Chapter 3
Armstrong N, McManus AM (eds): The Elite Young Athlete.
Med Sport Sci. Basel, Karger, 2011, vol 56, pp 47–58
Nutrition and Elite Young Athletes
Asker Jeukendrupa ⭈ Linda Cronina,b
a
School of Sport and Exercise Sciences, University of Birmingham, Birmingham, bDepartment of Life Sciences,
Roehampton University, London, UK
Nutrition can play an essential role in the health of elite
young athletes as well as exercise performance. Children
and adolescents need adequate energy intake to ensure
proper growth, development, and maturation. In addition, the requirements may further increase with increasing exercise training. There are, however, several metabolic differences that result in slightly different advice for
young versus adult athletes. For example, younger athletes generally rely more on fat as a fuel, have smaller glycogen stores and have a limited glycolytic capacity. This
would imply reduced carbohydrate requirements but a
greater capacity to oxidize fat. There are also differences
in thermoregulation, although the exact impact on fluid
requirements is not clear. The limited evidence suggests
that acute energy and fluid imbalances can be detrimental to performance and there may be benefits of ingesting
carbohydrate and fluid during exercise, especially during
more prolonged exercise. Exogenous carbohydrate oxidation rates have been reported to contribute more to
energy expenditure in children. This may, however, simply be a reflection of the fact that the oxidation of this carbohydrate is not limited by body size, but by absorption.
Absorption rates are likely to be similar in children and
adults and therefore exogenous carbohydrate oxidation
rates should be comparable. The relative contribution will
therefore be higher because of the lower absolute intensities in children. There are a large number of questions still
unanswered and sports nutrition advice to the elite young
athlete is largely extrapolated from the adult population.
Therefore, more research is needed in the years to come
to give better advice to these young athletes.
Copyright © 2011 S. Karger AG, Basel
For many children and adolescents who are strongly committed to sport, nutrition is not on the radar. However, nutrition is a major component of
their training. Nutrition interacts not only with
growth and development, but also with recovery,
performance, avoiding injury and problems that
may arise as a result of deficiencies. Nutrition is
important for both health and performance. This
chapter addresses some of the main nutritional issues of young athletes and discusses nutrition for
children from the age of 6 to 20 years. Where no
evidence is available, information on young adults
will be used.
Energy Requirements
The growth of pre-pubertal children (between 2
and 10 years) is linear and occurs at a relatively
constant rate of 6 cm per year. The median heights
and weights for boys and girls are similar, averaging 87 cm and 12 kg at the age of 2 years to 137
cm and 32 kg by the age of 10 years. Even in childhood, boys tend to have slightly greater lean tissue
mass and a lower proportion of body fat than girls.
Children and adolescents need adequate energy
intake to ensure proper growth, development, and
maturation. Dietary reference values (DRVs) have
been established for various ages. The athletic or
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Abstract
48
expenditure and exercise economy disappear [3].
Thermoregulation in children has been discussed
in more detail by Falk and Dotan [4].
It is important to educate children to eat a
healthy and balanced diet and to encourage good
eating habits. This can reinforce lifelong eating
habits that contribute to the overall well-being of
children and may enhance performance. On the
other hand, any bad habits are difficult to get rid
of later in a sporting career and should therefore
be avoided. There is an important role for both
coach and parents to encourage appropriate eating behaviours, but also to avoid bad habits such
as too much attention to body weight (see section
on weight management).
Exercise Metabolism in Children
The quality of the muscle – rather than the quantity – is a major determinant of substrate utilisation. Studies in adults clearly show a correlation
between mitochondrial density of the muscle and
fat metabolism (i.e. the more mitochondria, the
higher fat oxidation rates during exercise). There
also seems to be a correlation between muscle fibre type and substrate metabolism with higher
percentage type I fibres favouring fat metabolism.
