KEY WORDS: Nutrition, sport, adolescent athletes, dietary supplement.

Nutrition is an important aspect of an athlete's training program. Although exercise and athletic training is considered to increase nutrient needs in some athletes, a balanced diet with adequate calories can potentially provide the necessary nutrients (American Dietetic Association, 2000; Maughan et al., 2004). It is likely, however, that for various reasons, not all athletes are able to consume a diet that meets their nutritional needs and thus resort to nutritional supplements with the intention of preventing deficiencies and even enhancing performance (Maughan et al., 2004).

Today, the supplement industry is an international market worth billions of dollars. In 2000, it was reported that sales of dietary supplements in the US reached US$17.1 billion with an annual increase in consumer spending of more than 10% (Food and Drug Administration, 2002). Although many individuals use supplements, those engaged in sport and physical activity represent a substantial portion of the population purchasing supplements (Maughan et al., 2004).

A past review of supplement use among athletes was conducted by Sobal and Marquart, 1994a, which analyzed literature, published primarily in the 1980's. With the supplement market being what it is today, supplements are readily available to athletes and are more accepted within the athletic culture, thus investigation into current supplementation behaviours is warranted. This review will look at the current literature on supplement use among young athletes, a population particularly vulnerable to the hype and claims put forth by the supplement industry.

With the use of supplements increasing amongst the child and adolescent athlete population, it is important to investigate the reasons for the athletes adopting this behaviour. Burns et al., 2004 found that adult athletes reported using vitamin and mineral supplements frequently and considered them to have the highest impact on healing/rehabilitation, while protein supplements and Creatine were considered to have the highest perceived impact on sport performance. Other reasons for supplement use cited by adult and elite athletes include increased energy, enhanced performance, improved health, prevention of nutritional deficiencies, prevention of illness, increased muscle mass, and improved recovery (Erdman, et al., 2006; Froiland, et al., 2004; Kristiansen, et al., 2005; Slater et al., 2003).

In the child and adolescent athlete literature, the reasons for supplementation are similar. High school athletes have reported growth (48%), illness prevention (44%), illness treatment (37%), enhanced performance (31%), tiredness (28%), and muscle development (28%) as the important reasons for consuming supplements (Sobal and Marquart, 1994b).

Nieper, 2005 found that the primary reasons for UK junior national track and field athletes to consume supplements included health (45%), immune system (40%), and performance (25%). Enhancement of athletic performance is certainly on the minds of adolescent athletes, as a study conducted by Perko et al., 2000 found that adolescent athletes agreed that most athletes their age need dietary supplements a) to improve sports performance (17.7%), b) taking dietary supplements would help all athletes perform better (27.4%), c) taking supplements is a safe way to improve sports performance (32.7%), d) that supplements give you more energy (41%), and e) taking supplements is a good way to build muscle (44.8%).

The influence of elite and professional athletes engaging in supplement use cannot be ignored. Highly publicized accounts of athletes, who are idolized by children and adolescents, using supplements and ergogenic aids may glamorize supplement use and encourage young athletes to mimic their actions (Calfee and Fadale, 2006).

Young athletes are under great pressure to attain increasingly higher levels in sport, thus they may be enticed to seek alternative means to be able play at elite levels or to be noticed by professional scouts. The high economic value and social status of a professional athletic career may lure young athletes to engage in supplement use as a means for developing the competitive edge to 'make it' as a professional sports person (Calfee and Fadale, 2006).

The desire to play sport at the varsity or collegiate level is another justification for young athletes to engage in supplement use. Athletes expecting to play sports after secondary school are more likely to take dietary supplements (Sobal and Marquart, 1994b). In North America, playing sport at the university or college level comes at a high financial cost thus the quest to secure and maintain an athletic scholarship may be another motivator for using performance-enhancing substances (Calfee and Fadale, 2006). This may also become a factor in the UK, as universities are beginning to offer sports scholarships to attract and retain skilled athletes.

Supplement use tends to differ across genders for prevalence of use, types of supplements used, as well as reasons for use. Specific gender comparisons are largely confined to three of the reviewed studies, which explored the percentages of males and females using supplements. It was found that females engage in supplement use more frequently than their male counterparts which was consistent with the trend observed in previous reviews of the literature (Sobal and Marquart, 1994a; Sobal and Muncie, 1988) however in the current review, one study (Scofield and Unruh, 2006) found conflicting results (see Table 3). The other four original studies included in this review either accounted for gender differences only in particular aspects of data analysis (Kim and Keen, 1999; Perko et al., 2000; O'Dea, 2003; Sobal and Marquart, 1994b), or compared genders for individual supplements (Bell et al., 2004), not overall use.

In terms of reasons for supplement use, males tend to put more emphasis on the athletic performance-enhancing effects of supplements where as females tend to be more concerned with the health benefits (Slater et al., 2003; Sobal and Marquart, 1994b). Neiper (2005) confirmed this by revealing that female UK junior national track and field athletes reported taking supplements for health issues (33%) and strengthening the immune system (44%) with performance and strength (both 11%) reported as less important. The primary reason males consumed supplements was to improve performance (36%) and males (45%) consumed 'ergogenic aids' (defined as Creatine and caffeine) more often than females (11%).

