Young athletes

Helen O’Connor and Bronwen Lundy

Engagement in physical activity has a range of important physical, social and mental health benefits for young people. Sports participation can be at a range of levels, from active recreational engagement through to elite, internationally representative competition. Young prepubertal athletes can often be engaged in more than one sport, even at a relatively high level. However, as they progress into adolescence and reach national or internationally representative levels, sports participation becomes increasingly specialised, typically focusing on only one sport.

Nutrition is important for young athletes, to support overall growth and development as well as optimal sports performance. Engagement in sport has the potential to motivate improved dietary practices to support performance improvement. In developed countries, and increasingly in developing countries, young athletes experience an obesogenic environment and the literature suggests that athletes—even those at elite or professional levels—consume diets that are not consistent with public health or sports nutrition guidelines (although there is evidence that the dietary intakes of young athletes are superior to their non-athletic counterparts). Typical (Croll et al. 2006) food habits in childhood and adolescence—which include a preference for fast foods, inadequate vegetable intake and high intakes of nutrient-poor discretionary foods and sugar-sweetened beverages—are also reported in young athletes (Parnell et al. 2016). There is a need for nutrition education which includes skills in shopping and cooking throughout these developmental years. As young athletes may spend much of their spare time training, they often have less time to participate in or observe food preparation skills in the home.

Young athletes have special nutrition needs. They can also be at an increased risk of inadequate or inappropriate dietary intake or supplement use. These risks may result from the additional demands of serious training on their capacity to consume a diet that has sufficient energy and macro- and micronutrients. Additionally, pressure to attain a specific weight or body composition can encourage restrictive eating practices or use of inappropriate dietary supplements or ergogenic aids, contributing to this risk. Clearly, childhood and adolescence is an important life stage for growth and development, as well as an opportunity to establish healthy eating habits, a positive relationship with food and a robust body image.

LEARNING OUTCOMES

Upon completion of this chapter you will be able to:

• identify special nutritional requirements and considerations for young athletes

• understand the challenges of ensuring young athletes consume sufficient energy and nutrients to sustain growth as well as optimise training adaptations and sports performance

• outline the risks associated with energy restriction, including relative energy deficiency in sport (RED-S), and the associated negative physical, mental health and body-image consequences

• understand differences in thermoregulation between young and adult athletes and how this may impact hydration strategies and the risk of exertional heat illness

• understand how dietary supplements are attractive to young athletes but also the potential risks associated with their use at the early phase of their development and sports career.

THE ROLE OF NUTRITION IN SUPPORTING GROWTH AND DEVELOPMENT OF YOUNG ATHLETES

During childhood, growth and development is relatively steady and occurs at a similar rate in boys and girls. The physical strength and exercise capacity of prepubertal boys and girls is generally similar at this age stage and, although they often compete separately, many recreational sports permit girls and boys to compete together. During puberty, the rate of growth increases until peak height velocity is reached. This more rapid rate of growth is often referred to as the pubertal growth spurt, although it is important to recognise that the onset and growth rate during this ‘spurt’ varies widely. Girls generally commence their growth spurt and reach peak height velocity two years earlier (~12 years) than boys.

High-level engagement in sport can sometimes make it more difficult for young athletes to consume sufficient energy to meet the demands of training, especially during the pubertal growth spurt when there are additional energy demands for growth. Inadequate energy intake can result in delayed growth and, sometimes, increased fatigue, poor recovery and a range of more serious consequences if this is chronic. Although increased physical activity usually increases appetite to a level that helps the young athlete match their energy needs, young athletes will still need specific guidance to help plan their dietary intake around training demands. This can also sometimes be the case when there is a relatively rapid increase in the duration or intensity of training, which can occur when a young athlete is identified for a talented athlete program or if they move up to a new training level.

There are a number of practical factors that can also make it more difficult for young athletes to consume sufficient energy. The demands of busy training schedules, either before or after school, can reduce the time available for food preparation and consumption. Some young athletes avoid food and/or fluid close to, or even during, training sessions due to issues with gastrointestinal discomfort or the fear of experiencing a ‘stitch’. These issues are more common in sports that involve running, jumping or tumbling, as opposed to swimming and cycling where the torso experiences less impact. Young athletes usually need to consume additional food and fluids during school hours to meet their higher energy demands; preparing and carrying the additional food, or purchasing or accessing this at school, presents another challenge. Higher intakes can sometimes also result in comments from other, less active peers about the volume of food consumed, which can make some young athletes feel self-conscious. Fussy eating is a further factor challenging the attainment of sufficient energy intake, and addressing this merely by increasing the volume of a limited range of foods can make the diet extremely monotonous and unappetising. Younger athletes need support and encouragement to widen the range of foods consumed, which is valuable for positive longer-term health and performance outcomes.