For obvious reasons, very few studies have investigated muscle composition in children. However,
a study by Bell et al. [5] found a similar mitochondrial to myofibrillar volume ratio in children and
adults, indicating that with growth and maturation, increases in muscle mass are paralleled by an
increase in mitochondria within these fibres. The
maximal oxygen uptakes (V̇O2 max) of these children were similar to the V̇O2 max of an average
adult (45 ml • kg–1 • min–1) and it was argued that
the oxidative capacity of children could be further
developed later in life when an endurance training programme was followed (or reduced when
a sedentary lifestyle was adopted). In one study
muscle fibre type composition was determined in
a large number of 16-year-olds and reassessed 10
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very active child or adolescent generally will have
needs in excess of this level due to the greater energy expenditure from their higher levels of physical
activity. It is difficult to establish a DRV for energy
for this group because of very large inter-individual
variability. In adolescents in particular the onset of
the growth spurt, which is a major impetus for increased energy requirements, is unpredictable.
Prolonged inadequate energy intake may result in
short stature, delayed puberty, menstrual irregularities or absence, poor bone health and increased
risk of injuries [1]. Certain categories of young athletes are more at risk for developing eating disorders like distance runners, jumpers and gymnasts.
It is important to realise that it is impossible to
derive estimations of energy expenditure for children based on adult data. It has repeatedly been
demonstrated that children are less metabolically efficient during motor activities, resulting in
higher energy requirements per kilogram body
mass during activities. For example, one study
reported that children require 30% more energy
during running [2]. There may be several explanations for the higher energy expenditures. First,
children have a higher resting metabolic rate, but
they also have a disadvantageous stride frequency and stride length (imposed by shorter limbs).
Traditionally, it has been stated that children’s
lower mechanical efficiency would negatively
affect the regulation of their body temperature,
however this supposition was based upon studies that did not exercise children and adults at the
same relative exercise intensity and so children
were actually working at a higher exercise intensity than adults, consequently resulting in higher heat production. Similarly, the initial studies
did not account for children’s shorter leg length
(i.e. their higher stride frequency), nor account
for how this would affect the metabolic cost of
any exercise undertaken. However, if when comparing thermoregulation effects in children and
adults relative exercise intensity is calculated by
adjusting treadmill speed to stride frequency, the
differences between adults’ and children’s energy
Nutrition
children seems to exist until mid- to late puberty,
after which a more ‘adult-like metabolic profile’
seems to be evident [14]. It has been suggested
that as fat metabolism appears to be more dominant in exercising children than in adults, children may possibly have a reduced requirement for
dietary carbohydrates, particularly prepubescent
children [12, 15].
Protein
In order to support growth and development, children and adolescents have protein requirements
that are relatively high compared to adults. The
Recommended Daily Allowances (RDAs) for protein in the United States and Canada are displayed
in table 1. However, the protein requirements
for young elite athletes are likely to be higher.
Boisseau et al. [16] studied protein requirements
of 14-year-old soccer players, who played 10–12 h
per week, using nitrogen balance measurements.
The estimated daily protein needed to maintain
nitrogen balance was 1.04 g • kg–1 • day–1. It was
suggested that the RDA for protein for these young
athletes was 1.40 g • kg–1 • day–1 (or 75 g • day–1 in
this group), which would be well above the RDA for
non-athletic children (52 g • day–1) [17]. However,
as is the case with adult athletes [18] this requirement is quite easily met. The study in soccer players was performed in France and the suggested
RDA of 1.40 g • kg–1 • day–1 is still well below the
average protein intake by that age group in France
(2.07 g • kg–1 • day–1). In the United States [19] and
in Australia protein intake by children and adolescents are generally 2–3 times the RDA (USA) or
recommendations in the United Kingdom (table
1) or Australia (not listed). Even in sports where
elite young athletes were reported to restrict energy intakes, protein intakes were still between 1.5–
2.0 g • kg–1 • day–1 [1]. Although on the whole protein requirements seem to be no particular concern
in young athletes, it is important to be aware that
there may be individuals who, perhaps through a
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years later when they were biopsied again to determine the muscle fibre type composition [6, 7].
It was concluded that fibre type did not change
significantly (~52% type I fibres, 33% type IIA
and 15% type IIX). Based on the limited information available it seems fair to conclude that there
are no major shifts in muscle fibre type or composition with age, however, mitochondrial density
can be increased with specific training.
