Stress fractures are one of the most common and potentially serious overuse injuries reported in athletes, both elite and recreational. With this in mind, Andrew Hamilton looks at recent evidence linking stress fractures and nutritional status… MORE
Antioxidant protection for athletes: is it time to ditch the pills?
During the last decade, research into antioxidant nutrition and athletic performance has been one of the most rapidly evolving areas of sports nutrition. But while many athletes take antioxidant supplements, the most recent research suggests that there may be more effective approaches to protecting the athletic body. Andrew Hamilton investigates
Although there’s plenty of evidence for the theory of antioxidant protection from free radicaldamage to cells generally, the link between antioxidant supplements and protection during athletic performance is poorly understood. Some studies appear to show a benefit(1-4), others have shown little benefit(5-8) and some animal studies have even suggested that large doses of antioxidant vitamins may be detrimental (8-11). Thus the antioxidant story has been characterised by changing scientific consensus and confusion: do athletes really need extra antioxidant protection, and if so what kind and how much?
Nutrients as antioxidants
Much of the research into antioxidants and athletic protection/performance has centred on nutrients such as vitamins A, C and E and the mineral selenium. Not only are these essential for other functions in the body, they also activate some of the key antioxidant enzymes in the body, which help to defend cells against free radical damage.
Most studies into antioxidants and athletes have involved athletes taking large doses of one or more of these antioxidant nutrients and then observing the effect on a subsequent bout of exercise. In particular, researchers have been keen a) to investigate whether the administration of antioxidant nutrients reduces the amount of oxidative damage caused by exercise and b) to see whether antioxidants actually enhance performance.
The answer to the first part of this question is that there does seem to be evidence that extra antioxidant nutrients can reduce the markers of free radical damage during subsequent exercise, but, as mentioned above, this is by no means clear-cut. In terms of performance gain, there’s little evidence to date that antioxidant nutrients can enhance actual physical performance but there may be other benefits associated with taking them.
One potential benefit is the reduction in post-exercise muscle soreness. To recap briefly, we now know that the destructive power of oxygen free radicals can be harnessed positively by immune cells to help break down exercise-damaged muscle tissue as part of the process of tissue repair. We also know that this immune-cell-mediated free radical damage appears to peak roughly 24 hours after exercise, which explains why muscle soreness also peaks then.
However, an optimally functioning antioxidant defence system appears to minimise extraneous free radical damage to otherwise healthy tissue, and may therefore help to minimise the degree of post-exercise muscle soreness; studies have shown that mice fed a compound called PEG-SOD (an extremely powerful free radical deactivator) performing prolonged eccentric exercise exhibited much less delayed-onset free radical damage to otherwise healthy muscle tissue than controls(12).
In our previous review of this subject we reported that, while some studies on supplementing antioxidant nutrients had produced inconclusive results(13,14), others had reported positive results including:
- reduced muscle soreness after shuttle running when taking vitamin C (15);
- reduced exercise-induced DNA damage in immune cells in women when taking vitamins C and E(16);
- enhanced muscle damage repair in older runners running downhill when taking vitamin E(17).
But what does the latest research say on this subject? In an American study earlier this year on vitamin C supplementation and delayed onset muscle soreness (DOMS), 18 healthy men were randomly assigned to one of two groups(18); the vitamin C group took 3g per day (1g morning, noon and night) of vitamin C for two weeks prior to a heavy exercise session and for four days afterwards, while the control group took placebo (dummy) pills. Both groups then performed 70 eccentric action repetitions of elbow flexion to stress the triceps of the rear arm in such a way as to produce maximum post-exercise soreness. Blood samples were also taken in order to measure creatine kinase (an indicator of muscle breakdown) and oxidised glutathione/glutathione ratio (a measure of free radical damage due to oxidative stress). Among the key findings were the following:
- At all times during the 96-hour period following the exercise, muscle soreness levels were significantly reduced in the vitamin C group compared to the control group;
- The increase in creatine kinase in the vitamin C group was significantly less at 48+ hours after exercise than the control (indicating reduced muscle breakdown);
- The oxidised glutathione/glutathione ratio was lower in the vitamin C group at four and 24 hours post-exercise than the control group (indicating less oxidative damage).
Meanwhile, a study published just two months ago looked at the effects of giving an antioxidant supplemented carbohydrate/protein drink to cyclists riding to exhaustion at 70% VO2max and then again 24 hours later at 80% VO2max(19). Compared to an isocaloric carbohydrate drink without added antioxidants, the antioxidant supplement drink reduced post-exercise muscle soreness and markers of muscle damage, even though the cyclists consumed less carbohydrate on this regime than when taking the carbohydrate-only drink.
However, not all the latest research on supplementing antioxidant nutrients is positive. A study published in the New Year on 22 runners during and after a 50km ultramarathon showed that, compared to a placebo, taking 1,000mgs of vitamin C and 300mgs of vitamin E did not reduce markers of post-exercise muscle damage or the contractile ability of the quadriceps and hamstrings(20).