Zeigler et al. (2003) found that male and female elite figure skaters differed considerably in terms of their motivation for consuming supplements. Females used supplements to prevent illness (61%), increase energy (39%), and make up for an inadequate diet (28%). Males used supplements to increase energy (41%), prevent illness (34%), and enhance performance (21%). This is evident in the types of supplements used most frequently used by each gender. Females consumed multivitamin/mineral supplements most frequently (83%), followed by herbal supplements (48%), and multivitamin-only supplements (42%). These supplements could be categorized as health-related supplements, where males also used multivitamin/mineral (61%) and herbal supplements (44%), they were taken less frequently, and instead, males consumed protein bars (38%), protein powders (15%), amino acid powders (8%), and Creatine (3%) more frequently than their female counterparts, confirming that males tend to use supplements claiming a more ergogenic effect. The only exception to this is that females (35%) consumed energy drinks and bars slightly more frequently than males (33%).

Several limitations should be acknowledged whilst conducting this review of the supplement use literature. Most studies included in this review used a form of questionnaire to assess supplement use among athletes, however, one study (O'Dea, 2003) used focus groups, and another (Ziegler et al., 2003) used a combination of questionnaire and other anthropometric measurements. The questionnaires used were different, and although addressing similar matters, subtle differences could affect the overall results.

Another issue in reviewing the literature is that different studies investigated the use of different supplements in the questionnaires. For example, some included sports drinks, energy drinks, caffeine, herbal supplements while others included a few supplements, while others still focused only on vitamin and mineral supplements. To achieve an accurate account of the prevalence and frequency of use of all types of supplements, studies should incorporate a more uniform method of examining supplement use which would include a uniform categorization of supplements, as well as a standardized list of the types of supplements to include in the analysis to enable direct comparisons between the findings of various studies. This process would allow researchers to obtain a more complete interpretation of the current usage of supplements among the young athlete population.

Further research is needed regarding the effects of high toxicity levels of vitamins/minerals in adolescents, as they are the most frequently consumed supplement by the population. The nutritional component of the young athlete's training program is especially crucial as there are many connections between nutritional deficiencies, growth, development, athletic performance, and injury prevention (Petrie et al., 2004). A major concern for health professionals and the sporting community is that the effects of supplement use on the growth and development of children and adolescents are unclear. Minimal experimental research exists regarding the performance-enhancing qualities or adverse effects of ergogenic substances focusing exclusively on adolescent athletes under the age of 18 (American Academy of Pediatrics, 2005). Many physiological changes are occurring during these developmental stages thus making it difficult to fully understand the physiological implications of regular consumption of supplements by this age group (Calfee and Fadale, 2006). More direct evidence is also needed regarding potential physiological side effects of taking any type of supplement.

Another area requiring further research is the exact vitamin and mineral requirements of child and adolescent athletes. Most of the research currently available shows that athletes tend to have a greater food intake that negates any increased need for vitamins and minerals. The majority of these studies are however, citing results of studies conducted in the 1980's and 1990's. More current empirical evidence would be beneficial, as although in theory, the diets of athletes should be better and more complete than their non-athletic counterparts, this cannot be assumed. It is quite possible that that the diets of athletes are not supporting the increased requirements for vitamins and minerals. Nutrition knowledge of adolescent athletes has been found to be inadequate and laden with misconceptions regarding the nutritional requirements for maximal sport performance, and thus their dietary habits tend to be poor (Reading et al., 1999).

To further investigate the factors influencing athletes to engage in supplement use, it may be beneficial to explore the link between health and athletic performance. Although research has demonstrated that enhancement of sport performance is a primary reason for supplementation, overall health concerns also emerged as a key motivator, especially for females, and was often more prominent than sport performance (Kim and Keen, 1999; Sobal and Marquart, 1994b). Vitamins and minerals are considered a nutritional supplement, however, it is possible that athletes, who are generally concerned about their health and aware of the impact optimal health has on athletic performance, view vitamins and minerals as more of a pharmaceutical or medicine than a supplement. Since vitamin and mineral use is so prevalent among athletes, it is likely that there is a misunderstanding as to the role of vitamins and minerals in the diet, their function in maintaining overall health, their role in athletic performance, and how they are best obtained from the diet. Athletes want to be as healthy as possible and tend to consume vitamins and minerals to maintain health rather than boost performance therefore it may be advantageous to determine a different approach to studying and addressing vitamin and mineral use by athletes.

In conclusion, it is important to recognize sporting organizations such as UK Sport, who are acknowledging the need to educate young athletes about supplement use. To do this, UK Sport have decided to focus their efforts on their attitudes and values of supplement use in sport. UK Sport has commenced a research project involving over 1,000 young athletes in the UK (UK Sport, 2006). As well as increased education for young athletes regarding supplement use, parents and coaches also need to be targeted to help the athletes make the right choices.

Promoting physical activity in children and adolescents is particularly important at this time of increasing incidence of childhood obesity. Instilling a healthy attitude about exercise and sport in youth is imperative, but the message may become misconstrued if the emphasis is to excel at all costs, thus suggesting to young athletes that using supplements is the key to success. Supplement use among young athletes is a current and substantial issue that merits future study in order to provide as much accurate information as possible for both professionals and consumers.

I would like to thank the staff at the Children's Health and Exercise Research Centre, University of Exeter, for their warm hospitality during my work placement. I would especially like to thank Prof. Craig Williams for his guidance and support.

• Supplement use among the child and adolescent athlete population is widespread with the most frequently used supplement being a form of vitamin/mineral supplement.
• The effects of supplement use on the growth and development of children and adolescents remain unclear and thus use of supplements by this population should be discouraged.
• It is likely that there is a misunderstanding as to the role of vitamins and minerals in the diet, their function in maintaining overall health, their role in athletic performance, and how they are best obtained from the diet therefore further education for adolescent athletes and athletes in general is needed.