Even though the phenotypic characteristics of
the muscle may be similar in children and adults,
there do appear to be differences in substrate utilisation. These differences have been discussed in
more detail elsewhere [8] but include a lower glycolytic capacity, a higher oxidative capacity and
higher rates of fat oxidation. For more detailed
reviews the reader is referred to Riddell [8] and
Boisseau and Delamarche [9].
In brief, studies using indirect calorimetry suggest that the proportion of fatty acids to carbohydrates used for energy during exercise is different
in children than in adults, possibly due to children’s smaller endogenous carbohydrate stores.
In fact, the contribution of fatty acid oxidation
towards energy production has been reported
to be larger for both girls and boys than it is for
adults [8, 10–12], suggesting that children are well
equipped for sustained aerobic activity, but that
their capacity for anaerobic performance may be
limited by their maturation status [9, 13]. This difference, however, seems to diminish throughout
adolescence, especially in boys [8, 14], suggesting
that the hormones associated with puberty (i.e.
growth hormone, insulin-like growth factor, sex
steroids and catecholamines) play an influential
role in controlling the regulation of energy metabolism in children [9]. In fact, pubertal development has long been associated with a period of
insulin resistance and reduced insulin-stimulated
glucose disposal at rest in pubescent children,
when compared to pre-pubertal children and is
probably caused by the conservation of carbohydrate stores for the energy requirements of
growth. This difference in substrate utilisation in
Table 1. Recommended protein intake for boys and girls versus typical intake in USA and Australia
Gender and age
Males
Females
Recommendations
Examples of reported intake,
g/day
RDA USA and
Canada
g/kg/day
RDA, g/day
1–3 years
1.05
13
4–8 years
0.95
19
20 (4–6 years)
66
64
9-13 years
0.95
34
28-41 (7–10 years,
11–14 years)
81
75
14–18 years
0. 85
52
45 (14–16 years)
97
120
19–30 years
0.80
56
56
1–3 years
1.05
13
4–8 years
0.95
19
20 (4–6 years)
66
64
9–13 years
0.95
34
28–41 (7–10 years,
11–14 years)
68
75
14–18 years
0.85
46
45 (14–16 years)
68
80
19–30 years
0.80
46
45
72
GDA, UK g/day
P intake in USA
P intake in AUS
55
109
55
combination of energy restriction and a vegetarian
diet, have a very low protein intake.
Carbohydrates
It has been shown that carbohydrate ingestion in
adults both before and during exercise can delay fatigue and improve endurance performance.
Unlike protein which has a quite general recommendation, recommendations for carbohydrate
intake highly depend on the intensity, type and
50
duration of exercise that is performed by young
athletes. Carbohydrate loading is a technique
that is often used by adult athletes to maximize
muscle glycogen stores and enhance endurance
exercise performance. Glycogen loading is not
advised for children [22] but since most events
will be shorter and glycolytic capacity is limited,
it must be questioned whether such a strategy
would be beneficial at all. A relatively high carbohydrate diet is advised but there is probably
no need to follow a dedicated glycogen loading
regimen.
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These data show that recommendations are exceeded and, at least at the group level, protein intake is more than adequate. RDA data derived from Dietary Reference Intakes (DRIs): Recommended Intakes for Individuals, Macronutrients.
Food and Nutrition Board, Institute of Medicine, National Academies, 2005 [17] and Working Group Report. Guideline
Daily Amounts (GDA) derived from report of the IGD/PIC Industry Nutrition Strategy Group Technical Working Group
on Guideline Daily Amounts (GDAs). Watford, UK. IGD, 2005 [20]. Intake data from references [19] USA and [21]
Australia.
Nutrition
aged 13–19 years periodically drank a carbohydrate drink (2% glucose and 4% sucrose), whilst
completing a cycle test at 60% V̇O2 max, their subjective rating of exercise intensity was significantly decreased by 1–2 points (RPE scale).
Beneficial effects of carbohydrate ingestion on
subjective ratings of perceived exertion have been
reported in healthy adults too, although interestingly in the boys the effect occurred after just 60
min of moderate intensity cycling, rather than
after 90–120 min, commonly reported in adult
studies. This effect occurring earlier in children
could be due to be the result of smaller carbohydrate stores in children and therefore their earlier
reliance on exogenous carbohydrate. It must be
noted, however, that this finding was not reproduced in a more recent study, where ingestion of
a 6% carbohydrate-electrolyte solution was found
to have no effect on RPE in boys during a 60 min
cycling test at ~70% peak V̇O2 [12].