The fruit and vegetable connection
The weight of evidence for supplementing antioxidant nutrients is on balance more favourable than not, but still far from clear-cut. One possible reason for the mixed results in these studies is that, until recently, researchers have focused on supplementing antioxidant nutrients but have paid scant attention to a huge range of naturally occurring compounds in plant foods called phytochemicals.
Many nutritionists now believe that dietary phytochemicals are at least as important as the antioxidant nutrients (if not more so) in protecting cells from free radical damage. There’s good evidence for this in a US study that looked at the effects of supplemental vitamin C (500mg per day) and vitamin E (400IUs per day) for two months on oxidative damage to DNA by measuring the levels of a marker substance called 8-hydroxy-2-deoxyguanosine (8-OHdG) excreted in the urine in 184 subjects(21).
Compared to placebo, neither vitamin reduced the level of markers of oxidative DNA damage. However, a closer analysis of the subjects’ diets showed that higher intakes of fruit and vegetables did reduce the amount of DNA damage, regardless of whether they were taking vitamins or placebo – persuasive evidence that the phytochemical content of the diet exerted more of a protective effect than supplemented antioxidant nutrients.
Further evidence comes from a very recent study this year on oxidative stress during exercise (30-minute run at 80% of VO2max), which compared the protective effects of daily supplemented antioxidant nutrients (400IUs of vitamin E and 1,000mgs of C) with a mixed fruit and vegetable juice powder concentrate containing 108IUs of vitamin E and 276mgs of vitamin C(22). The results showed that while only the vitamin supplements raised blood vitamin levels, both treatments reduced the amount of a marker of oxidative stress called protein carbonyl and by similar amounts. Compared to the vitamin supplements, the fruit/vegetable powder contained less than a quarter of the vitamins C and E, which suggests that additional antioxidant activity in the fruit/vegetable extract (ie phytochemicals) may have been important.
Fruit and vegetable research
Unsurprisingly, some researchers have begun to investigate whether diets or fruit/vegetable extracts containing high levels of phytochemicals offer superior antioxidant protection to athletes compared to conventional supplements, and the results so far look promising.
For example, a Spanish study last year tested the effects of an antioxidant-rich beverage containing black grape (81 grams per litre [g/L]), raspberry (93g/L) and redcurrant (39g/L) concentrates on exercise-induced oxidative stress in 26 cyclists(23). Half the group were randomly allocated to receive the antioxidant beverage 15 minutes pre-exercise and during a 90-minute bicycle ergometer test at 70% VO2max, while the other half received placebo. Measured protein carbonyl levels were 29% less in the fruit juice concentrate group. Moreover, 8-OHdG increased by 21% in the placebo group, but did not increase in the juice concentrate group.
More evidence for the benefits of brightly coloured fruits and vegetables comes from a Polish study on rowers carried out at the end of last year, which investigated the effect of consuming an increased intake of phytochemicals called anthocyanins (contained in chokeberry juice) on the measures of oxidative stress (free radical damage at a molecular level) in rowers performing intense workouts during a one-month training camp(24).
The rowers were randomly assigned to receive either 150mls of chokeberry juice daily (containing around 34mgs of active anthocyanins) or a placebo. Before and after the supplementation period, the subjects performed a 20-minute incremental rowing exercise test starting at 40% and increasing to 90% of VO2max. Compared to the placebo, taking the chokeberry juice produced a significant drop in the measures of free radical damaged induced by the strenuous exercise, and this was confirmed by lower levels of activity of an enzyme called glutathione peroxidase, which fights oxygen free radical species in the body.
Although relatively few studies have been conducted into the protective effects of enhanced fruit and vegetable intake in athletes, those that have appear to have produced far more positive results than those using single antioxidant nutrients. But do fruit and vegetables and their extract/juices actually enhance performance?
As we reported in a recent ‘What The Papers Say’ (PP 235), US researchers have looked at the effects of drinking cherry juice on post-exercise muscle damage and soreness(25). Volunteers drank 12 fl oz of either the cherry juice blend (equivalent to 120 cherries) or a placebo drink twice a day for eight consecutive days and on the fourth day they performed a bout of 2 x 20 maximum eccentric elbow flexion contractions, designed to induce muscle damage and soreness. After the exercises, strength losses averaged 22% with the placebo but only 4% with the cherry juice. Moreover, pain in the elbow flexors peaked at 24 hours with the cherry juice trial whereas it continued to increase in the placebo trial to peak at 48 hours, indicating reduced levels of oxidative damage in the cherry group.
A fascinating study meanwhile examined the relationship between reduced levels of dietary antioxidants and levels of free fatty acids in the blood (a major fuel source for humans at rest and during moderate intensity exercise)(26). Seventeen trained athletes followed a restricted antioxidant diet (containing about a third of the antioxidant content of a ‘high-antioxidant’ diet) for two weeks then underwent submaximal and incremental exercise testing to exhaustion. These results were compared to an initial test conducted while they were consuming their habitual high antioxidant diet.
