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Nutrition and Performance in Sport

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Christophe Hausswirth

Chapter 3. Nutrition and lifestyle

Topic 2. The use and abuse of dietary supplements by athletes

RJ Maughan

Résumé

Regular intensive training and the physical and mental stresses of competition in high-level sport can markedly increase the need for dietary energy intake and for macronutrient and micronutrient intake. This requirement can generally be met by dietary management without the need for dietary supplements. Nonetheless, a small number of supplements may be useful to some athletes. Where there is a diagnosed deficiency of a specific vitamin or mineral, a supplement may be a useful short-term solution while a longer-term dietary solution is implemented. Dietary creatine supplementation (at a dose of 3 to 5 g per day for about 30 days or higher doses for shorter periods) can increase muscle power output, most particularly during intermittent sprint exercises. Furthermore, creatine intake can augment muscle adaptations to resistance training. Buffering agents, including bicarbonate and β-alanine, may be of use in high-intensity events where metabolic acidosis is a limiting factor. The potential of low-dose caffeine ingestion (2 to 5 mg/kg body weight) to enhance endurance performance is well established. Recent evidence supports the use of dietary nitrate supplements, which can reduce the oxygen cost of exercise and thus enhance performance. Successful performance also depends on injury prevention and health maintenance. Glucosamine and chondroitin may be useful in the treatment of joint pain and osteoarthritis in some athletes, but there is no evidence that administration of these supplements will be preventative. The efficacy of antioxidant or vitamin C intake in excess of the normal recommended dietary dose is equivocal. Poor quality assurance in the manufacture of supplements may result in the absence of the active ingredient in some commercial supplements and in the presence of prohibited substances that will cause the athlete to fail a doping test. It is recommended that dietary supplements are only used on the advice of a qualified sports nutrition professional.

Texte intégral

1. Introduction

1Although many athletes, at all levels of competition, place great emphasis on the use of dietary supplements, it is important to recognize that, of all the factors that determine athletic performance, supplements can play only a very small role. Compared with factors such as talent, training, tactics and motivation, nutrition has a relatively small effect on performance and supplements are only a small part of the athlete’s nutrition strategy. Nevertheless, at any level of competition, athletes are likely to be fairly evenly matched. At the highest levels of competition, all of the participants will be genetically gifted, will have trained intensively and will be highly motivated. Where the margins between success and defeat are small, the small factors can become the ones that determine the outcome of sporting contests. This perhaps explains why athletes are constantly searching for any opportunity to gain an advantage over their competitors.

2. The point on the recent studies

2Many published surveys have shown a high prevalence of use of supplements among athletes. An unpublished survey by Depiesse et al of 310 competitors at the IAAF World Championships revealed that supplements were used by 86% of the athletes: this included 83% of men and 89% of women surveyed. Reasons for using supplements given by these athletes were:

– to aid recovery from training

71%

– for health

52%

– to improve performance

46%

– to prevent or treat an illness

40%

– to compensate for a poor diet

29%

3Hekkinen et al. (2011) also reported a high prevalence of supplement use in elite Finnish athletes: (81% in 2002 and 73% in 2009). In contrast to this, however, Karimian et al. (2011) examined supplement use in 250 male and 250 female bodybuilders in Iran: supplement use was reported by 87% of the men but by only 11% of the women: reasons for using supplements were reported to be for health (45%), enhancing the immune system (40%) and improving athletic performance (25%). The prevalence of use does seem to vary widely, and this is explained in part by the lack of agreement as to what constitutes a dietary supplement. Some surveys have included sports drinks, energy bars, gels and other sports foods, while others adopt a more restricted definition. Some of the reasons given by athletes for their use of supplements are the same as those given by health-conscious members of the general population: there are concerns that food quality has declined due to intensive farming and modern food-production methods. The majority of supplement use is by those who exercise for health, and the focus of supplement use is on health rather than on performance. The serious athlete must be concerned about health as well as performance. Illness or injury that requires a break from training can disrupt preparations, and illness at the time of key competitions can ruin an athlete’s competitive season.