It has been suggested that the ingestion of
carbohydrates may alter the substrates used by
exercising children. Indeed, Riddell et al. [33]
reported that the ingestion of a 13C-labeled glucose solution induced a sparing of endogenous
glucose utilization in boys (from 68 to 59% of
total energy utilization) and decreased fat utilization (from 32 to 18% of total energy expended). Furthermore, subsequent studies have also
reported that children appear to oxidize relatively more exogenous carbohydrate during exercise
than do adults [11, 12], despite their lower whole
body rate of carbohydrate oxidation and much
higher rate of fat oxidation. In fact, one study
looking at the effect of pubertal status and age on
exogenous carbohydrate oxidation reported that
exogenous carbohydrate oxidation contributed
to ~30% of the total energy expenditure in the
pre-pubertal and early pubertal boys, compared
to only ~24% in the mid-to-late pubertal boys.
It was suggested that the reliance on exogenous
carbohydrate oxidation during exercise is sensitive to pubertal status, rather than just chronological age [12].
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During exercise in adults carbohydrates can
help by maintaining high rates of total carbohydrate oxidation, sparing endogenous muscle
glycogen stores and maintaining blood glucose
concentrations, particularly in the later stages
of exercise [for reviews, see 23, 24]. In addition
to these metabolic effects there is also evidence
to suggest that carbohydrates may affect central
drive (or motivation) too, possibly via oral carbohydrate receptors [25–27].
In contrast, neither glucose consumption before [28] or during [11] exercise has been reported
to improve performance time in male adolescents.
In fact, in a study by Riddell et al. [11], twelve 10to 14-year-old boys intermittently drank either
water or a 6% glucose drink whilst undertaking
90 min of cycling at 55% peak V̇O2, followed by
an all-out performance ride to volitional exhaustion at 90% peak power. Although there was a
trend for the glucose solution to improve performance time (occurring in seven of the 12 boys),
this finding was not consistent and no significant
differences were found in performance between
the two drinks (water session: 142 ± 37 s vs. glucose: 177 ± 33 s). Interestingly though, the same
study showed that drinking a 3% glucose plus 3%
fructose solution did result in cycling time significantly improving by ~40% (202 ± 40 s). Although
this finding of an improved performance with a
glucose plus fructose drink has also been reported
in adult studies [29], the mechanisms may be very
different, as the amounts of carbohydrate ingested
in these studies were much larger and the exercise
duration longer. In addition to enhancing exercise
performance in endurance exercise, carbohydrate
ingestion has also been shown to increase performance in intermittent, high-intensity exercise
[30] and increase explosive strength and speed, as
well as shooting skill performance [31] in a basketball skill test.
Furthermore, the subjective rating of perceived
exertion (RPE) may also be influenced by carbohydrate ingestion. In another study by Riddell et
al. [32], it was reported that when healthy boys
52
maturation dependent decline in muscle glucose
uptake has also been reported [39].
Alternatively the increased reliance on exogenous carbohydrate oxidation could be because
the ability of exercising muscle to catabolise endogenous glycogen as an energy substrate is less
in boys than in men [40]. Interestingly though,
this same pubertal effect has not been found in
girls [34].
Carbohydrate intake during exercise has been
linked with improved performance, but also with
increases in gastrointestinal symptoms. When 18
adolescents undertook intermittent high-intensity
exercise for 48 min (treadmill sprinting, lateral
hops and shuttle run), ingestion of an 8% CHO
drink caused a higher rating of gastrointestinal
discomfort (stomach upset and side ache) than
ingestion of a 6% drink [41]. The use of carbohydrate drinks during such short duration and highintensity activities should be questioned anyway
as it is unlikely to give any metabolic advantage.
To date, there have been no studies investigating either the performance effects of carbohydrate
or the rate of carbohydrate oxidation in highly
trained elite young athletes. Due to this dearth of
information, we have relied upon studies undertaken on healthy active boys and girls to inform
this section; however, as it is known that both insulin sensitivity and substrate utilization during
exercise are affected by training, it is possible that
the outcomes of such studies on elite young athletes could differ from those obtained on active
children. Also, the exercise capacity and the absolute intensities these athletes would exercise at are
higher than their untrained counterparts and it is
therefore more likely that their metabolism will
behave more like that of older children.