SPECIAL NUTRIENT REQUIREMENTS OF YOUNG ATHLETES

Protein

Children and adolescents need additional protein (compared to adults) to support growth (for example, the Australian recommended dietary intake for protein is 0.75 and 0.84 g/kg BM/day for adult women and men respectively, versus 0.77–0.91 and 0.91–0.99 g/kg BM/day for girls and boys respectively [NHMRC 2006]). Adult athletes need additional protein to assist in the growth and/or maintenance of lean body mass. Although few studies have been performed with young athletes, there is some evidence for an increased protein requirement (1.35–1.6 g/kg BM/ day), especially in adolescents with high musculature or undergoing heavy training (Aerenhouts et al. 2011). As young athletes in developed countries typically consume around 1.2–1.6 g/kg BM/day of protein, it is anticipated that almost all would obtain sufficient protein from food and not require supplemental protein (Desbrow & Leveritt 2015).

Adequate energy intake is critical to the maintenance of positive nitrogen balance in athletes. In practice, inadequate energy is more often a factor limiting muscle gain than inadequate protein; however, younger athletes on restrictive diets may be at risk of both inadequate energy and protein intake. Although the research on protein requirements in athletes focuses on adult athletes, it seems likely that younger athletes would benefit from strategies used by adults to optimise development and maintenance of muscle, including distribution of protein over the day and protein intake around the time of training (see Chapter 9).

Carbohydrate

As with protein requirements, there is limited research on how the carbohydrate requirements of young athletes differ from those of adults. Early muscle biopsy studies found that young athletes have greater oxidative enzyme concentration and aerobic capacity, and less adaptation to anaerobic enzyme capacity (Erickson & Saltin 1974). They have been reported to rely more on oxidative (aerobic) metabolism during exercise (Taylor et al. 1997). Other studies have shown no difference in adaptation compared to adults (Haralambie 1982). Taken together, there is little evidence to support major differences in carbohydrate adaptation or fuel utilisation between younger and adult athletes.

Athletes should focus on planning carbohydrate intake around exercise duration and intensity (see Chapter 9 for recommended intake ranges). Young athletes typically have lower training volumes than adults, although this depends on both the sport and the individual athlete. It is usually the case for endurance and team sports, where training loads are built gradually until training loads are similar to those of adults in late adolescence. For this reason, carbohydrate loading is not necessary for young athletes until they undertake longer-duration endurance events in late adolescence.

Micronutrients at risk in young athletes

Young athletes commonly have low levels of calcium and iron (Desbrow & Leveritt 2015), especially female athletes after menarche when menstrual loss increases iron requirements. Iron loss may also be greater in athletes participating in endurance sports (see Chapter 14). Iron intake is usually lower in athletes restricting energy intake and sometimes in those who are vegetarian or eat less animal protein (see Chapter 5).

Calcium is another key nutrient, given the increased requirement during childhood and adolescence to support bone development. Inadequate calcium intake is often reported during childhood and adolescence in the general population and is also commonly reported in studies of young athletes. The risk of inadequate intake is increased in those who avoid or restrict dairy products.

Vitamin D is another nutrient which may be lower in athletes who train longer hours indoors or in latitudes where there is less sunlight and foods are not fortified with vitamin D. Sunscreen use, while important to prevent sun damage, does limit synthesis of vitamin D from the skin, so sun exposure without burning, where possible, is a valuable strategy to support adequate vitamin D levels. Athletes with darker skin and those who wear full-length clothing will produce less vitamin D from sun exposure and may be more reliant on obtaining vitamin D from dietary sources.

It is important to recognise that use of supplements to treat an existing nutrient deficiency may be warranted. This should be undertaken after a medical diagnosis and in conjunction with professional support from a sports dietitian to assist the athlete to improve dietary intake and prevent future deficiency through a balanced intake of whole foods.