4Sport-specific reasons for supplement use include a belief that the stress of intense training/competition cannot be met by food alone, and that supplements can offer a specific advantage in either training or competition. There is also an awareness that successful competitors are using supplements, and the use of supplements is often endorsed or encouraged by influential individuals in the athlete’s circle, including coaches and training partners. There is also some evidence – largely anecdotal – that the amount of supplement used by athletes often exceeds the recommended amount. This “more is better” philosophy is encouraged by the belief that rivals are using even higher doses. Even elite athletes may not have access to professional advice, however. Hekkinen et al. (2011) reported that only 27% of the 372 elite Finnish athletes they surveyed in 2009 had an opportunity to consult a sports nutrition professional.

5There are several key questions that any athlete should ask before embarking on a programme of supplement use. The first is an assessment of the evidence base relating to efficacy. Athletes should look for a good reason for using a supplement and this means that there should be evidence of likely benefits from its use. This means looking for evidence relevant to their own sport, preferably with data collected from athletes competing at a similar standard. It is often the case, however, that the evidence base is limited. For some of the products on sale, it is impossible to find any good evidence in the form of randomized, placebo-controlled intervention trials. Supplement sales often rely instead on endorsements by successful athletes, who may or may not have used the product. Where evidence does exist, measurements are seldom made on elite athletes and it is very likely that the factors that limit performance may be different in the elite athlete than in the untrained or recreationally active individuals who most often participate in laboratory studies. The exercise models used in laboratory studies may or may not be relevant to the athlete’s sport and may not even be real measurements of exercise performance. The “anaerobic threshold”, for example can be shown to correlate with endurance performance in a heterogeneous population, but it is not a good marker of performance and it can change in the opposite direction from performance changes in response to nutritional interventions (Kelman et al. 1975). In spite of this, however, changes in this measure in response to a supplement intervention are sometimes used as evidence of a performance effect in studies of the efficacy of dietary supplements (Graef et al. 2009). Similarly, the physical working capacity at fatigue threshold (PWCFT)) is an indirect measure, based on electromyographic measurements made during an incremental test that may or may not relate to exercise performance. Nevertheless, studies using the PWCFT have concluded that a period of supplementation with βeta-alanine can improve muscle endurance (Stout et al. 2008) though no real measure of performance was made.

6No serious attempt has been made to evaluate the efficacy of many of the supplements used by athletes, nor indeed many of those that are popular with the general population. This is at least in part due to the difficulties involved in assessing efficacy. Many supplements are targeted at elite athletes, or at least at those training seriously and competing at a high level. However, these individuals are seldom willing to participate in laboratory trials because of the inevitable disruption to training and competition schedules. The relevance of tests carried out on recreational athletes to the elite performer, is, however, doubtful. It seems quite possible that the factors that limit performance differ in the truly elite athlete from those in the untrained or recreationally-active individual.

7There are also some real difficulties in assessing efficacy of supplements. Even something as simple as performance cannot be measured without some degree of uncertainty due to the inherent variability in performance. The validity, reliability and sensitivity of tests commonly employed as a measure of exercise performance has generated considerable controversy in recent years. Constant power tests to volitional exhaustion have been employed to examine the influence of various interventions on performance, but this method of testing has been criticized for a lack of ecological validity and poor test-retest reliability. This view is supported by the findings of Jeukendrup et al. (1996), who found a large day-to-day variability (CV 27%) in time to exhaustion tests, and a much smaller variability in a time trial protocol (< 4%). The variation in performance of some of the subjects in that study, however, seems improbably high, and data from our research group report more consistent performance in a constant power to fatigue test (CV 6%; Maughan et al, 1989). More recent reports have highlighted similar errors of measurement when changes in performance are normalized across tests (Hinckson and Hopkins, 2005). A key factor to consider when selecting an appropriate exercise test is its sensitivity, and the smallest worthwhile effect that can be reliability detected. Amann et al. (2008) demonstrated that time to exhaustion and time trial protocols display a similar sensitivity to the effects of hypoxia and hyperoxia on performance, and suggest that this finding will extend to other factors influencing performance. This is brought about by larger effects on performance in response to an intervention with constant power tests than are typically observed in time trial protocols: this compensates for the larger test-retest variability, resulting in a very similar signal-to-noise ratio to that seen with time trial protocols (Amman et al. 2008; Currell and Jeukendrup, 2008). In some research situations, the obvious limitation of time trial-type test is a difficulty in comparing the effect of an intervention on the physiological response to exercise, since at any given time one volunteer’s relative workload may vary greatly from that of other participants. It should also be recognized that a laboratory time trial in no way resembles a competitive race situation in which the aim is to win, rather than to run or cycle as fast as possible. Most runners or cyclists would say that a race is like a time to fatigue test: the point at which you can no longer keep up with the leading group corresponds to the time to exhaustion in the laboratory test – the only difference is that instead of stopping you simply lose contact with the faster performers.

