High-Dose Antioxidant Supplementation May Limit Training Adaptation

Antioxidants are known to protect the body against the potentially damaging effects of free radicals and their metabolic products such as reactive oxygen species (ROS). These products are managed in the body through intercellular synthesized enzymatic antioxidants as well as from ingested forms such as vitamins A, C, and E, carotenoids including β-carotene, and other plant-based polyphenols. Free-radical ROS have at least one unpaired electron that can cause oxidative damage to cellular membranes, tissue proteins, DNA, and other components of the body.

Antioxidants function to prevent this damage by the following actions:

• Preventing ROS formation
• Intercepting ROS attack by scavenging reactive metabolites and converting them into less reactive molecules
• Binding to free-radical catalysts to prevent the initiation of free-radical reactions
• Preserving cell membrane stability
• Providing a favorable environment for the effective functioning of other antioxidants or production of new antioxidants

Free radicals are internally produced (or activated) through various means including ultraviolet light (UV rays), cigarette smoke, alcohol, high-fat diets, and the metabolic actions of white blood cells (WBCs) that occur during the eradication of foreign materials or bacteria; without even considering exercise-induced stimuli. Concerning exercise, 2-5% of oxygen consumed during any activity forms ROS. Free-radicals are produced as a byproduct of aerobic metabolism within the mitochondrion and during strenuous resistance training (particularly eccentric loading) in conjunction with the considerable microscopic damage that occurs to myofibrils. Specifically, damaged muscle provokes an immune response (WBCs are attracted to the tissue and begin breaking down damaged fibers to initiate repair) which initiates the production of free-radical ROS. This causal relationship between damaged myofibrils and ROS is believed to be an underlying cause of disrupted muscle function as well as soreness.

According to a collection of literature published from 1990-2006, the negative effects of exercise-induced muscle damage (consequently associated with free-radical ROS action) include:

• Muscle pain, soreness, and stiffness
• Reduced range of motion
• Higher than normal blood lactate concentration
• Higher perceived exertion during exercise
• Loss of strength and dynamic power output that can last for days
• Impaired restoration of muscle glycogen due to temporarily impaired glucose uptake capability

Due to the knowledge of the previously examined concepts, it has been presumed (based on sales of supplements) by athletes, body builders and fitness enthusiasts that antioxidant supplementation (oftentimes in daily dosages that equate to 1000% of the standardized RDA value) may serve as a potentially useful ergogenic aid. The idea is that antioxidants promote optimal recovery and support the immune system during strenuous periods of training. Research has thoroughly examined this potential recovery strategy with a mixed outcome. The literature shows a number of inconsistencies and contradictory findings making the use of vitamin antioxidant supplements to reduce free-radical-mediated damage unclear as an effective ergogenic aid. Updated studies actually reveal antioxidants inhibiting the potentially beneficial effects of ROS after exercise-induced damage. Novel research has revealed the important role of ROS as signaling molecules that alter contractile function and adaptive processes in muscle promoting gains. Specifically, ROS seem to be involved in the modulation of gene expression (DNA code) onto new muscle fibers. This means that free radicals actually play a role in promoting training adaptations, rather than preventing them. Therefore, consumption of high-dose antioxidant supplements could actually impair the ability of an individual to adapt to a training stimulus. Further research appears to be needed to document the effects on training adaptations from long-term use of antioxidants, but current recommendations seem to be adequate.

Whereas the current recommendations relative to activity demonstrate a level of efficacy, excess has not shown the purported benefits. Whereas antioxidants consumed from food sources like fruits and vegetables do not present issues, supplemented forms are linked to toxicity risk. Therefore one should consider the potential dangers of high-dose antioxidant supplementation, rather than hoping that more is better and research just needs to catch up to practice. Some examples of potential danger and negative effects include:

• Increased intake of vitamin E and β-carotene among smokers has actually been shown to increase the risk of lung cancer
  • Excess antioxidant action may interfere with important process needed to breakdown damaged/hazardous cells in the body (in this case, the killing of cancer cells)
• Large doses of vitamin C are associated with urinary stone formation, impaired copper absorption, and diarrhea
• Large doses of vitamin A by pregnant women can cause birth defects
• Large doses of vitamin E can impair absorption of vitamins A and K

As stated previously, more research is needed to properly document the effects of long-term antioxidant use, but the following facts may help personal trainers to decide whether to recommend antioxidant supplementation to clients on rigorous training regimens or not:

1. Numerous studies indicate that the body’s natural antioxidant defense system is enhanced as an adaptation to exercise
2. High dose supplementation does not directly improve exercise performance and recent studies indicate that it may impair the adaptive response to exercise training
3. Individuals who exercise regularly have a lower incidence of CHD, obesity, diabetes, and some types of cancer compared with sedentary people which suggests that the benefits of regular exercise outweigh the risks of free-radical-mediated damage
4. Mega-doses of antioxidant vitamins can have undesirable side effects in some individuals and may reach toxic levels
5. Clients can obtain sufficient intakes of natural antioxidants by consuming a well-balanced diet that is rich in a variety of fruits and vegetables