THERMOREGULATION AND HYDRATION IN YOUNG ATHLETES

We used to believe that children were less able to regulate their body temperature than adults, but we now know that this is not the case. Children’s greater surface area-to-mass ratio is actually an advantage for heat loss in most circumstances, except when environmental temperature is greater than skin temperature (>3^oC) (Rowland 2008). In practice, younger athletes do not usually train or compete at the same work rates as adults; rather, they exercise at loads commensurate with their age and body size, which protects them from excessive heat storage (Rowland 2008). Children also have higher skin blood flow during exercise, and this promotes increased convective heat loss. Children do sweat less than adults but this actually helps to reduce the risk of hypohydration. Relative to their body mass, prepubertal athletes have been shown to have better evaporative cooling than young adults. The smaller, more diffuse sweat drops produced in prepubertal children promote better evaporation than larger sweat drops in adults, which tend to join together and drip rather than evaporate from the body. The lower body mass in children essentially means they need to produce less sweat than adults to maintain heat balance for the same change in core temperature (Rowland 2008).

In 2011, the American Academy of Pediatrics released a policy statement (American Academy of Pediatrics et al. 2011) concluding that young athletes do not have less effective thermoregulatory ability, insufficient cardiovascular capacity or lower physical exertion tolerance compared with adults during exercise in the heat, as long as adequate hydration is maintained. Aside from inadequate hydration, the primary determinants of reduced performance and exertional heat-illness risk in youth during sports in hot environments include undue physical exertion, insufficient recovery between repeated bouts of exercise and closely scheduled same-day training sessions or competition rounds. Inappropriate clothing, uniforms and protective equipment also plays a role in excessive heat retention. In practice, serious heat illness in young athletes is infrequently reported in the medical literature, suggesting that such events are rare (Rowland 2008). In practice, many of the strategies for hydration and competition refuelling used for adults are appropriate to young athletes. Although the volumes of fluid required are less for young athletes, they should still aim to reduce net fluid loss to <2 per cent of body weight (see also Chapter 11).

RISKS AND CHALLENGES ASSOCIATED WITH OPTIMISING PHYSIQUE ATTRIBUTES IN YOUNG ATHLETES

Physique is an important attribute for success in many sports. Characteristics such as larger stature and arm span are important for shooting and reach in sports such as basketball, swimming and tennis, while in gymnastics, diving and figure skating a shorter, compact frame facilitates the ease of aerial rotation. Other physique characteristics important to many sports include increased muscularity and lower levels of body fat. Although these can be modified by diet and training, they are also under genetic control and so there are limits to the capacity for change. Natural physique attributes are often early influencers of sport selection. Boys who are muscular and tall for their age may be attracted to sports such as rugby union rather than gymnastics.

The timing of the onset of puberty may also influence sport success. In contact football sports such as rugby, there is concern that early pubertal development provides an unfair advantage for talent identification over athletes of the same age who are relatively prepubertal but similarly or more talented than their earlier-developing counterparts. The reverse is also true in some women’s sports, where the desired body shape and size is closer to the prepubertal physique. Normal changes that occur during puberty, including an increase in both muscularity and, predominantly in females, acquisition of body fat, may result in some talented athletes developing a physique that is less desirable for their sport (Cobley et al. 2009).

RESTRICTIVE EATING, DIETING, DISORDERED EATING AND ENERGY DEFICIENCY IN YOUNG ATHLETES

Risks of restrictive eating and dieting

Young athletes in sports where leanness is highly desirable (such as gymnastics, ballet, diving or figure skating) or where they need to make weight to compete (as in lightweight rowing, boxing and martial arts) are at an increased risk of restrictive eating, which can have negative short- and long-term consequences. In the short term, they may consume inadequate energy to train effectively, recover, adapt and improve performance. Growth may also be compromised. Inadequate carbohydrate intake may result in glycogen depletion, and this can increase fatigue and reduce training capacity and the potential for optimal metabolic adaptation. Inadequate protein intake may compromise growth and lean mass development. When overall food intake is reduced, there is also an increased risk of deficiency of key micronutrients.

Disordered eating and eating disorders

In the longer term, short-term dieting may develop into disordered eating. This can happen gradually and without the awareness of the young athlete, coach or parent. The disordered eating patterns can eventually progress to an eating disorder such as anorexia or bulimia nervosa. Although prevention should always be the primary aim, when disordered eating behaviours develop, early intervention is essential and is associated with significantly better longer-term outcomes. Although disordered eating can be difficult to identify in its early stages, athlete, coach and parent education can assist with earlier recognition of the problem (Jeacocke & Beals 2015).