8It is essential that those who consider the use of supplements understand the limitations of the experimental evidence. The margin between first and second in an Olympic final is very small, and even the difference between first and last in an event may be smaller than the sensitivity of the laboratory tests of performance. Showing that a supplement does not result in a statistically significant improvement in performance does not mean that it may not have worthwhile benefits for some athletes. There is also increasing discussion in the published literature of “responders” and “non-responders” in populations of subjects who participate in any given study. To truly establish whether an individual falls into one or other of these categories, however, requires multiple tests to establish that the response is consistent. Where some individuals improve and some do not in a single test, the explanation is more likely to reflect the random variation in performance that normally occurs.

9If something as simple as endurance performance cannot be measured reliably, the difficulties in assessing efficacy are magnified many times when there is a less well-defined endpoint. Even with a very large financial investment, it is unrealistic to expect clear evidence of efficacy in many of the areas that are important to athletes, such as wound healing, muscle soreness, immune function and joint health. The Glucosamine/Chondroitin Arthritis Intervention Trial (GAIT) lasted two years and cost over US$ 12.5 million, but provided rather inconclusive results. Phase 1 of the Study concluded that the dietary supplement combination of glucosamine plus chondroitin sulphate did not provide significant relief from osteoarthritis pain among all participants, although a small subgroup of study participants with moderate-to-severe pain showed significant relief with the combined supplements (Clegg et al. 2006). Phase 2 of the trial concluded that the supplements, either together or alone, appeared to fare no better than placebo in slowing loss of cartilage in osteoarthritis of the knee (Sawitzke et al. 2008). Supplements companies are understandably reluctant to make such large investments in research where the outcome is uncertain.

10It is also important to consider not only the potential benefits of supplement use but also the costs and risks (Maughan et al. 2011). Some supplements are expensive and do not represent a good financial investment for the average athlete. Others may pose a risk to health or performance, while some may cause the athlete to fail a drugs test. The issue of supplements and drugs tests will be discussed in more detail later, but it is clearly difficult tor the athlete to make an informed decision on supplement use as many of the issues at stake are unknown. The literature contains an increasing number of reports of adverse health outcomes linked to the use of dietary supplements and in many cases these relate to the present of undeclared compounds (Krishnan et al. 2009).

11Of the many hundreds, or even thousands, of supplements on sale, there is good evidence to support the use of only a very few, including creatine, caffeine, buffering agents (bicarbonate and perhaps also citrate), and perhaps also nitrate. There is some evidence for performance related or health-related effects from a few others, including arginine, glutamine, β-alanine, echinacea, glucosamine, antioxidants, carnitine and zinc, but prospective, randomized clinical trials of these products are almost completely absent. The athlete should recognize also that some of these supplements may have benefits in some exercise contexts, but not in others. It is important to know whether the supplement is likely to offer a benefit in the athlete’s own sport and how and when it should be used, i. e. whether in training or acutely in competition. It is beyond the scope of this brief review to assess the evidence for efficacy of these various supplements, and the athlete should seek specialist advice from a qualified sports nutrition professional.

