Antioxidants in sport: current thinking
Issue: Volume 28 Number 1
Trent Watson recently made a presentation on exercise and antioxidants at the Sports Dietitians Australia one-day update. Trent is a sports dietitian who has recently completed doctoral studies investigating the benefits of antioxidant supplements, exercise performance and recovery following exercise. This article summarises our current understanding of the role dietary antioxidants play in moderating the oxidative stresses that result from undertaking strenuous exercise, and provides an insight into the latest research investigating the benefits of antioxidant supplements.
Oxidative stress
Ninety-five to 98 per cent of oxygen that enters the mitochondria (the energy production control centre in cells) is converted to water in order to produce energy for the body. The remaining 2-5 per cent of oxygen is used to generate free radicals. These free radicals are called reactive oxygen species (ROS). Oxidative stress occurs when free radicals outnumber your antioxidant defence mechanisms.
Despite some confusion about techniques that measure oxidative stress, it appears that exercise increases oxidative stress. During exercise there is an increase in free radical generation of about 10-15 times overall, and about 100 times within the working muscles. This increase is likely to affect exercise performance adversely, and has been linked to increased fatigue, muscle damage and reduced immune function.
Antioxidants
Antioxidants produced within the body include superoxide dismutase, glutathione peroxidase, catalase and uric acid. Other antioxidants are consumed either via foods in the diet or from supplements; these include vitamin E, vitamin C, carotenoids and flavonoids. There are two others worth mentioning: co-enzyme Q10 and glutathione, which are produced within the body and can be consumed as supplements.
Antioxidants produced in the body are mobilised from tissue stores during exercise to combat the increased production of free radicals. Further to this, antioxidant production is regulated as a result of increased oxidative stress associated with exercise- which all makes perfect sense! Your body obviously recognises a crisis and acts accordingly. However, what about dietary sources of antioxidants- do these need to increase with an increased exercise load? A couple of questions come to mind:
- Does a diet high in antioxidants protect against the increased oxidative stress associated with exercise?
- Can antioxidant supplements improve the bodyís response to an increase in exercise?
The weight of current evidence suggests that antioxidant supplementation reduces markers of oxidative stress associated with exercise. However, only one study (performed at high altitude) has shown that antioxidant supplementation improves exercise performance.
The latest research
Trent Watsonís initial project investigated the differences between a diet rich in antioxidants with a reduced antioxidant diet. Subjects consumed either their normal diet (which equated to a high antioxidant diet) for two weeks, or a restricted antioxidant diet (reduced intakes of fruit and vegetables) prior to undertaking a strenuous exercise test and blood analysis. Not surprisingly, markers of oxidative stress were significantly higher following exercise during the restricted antioxidant diet; however, there were no differences at rest between the two trials.
Trent concluded his presentation by saying that 'The findings suggest that a diet rich in high antioxidant foods is capable of protecting against oxidative stress during exercise'. On quizzing Trent regarding the subjects' normal diets (high antioxidant diet), he reported that most subjects consumed five to six pieces of fruit daily, plus vegetables. I found that interesting, as many athletes I work with, particularly male adolescent athletes, would not consume that quantity of antioxidant-rich fruit and vegetables relative to their total energy intakes.
In a follow-up study, Trent had subjects undertake a two-week period of their normal training diet, followed by a two-week period of vitamin E supplementation, with either fish oils or olive oil. Following both two-week blocks, subjects underwent a strenuous exercise test and blood analysis. Interestingly, the results indicated that vitamin E supplementation increased markers of oxidative stress during exercise, the opposite to what one might think! Certainly, there is a clear need for more research into this area to further increase our understanding of the role of antioxidant supplements.
So, what do I take from this research? Well - the same as every good sports dietitian:
- A well-chosen diet, rich in foods with high antioxidant levels, is an athlete's best defence against the increased oxidative stress associated with strenuous exercise.
- Athletes should include at least two to three serves of fruit daily and at least four to five serves of vegetables (cooked or salad vegetables) in their diet.
- In essence, an athlete should increase their intake of antioxidant-rich foods (fruit and vegetables) to reflect their total energy/dietary intake.
Reference
Watson, TA, Callister, R, Taylor, RD, Sibbritt, DW, MacDonald-Wicks, LK, Garg, ML 2005. 'Antioxidant restriction and oxidative stress in short-duration exhaustive exercise,' Medicine and Science in Sports and Exercise, 37(1): 63-67.