Young athletes should not be encouraged to reduce weight or body fat without serious consideration of the potential negative effects. Weight management in young athletes requires the clinical expertise and professional support from a sports dietitian. Critical comments about weight or body composition often initiate inappropriate dieting, and this increases the risk of adverse outcomes in young athletes who are vulnerable to misinformation and may seek to rectify their weight ‘problems’ with ‘fad’ diets or non-evidence-based approaches. Once disordered eating practices develop they are difficult to reverse and intensive clinical intervention from a psychologist/psychiatrist and a dietitian is required. Medication, family therapy and, sometimes, hospitalisation may be needed. The development of an eating disorder can seriously jeopardise the future sports prospects of the athlete and lead to poorer longer-term physical and mental health. Making the decision about whether a young athlete with disordered eating should continue participating in sport can be challenging. Guidelines for sport exclusion and return to play have been developed on a score-based system and can help coaches and practitioners make objective decisions (De Souza et al. 2014).

RELATIVE ENERGY DEFICIENCY IN SPORT

There has been awareness for some time that young athletes, most often females, may experience health issues related to insufficient energy intake to fuel their training and maintain other essential body functions such as growth, repair of tissues such as bone and normal reproductive function. This issue was initially described as the ‘Female Athlete Triad’, which acknowledged the cluster of symptoms observed, including low bone-mineral density, low energy availability, eating disorder or disordered eating and menstrual dysfunction (Drinkwater et al. 2005). Energy availability (EA) is a relatively new concept that describes the energy remaining to maintain essential body functions after accounting for expenditure for exercise training (Loucks 2003).

EA is defined by the following equation:

EA (kJ/kg FFM) = (energy intake – energy cost of exercise) / fat-free mass

Low energy availability (LEA) may occur accidentally through a misunderstanding of the energy needs for sport, as a result of dietary restraint or as the consequence of disordered eating (Mountjoy et al. 2014). LEA is considered to occur when EA drops below a threshold of 125 kJ/kg FFM.

Awareness and improved understanding of the impact of LEA has continued to evolve and it is now clear that the consequences go beyond menstrual dysfunction and decreased bone health (Mountjoy et al. 2014). Evidence of negative consequences is also evident in male athletes, although this is less well characterised than in females. The term ‘relative energy deficiency in sport’ (RED-S) (Mountjoy et al. 2014) expands and reconceptualises the ‘Female Athlete Triad’ issues, acknowledging that both male and female athletes may be affected and that LEA may affect multiple body systems, including gastrointestinal, immunological, cardiovascular and endocrine function. In young athletes, growth and the attainment of optimal peak bone mass may also be affected (Box 18.1).

Identifying RED-S in young individual athletes and teams

Identification and assessment of LEA in field settings is difficult due to challenges in accurately measuring energy intake and exercise energy expenditure (Melin & Lundy 2015). Even when accurate measurements can be obtained, these assessments represent only a single point in time and cannot identify whether the reported energy availability is representative of the usual diet or is a short-term change. Measurement of resting metabolic rate can be a useful screening tool as it is often suppressed in LEA. Relevant indicators in the athlete’s clinical history may also help to identify RED-S (Box 18.2). A helpful screening tool, the LEAF-Q (Low Energy Availability in Females questionnaire) is freely available, non-threatening and quick to complete (Melin et al. 2014).

Box 18.1: Consequences of RED-S

• Metabolic

• Endocrine

• Bone health

• Menstrual function

• Immunological

• Gastrointestinal

• Cardiovascular

• Psychological

• Growth and development

• Haematological

Source: Adapted from Mountjoy et al. 2014.

Box 18.2: Questions to consider if RED-S is suspected

1. Does the athlete have a history of frequent illness or injury?

2. For females, do they have a normal menstrual cycle?

3. Has body composition been a focus of their training or personally?

4. Do they have normal bone health?

5. Subjectively, what is their diet plan like?

6. Do they have a strong focus on healthy eating?

7. Do they adequately fuel training sessions?

For teams or squads of young female athletes, this may be helpful to identify those who require further medical and or nutrition support and follow up. The development of a similar tool for male athletes would be welcomed.

SPORTS SUPPLEMENT USE IN YOUNG ATHLETES

Young adolescent athletes in sports where larger mass and muscularity are important may be attracted to use supplements to support lean mass gain. While cautious use of supplements, particularly balanced products such as liquid meals (for example, Sustagen^TM) can assist young athletes in meeting energy needs more easily, heavy use of single nutrient supplements, such as protein powders and amino acids, may displace healthy foods and potentially increase the risk of ingestion of substances prohibited for use in sport. Even if the athlete is not yet undergoing drug testing, some of these substances (for example, stimulants and anabolic steroids) are detrimental to health. Stimulant abuse can result in a wide range of negative health consequences, including addiction/dependence, headaches, gastrointestinal upset and interrupted sleep patterns. Anabolic steroids can result in increased aggression, liver or heart damage along with a range of other serious effects.