12Some products contain impurities (lead, broken glass, animal faeces, etc) because of poor manufacturing practice. Some products do not to contain expensive ingredients listed on the label but only inexpensive materials. Athletes who are liable for drug testing under the rules of their sport must also be aware that some products contain doping agents that are not declared on the label (Maughan, 2005). In some cases, it appears that this contamination may be accidental, as a result of traces of product being transferred during the manufacturing of the supplement. In other cases, however, the levels of prohibited substances may exceed the therapeutic dose and it appears that there are instances of deliberate adulteration of otherwise ineffective products with highly effective pharmacological agents (Gmeiner, 2002). It cannot be accidental that anabolic agents are most commonly found in muscle building products, anorectic agents in weight-loss products and stimulants in tonics. For many prohibited substances, the amount necessary to produce a positive doping result is extremely small. In a study by Watson et al. (2009), 5 of 20 subjects given a 2.5 µg dose of a nandrolone precursor produced at least one urine sample that contained nandrolone metabolites at a level that would cause a positive finding for nandrolone use: when a 5 µg dose was given, 15 of the same 20 subjects would have produced a positive finding. The nandrolone precursor (19-norandrostenedione) was administered in 500 ml of water containing 5 g of creatine: 2.5 µg in 500 g represents a contamination level of 0.0000005%, which is far below the limit of contamination that is deemed acceptable in pharmaceutical manufacturing. The principle of strict liability that applies in drug control means that the athlete cannot use the fact that the presence of the prohibited substance was not declared on the product label as an excuse. In a study carried out by the Cologne doping control laboratory, 634 non-hormonal nutritional supplements were obtained in 13 countries from 215 different suppliers (Geyer et al. 2004). Upon analysis, 11 different anabolic androgenic steroids were found: 94 (14.8%) samples contained prohormones not declared on the label (“positive supplements”). No reliable data were obtained for further 66 samples (10.4%) because of matrix effects, so as many as 25% of the samples purchased may have contained prohibited anabolic-androgenic steroids. These results have been repeated many times and must be a cause for serious concern among athletes who are liable for drug testing.

13In response to this problem, various supplement certification programmes have been established to allow manufacturers to have raw ingredients and finished products tested for the presence of prohibited substances. Some of these programmes allow athletes to trace the batch number of the product they propose to use and to establish whether or not a certificate of analysis is available for that batch. These systems cannot provide an absolute guarantee that any particular product will be safe for athletes to use, but they do greatly reduce the risk that a supplement will contain prohibited substances. Limitations include the facts that:

  • not all prohibited substances are tested for;

  • there is a limit of detection below which the analytical process will not detect a specified substance that might be present;

  • for products sold as tablets or capsules, there may be a variation between individual doses. One may be contaminated, while others in the same bottle are not;

  • the presence of the active ingredient(s) is not verified by these programmes;

14All athletes should nevertheless be encouraged to view these programmes as a risk-management strategy and to support the manufacturers who submit their products to these programmes in an effort to ensure that supplements are safe for athletes to use.

15It should not be thought that problems of contamination are unique to dietary supplements. Food-product recalls are common in most countries, for reasons similar to those that apply to supplements, i.e. poor quality control leading to the presence of undeclared ingredients and failure to comply with labelling requirements. The difference, perhaps, lies in the scope for the presence of deliberate adulteration of otherwise ineffective supplements with pharmaceuticals whose use is prohibited in sport. Even then, the presence of growth promoters (specifically clenbuterol: Geyer et al. 2014) and veterinary products (phenylbutazone: Wise, 2013) in meat products is increasingly recognized. Some natural products that contain endogenous anabolic androgenic steroids are also used in food products and traditional medicines as well as in supplements (Geyer et al. 2014).

16In summary, the use of dietary supplements is no substitute for a good diet, and athletes are cautioned against indiscriminate use of supplements. Where energy intake or food choice is restricted, a low-dose, broad-spectrum vitamin-mineral supplement may help. A few supplements may have role where a specific need is demonstrated, but a careful analysis of the potential benefits of supplement use must be balanced against the potential for harmful effects.

Bibliographie

3. Bibliographic references

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Auteur

PhD. School of Sport, Exercise and Health Sciences, Loughborough University, Loughborough LE11 3TU, United kingdom. Email: r.maughan@lboro.ac.uk

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