Although the same types and amounts of fats were consumed during both diets, the results showed that circulating blood levels of the fatty acids omega-3 and omega-6 were significantly reduced on the low antioxidant diet and, while the exercise time to exhaustion was the same for both diets, athletes reported a higher perceived rate of exertion during submaximal exercise on the low antioxidant diet.
How can the athlete make best use of the current antioxidant knowledge to maximise protection during training and competition? The first thing to say is that the evidence that taking single doses of antioxidant nutrients such as vitamin C or vitamin E is beneficial is rather patchy; some studies show that single nutrient supplementation can reduce levels of muscle damage and a couple of studies have indicated that vitamin C may help reduce post-exercise muscle soreness. However, plenty of other studies have produced inconclusive results. Supplementing combinations of antioxidant nutrients (eg vitamins A, C, E and selenium) may be more beneficial as antioxidant nutrients do not work in isolation in the body but synergistically; a multi-antioxidant nutrient supplement probably makes more sense.
However, athletes should take note of the rapidly growing body of evidence pointing to the protective benefits of phytochemical-rich foods, such as brightly coloured fruits and vegetables. These not only contain antioxidant nutrients but hundreds of other naturally occurring powerful antioxidant compounds.
While the strength and depth of colour gives a very rough rule-of-thumb guide to the antioxidant activity of plant foods, a more scientific approach has been developed that measures the Oxygen Radical Absorbance Capacity (ORAC) of foods. The higher the ORAC score, the higher the potential capacity of a food to ‘quench’ oxygen free radicals and render them harmless. Natural fruits typically score between 500 and 900 ORAC units per 100 grams and the US Food and Drug Administration (FDA) has recently suggested that a daily consumption of around 7,000 ORAC units may provide optimum antioxidant protection (that’s around 5-10 servings of fruit and vegetables per day!).
However, some athletes with a high volume of training may struggle to include such large amounts of fruit and vegetables in their diet. This is because these foods are bulky and tend to contain relatively large amounts of water but low amounts of carbohydrate and very small quantities of protein. A large intake of fruits and vegetables increases satiety and could displace carbohydrate and protein-rich foods from the diet, making the task of muscle glycogen replenishment and recovery more difficult. The key then is to emphasise foods that are especially rich in antioxidant activity – ie with high ORAC scores.
But while ORAC scores give a better indication of antioxidant capacity of foods in vitro than mere colour, it’s important to realise that the relationship between ORAC scores and antioxidant activity in the body is still poorly understood; for this reason, it’s important not to sacrifice variety by consuming just one or two high ORAC foods in order to boost ORAC unit intake. Many lower scoring foods may offer particular benefits and work synergistically with other foods. Also beware of relying on some of the very high ORAC food extracts now coming onto the market claiming 20,000 ORAC units or more per 100g. It’s not yet known whether such values are accurate or if such concentrated antioxidants can be absorbed by the human body as effectively as those found in natural foods.
Andrew Hamilton BSc, MRSC, trained as a chemist and is now a consultant to the fitness industry and an experienced science writer
1. Biol Trace Element Res 1995;47:279-85
2. Int J Sport Nutr 1994;4:253-64
3. J Appl Physiol 1978;45:927-32
4. Acta Physiol Scand 1994;151:149-58
5. J Sports Med Phys Fitness 1999; 38(4):281-5
6. Am J Clin Nutr 1997; 65(4):1052-6
7. Cancer Epidemiol Biomarkers Prev 2000; 9(7):647-52
8. J. Nutr 2002;132:1616S-1621S
9. FASEB J, 2001; 15:A990
10. J Appl Physiol 2001; 90:1424-1430
11. Free Radic Biol Med 2001; 31:745-753
12. Am J Physiol 1990; 258: C429-35
13. Eur J Appl Physiol 2004; 92(1-2):133-8
14. Int J Sports Med 2002; 23(1):10-5
15. Int J Sport Nutr Exerc Metab 2001; 11(4):466-81
16. Free Radic Biol Med 2004; 36(8):966-75
17. Am J Physiol 1990; 259:R1214-9
18. Int J of Sport Nutr Exerc Metab 2006; 16;270-280
19. Med Sci Sports Exerc 2006 Sep; 38(9):1608-16
20. Med Sci Sports Exerc 2006 Jan; 38(1):72-80
21. Cancer Epidemiol Biomarkers Prev 2000; 9(7): 647-52
22. Med Sci Sports Exerc 2006; 38:6, pp1098-1105
23. Eur J Appl Physiol 2005 Dec; 95(5-6):543-9
24. Int J Sport Nutr Exerc Metab 2005; 15(1):
25. Br J Sports Med 2006; 40:679-683
26. Lipids, 2005; 40(4):433-5