These substances may be contaminants in the product and not disclosed on the label. There is also evidence that early use of supplements for lean mass gain may later influence inclination to use substances prohibited by sports drug agencies. For these reasons, a ‘food first’ approach is recommended for athletes younger than 18, with supplement use limited to sports foods (carbohydrate-electrolyte drinks, gels, sports bars and liquid meals) rather than ergogenic aids (Barkoukis et al. 2015). Young athletes have so much development potential from training, and the use of ergogenic aids at this stage introduces additional risk; performance assistance is best incorporated after optimising young athletes’ preparation through a well-designed eating plan, effective training, psychological strategies and technical development. Supplements are often viewed as the ‘magic bullet’ and can contribute to a ‘win at all costs’ mentality. The risks of supplement use often remain poorly understood at this developmental stage (Desbrow & Leveritt 2015).

SUMMARY AND KEY MESSAGES

Young athletes have special nutrition needs to support growth and development as well as training. These mostly revolve around the need for additional energy and for key nutrients such as protein, carbohydrate, iron and calcium. The additional needs for protein and carbohydrate are generally consistent with adult athletes per kilogram of body weight, although more research is required. Strategies such as carbohydrate loading are not needed until late adolescence, as the durations and distances of endurance events are shorter for young athletes. At this stage, developing athletes are often conscious of body image and can be vulnerable to restrictive eating, energy and nutrient deficiency. Support to maintain a positive body image and a healthy diet is crucial to optimal physical and mental health. Where warning signs of RED-S or disordered eating emerge, early professional intervention supports more positive longer-term outcomes. Despite earlier concerns, younger athletes are not at a greater risk of exertional heat illness than adults. Finally, young athletes are often attracted to dietary supplements and ergogenic aids. At this age, a ‘food first’ approach is recommended with an overarching philosophy of encouraging healthy eating and physical, mental and technical development over the use of supplements (unless there is a deficiency), particularly ergogenic aids.

Key messages

• Young athletes have special nutrition needs. There is an increased requirement for energy, especially during adolescence, to support the accelerated rate in growth and development, in addition to the needs of training.

• Young athletes may undertake restrictive eating to reduce weight or body fat and this can result in insufficient energy consumption. This not only increases fatigue and compromises training adaptations and performance but also places the young athlete at increased risk of relative energy deficiency in sport (RED-S).

• RED-S compromises reproductive, immune and cardiac function as well as bone health, with some of these negative outcomes irreversible.

• Young athletes on restrictive diets are at risk for micronutrient deficiency, as requirements for nutrients such as iron and calcium are increased during growth and development.

• Eating disorders, and restrictive eating that progresses to disordered eating, pose a serious risk to both physical and mental health. Disordered eating can be triggered by negative comments about weight or shape or the recommendation to lose weight or fat without support from a qualified health professional. Young athletes are at a vulnerable stage of life and a positive body image must be nurtured.

• Limited research in young athletes indicates that macronutrient requirements per kilogram of body weight are similar to those for adult athletes, although, as they usually train and compete for shorter durations, carbohydrate intake should be periodised to training loads and strategies such as glycogen loading are not needed until late adolescence.

• Although young athletes had initially been reported to thermoregulate less effectively than adults, recent research indicates they are not at significantly greater risk of exertional heat stress when compared to adults. Many of the strategies used for hydration and competition fuelling can also be applied in principle to young athletes.

• Ergogenic aids, while popular at this stage, should generally be avoided and a ‘food first’ approach encouraged. A key strategy for sports nutrition at this phase of development is to ensure the athlete develops knowledge, skills and increasing independence in selecting a healthy diet.

REFERENCES

Aerenhouts, D., Deriemaeker, P., Hebbelinck, M. et al., 2011, ‘Energy and macronutrient intake in adolescent sprint athletes: A follow-up study’, Journal of Sports Science, vol. 29, no.1, pp. 73–82.

American Academy of Pediatrics, Council on Sports Medicine Fitness, Council on School Health et al., 2011, ‘Policy statement: Climatic heat stress and exercising children and adolescents’, Pediatrics, vol. 128, no. 3, pp. e741–7.

