention of ageing or various diseases associated with oxidative damage, it has also been recognised that exercise is a stimulator of the generation of oxygen radical species. Hence, there has been interest in whether athletes might have additional needs for antioxidant vitamins to counteract such damage. Because of the considerable and long-term interest in this topic, we have devoted an entire part of this series on supplements to the antioxidant vitamins, A, C and E.
Vitamins A, C and E
D S Senchina
Vitamins A, C and E are being discussed together in this series because all three have antioxidant properties, though they differ in many other respects (see table 1). Recent research has focused on these vitamins’ antioxidant prospects because exercise induces free radical (ROS, reactive oxygen species) release which leads to cellular damage, fatigue and overtraining. Literally hundreds of studies on antioxidant vitamin supplementation (AVS) and exercise have been published. In the representative sample reviewed here, vitamin A (V-A) was given as β-carotene, vitamin C (V-C) as ascorbic acid and vitamin E (V-E) as α-tocopherol.
Systematic training causes the body to develop adaptations naturally to the increase in exercise-induced ROS,1 but it has been hypothesised that AVS may provide additional benefits.2 Studies validate as well as refute this possibility. Validating this possibility, in one study nine sedentary and nine active young adult males were given 50 mg/day V-A+1000 mg/day V-C+800 mg/day V-E for 2 months and performed a treadmill ergometer VO2max test before and after the treatment period.3 Before supplementation, sedentary subjects (but not trained subjects) exhibited exercise-induced increased white blood cell and granulocyte levels and increased erythrocyte aggregation and deformability; after supplementation, these phenomena did not occur. Both sedentary and trained subjects showed an elevated erythrocyte lipid peroxidation after the VO2max test pre-supplementation but not post-supplementation. This study suggested that AVS augmented the already increased antioxidant capacity found in trained individuals. In a study of adolescent male basketball players undergoing intense training for 35 days, athletes dosed with 500 mg/day V-C+150 mg/day V-E demonstrated higher plasma content of V-A (retinol), V-C and V-E concomitant with elevated erythrocyte glutathione peroxidase (GSH-Px) and reduced glutathione (GSH; both important antioxidant molecules) compared with non-treated controls.4
Refuting the prospect of AVS providing additional physiological benefits in trained athletes, a different study supplemented 21 sedentary young adult males with 500 mg/day V-C+400 IU/day V-E for 16 weeks. After the first 4 weeks, subjects also performed endurance training for 12 weeks.5 With results from previous studies, the researchers noted that, although AVS reduced plasma IL-6 levels in response to an acute exercise bout (cycle Pmax test) before training, AVS did not show the same effect after training. Plasma IL-6 levels were, in fact, higher in the supplemented/trained group than the placebo/trained group, indicating that either the supplement only worked in the sedentary condition or that the supplement attenuated the effects of training. Using a model of eccentric training, another team of researchers supplemented 14 young adult males with 1 g/day V-C+400 IU/day V-E for 11 weeks combined with 7 weeks of resistance training and found no differences in acute exercise-induced plasma GSH or other markers of antioxidant capacity and redox status compared with similarly trained but non-supplemented controls.6
The effects of AVS on performance have also been studied. Using subjects discussed previously,5 one team showed that maximal performance parameters and muscle biopsy-determined glycogen, citrate synthase and β-hydroxyacyl-CoA activity levels were no different in AVS-supplemented versus non-supplemented groups.7 One study of eight active men given 1 g/day V-C for 4 weeks concomitant with aerobic training showed that AVS had no impact on running performance.8 Another team reported that 10 well-trained young adult male runners given either 500 mg/day V-C+100 IU/day V-E or placebo for 2 weeks in a cross-over design showed no differences in 8 km time trial performance.9 Intriguingly, this latter study used an environmental chamber that simulated a hot, humid, ozone-polluted environment for the time trials and also reported that total plasma antioxidant V-C and V-E concentrations were higher, whereas plasma and nasal lavage CC16 (Clara cell protein; a marker for lung cell damage) levels were lower in the AVS-treated compared with non-treated trials. Addressing just V-C, it appears that those studies dosing athletes at 1 g/day or greater demonstrate negative effects on performance more often than not.10
Although the data together may appear ‘equivocal’, it would be premature to draw any conclusions regarding the effectiveness or lack of AVS in athletes. Experimental parameters (including subject, exercise and dosing characteristics) are variable across studies, rendering it difficult to make direct comparisons. Regardless of this, athletes who consume well-balanced meals congruent with contemporary guidelines should be receiving adequate supplies of these vitamins.11 Several have opined that athletes should not be likely to receive any additional benefits from exceeding recommended daily intakes (RDI)12 ,13 and that additional supplementation may only be necessary in cases of extreme, prolonged physical exertion under adverse environmental conditions.9 ,14 Nevertheless, multivitamin supplements are readily available to athletes, vary widely in their vitamin doses, and often contain antioxidant vitamins at RDI or greater levels. For example, in an impromptu survey of a dozen different multivitamin supplements available at a central Iowa grocery store in mid-September, V-A content ranged from 1750–6100 IU (35–120% RDI), V-C 60–250 mg (100–417% RDI) and V-E 22.5–200 IU (75–667%). Over-supplementation (toxicity) is more likely to occur with the fat-soluble vitamins. Excess antioxidant vitamins may be detrimental to athletes when they interfere with normal levels of ROS-mediated intracellular signalling, disrupting redox balance and interfering with normal muscle cell function15 and performance. The current data only suggest that AVS, in the doses used across studies, either improved or had no effect on physiologically relevant markers of antioxidant capacity and either decreased or had no effect on performance outcomes.
There is no question that athletes experience increased oxidative stress as a result of their activity levels, but the issue of whether or not athletes need higher dietary antioxidants than less active counterparts remains under debate. There are hypotheses as well as data to support a beneficial use, a neutral outcome in people who are already achieving intakes at recommended levels from dietary sources, and even a detrimental effect on training adaptations. Athletes consuming a balanced diet commensurate with their caloric needs, including vegetables, fruits and whole grains, are likely receiving adequate levels of antioxidant vitamins. For those athletes ‘erring on the side of caution’, food-based sources of antioxidant vitamins may be preferable to supplement-based sources because the possibility of toxicity is lower when foods are utilised. Although daily multivitamin supplements are generally regarded as safe, over-supplementation may result in disrupted redox balance or diminished performance. Further studies may help to clarify which of these outcomes is the most likely, given the various scenarios of training, environment and diet in which athletes may find themselves.
Competing interests None.
Provenance and peer review Commissioned; not externally peer reviewed.
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