Barkoukis, V., Lazuras, L., Lucidi, F. et al., 2015, ‘Nutritional supplement and doping use in sport: Possible underlying social cognitive processes’, Scandavian Journal of Medical Science in Sports, vol. 25, no. 6, pp. e582–8.

Cobley, S., Baker, J., Wattie, N. et al., 2009, ‘Annual age-grouping and athlete development: A meta-analytical review of relative age effects in sport’, Sports Medicine, vol. 39, no. 3, pp. 235–56.

Croll, J.K., Neumark-Sztainer, D., Story, M. et al., 2006, ‘Adolescents involved in weight-related and power team sports have better eating patterns and nutrient intakes than non-sport-involved adolescents’, Journal of American Dietetic Association, vol. 106, no. 5, pp. 709–17.

De Souza, M.J., Nattiv, A., Joy, E. et al., 2014, ‘Female Athlete Triad Coalition Consensus Statement on Treatment and Return to Play of the Female Athlete Triad: 1st International Conference held in San Francisco, California, May 2012 and 2nd International Conference held in Indianapolis, Indiana, May 2013’, British Journal of Sports Medicine, vol. 48, no. 4, p. 289.

Desbrow, B. & Leveritt, M., 2015, ‘Nutritional issues for young athletes: Children and adolescents’, in Burke, L.M. & Deakin, V. (eds), Clinical Sports Nutrition, 5th edn, North Ryde, NSW: McGraw-Hill Education, pp. 592–618.

Drinkwater, B.L., Loucks, A.B., Sherman, R.T. et al., 2005, ‘Position Stand on the female athlete triad’, in Sangenis, P. (ed.), IOC Medical Commission Working Group Women in Sport, International Olympic Committee, Lausanne, Switzerland, pp. 2–46.

Erickson, B.O. & Saltin, B., 1974, ‘Muscle metabolism in boys aged 11–16 years’, Acta Pediatrica Belgica, vol. 28, pp. 257–65.

Haralambie, G., 1982, ‘Enzyme activities in skeletal muscle of 13–15 years old adolescents’, Bulletin Europeen de Physiopathologie Respiratoire, vol. 18, no. 1. pp. 65–74.

Jeacocke, N. & Beals, K.A., 2015, ‘Eating disorders and disordered eating in athletes’, in Burke, L.M. & Deakin, V. (eds), Clinical Sports Nutrition, 5th edn, North Ryde, NSW: McGraw-Hill Education, pp. 213–232.

Loucks, A.B., 2003, ‘Introduction to menstrual disturbances in athletes’, Medicine & Science in Sports & Exercise, vol. 35, pp. 1551–2.

Melin, A. & Lundy, B., 2015, ‘Measuring energy availability’, in Burke, L.M. & Deakin, V. (eds), Clinical Sports Nutrition, 5th edn, North Ryde, NSW: McGraw-Hill Education, pp. 146–157.

Melin, A., Tornberg, A.B., Skouby, S. et al., 2014, ‘The LEAF questionnaire: A screening tool for the identification of female athletes at risk for the female athlete triad’, British Journal of Sports Medicine, vol. 48, no. 7, pp. 540–5.

Mountjoy, M., Sundgot-Borgen, J., Burke, L. et al., 2014, ‘The IOC consensus statement: Beyond the female athlete triad—Relative energy deficiency in sport (RED-S)’, British Journal of Sports Medicine, vol. 48, no. 7, pp. 491–7.

National Health and Medical Research Council (NHMRC), 2006, Nutrient reference values for Australia and New Zealand including recommended dietary intakes, Canberra, ACT: Commonwealth of Australia, pp. 27–8, retrieved from <https://nhmrc.gov.au/sites/default/files/images/nutrient-refererence-dietary-intakes.pdf>.

Parnell, J.A., Wiens, K.P. & Erdman, K.A., 2016, ‘Dietary intakes and supplement use in pre-adolescent and adolescent Canadian athletes’, Nutrients, vol. 8, no. 9, pp. 526.

Rowland, T., 2008, ‘Thermoregulation during exercise in the heat in children: Old concepts revisited’, Journal of Applied Physiology, vol. 105, no. 2, pp. 718–24.

Taylor D.J., Kemp G.J., Thompson C.H. & Radda G.K., 1997, ‘Ageing: Effects on oxidative function of skeletal muscle in vivo’, in Gellerich, F.N. & Zierz, S. (eds), Detection of Mitochondrial Diseases. Developments in Molecular and Cellular Biochemistry, vol. 21, Boston, MA: Springer.