Fat
Very few studies have investigated fat intake or fat
requirements in active children. Although certain
fats are important for growth and development, the
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Timmons et al. [34] have suggested that this
higher rate of carbohydrate oxidation may be due
to an increased recruitment and rate of translocation of insulin-sensitive GLUT4 glucose protein
transporters to the muscle membrane cell, resulting in a higher glucose uptake in exercising children. However, there may be another explanation
that is much simpler. In adult studies, it has clearly
been demonstrated that exogenous carbohydrate
oxidation is limited by absorption rather than a
muscle phenomenon [23, 24, 35]. The delivery
of carbohydrate to the muscle is the rate-limiting
step, not the oxidation in the muscle itself [for reviews, see 23, 24, 35]. It is not unlikely that in children the same limitation applies.
It has been found that in children under the
age of 5 years, the efficiency of carbohydrate absorption is lower than in adults, but this gradually
increases with age until the fifth year. The rate of
absorption also varies depending upon the ratio
of glucose to fructose in the drink [36]. Although
there is limited information regarding the rate of
intestinal absorption of carbohydrates in exercising conditions in children, some studies comparing oral 13C-bicarbonate dynamics between children and adults have suggested no differences
between children and adults in the absorption
of the tracer either at rest [37] or during exercise
[38]. If there are indeed no differences in carbohydrate absorption between children and adults,
this would provide an explanation for many of
the findings in previous studies. Similar absorption rates in children and adults would result in
similar exogenous carbohydrate oxidation (in
g • min–1). Because adults exercise at a higher exercise intensity and energy expenditure is greater,
the same exogenous carbohydrate oxidation results in a smaller contribution to energy expenditure. In contrast exogenous carbohydrate oxidation in children will contribute more to energy
expenditure. With the current limited evidence, it
is impossible to definitively determine what the
mechanism is. A muscle phenomenon cannot
be excluded, however, because in growing rats a
Thermoregulation and Fluid Requirements
One of the main ways that humans lose heat is
through the evaporation of sweat and the associated convection of body heat away from the surface of the skin. As children have a higher ratio of
body surface area to body mass [43] (at the age of
8 years it is approximately 50% higher than that
of an adult) it has been suggested that exercising
children should be able to dissipate heat quicker
than adults, giving their thermal homeostasis an
advantage over that of exercising adults, at least
up to the point at which ambient temperature exceeds skin temperature, after which this advantage is supposedly reversed. In practice, however, this has not been found to be the case and
instead adults and active children seem to experience similar core temperatures, even when exercising at high temperatures [44]. Whether the
same finding would occur in elite young athletes,
as compared to these active but not competitive
children, is yet to be determined.
Heat loss through the evaporation of sweat
can result in large fluid and electrolyte losses.
In adults, the dehydration caused by this fluid
loss has been shown to be detrimental to motor
Nutrition
control and physical performance [45], so adults
are advised to balance any fluids lost from sweating, with fluid intake. However, there are large
differences in sweat rates between children and
adults. In fact, when investigated in boys aged
9 years in hot and humid conditions (45°C and
97% relative humidity), it was reported that their
sweat rate was only half that of men. This muted
response, which also is similar to that seen in both
young girls and adult females, is probably due to
the underdevelopment of the peripheral sweating
mechanism in younger boys. In fact once the circulating levels of the male sex hormones start to
increase during puberty, the sweat rate is seen to
increase rapidly.
It seems tempting to conclude that if the young
athlete’s ability to sweat is reduced, particularly before puberty, then their risk of developing
sweat induced dehydration will also be reduced.
However, as sweating is the main way of dissipating heat during exercise, it is possible that children’s core body temperature could increase at a
more rapid rate than an adult’s. Ultimately, however, it seems that the reduced sweat rate does not
impair children’s heat loss during exercise [44].
Instead, it seems that children utilize different,
but just as effective, thermoregulatory mechanisms [44, 46–47] (see Falk and Dotan [4] for a
more in-depth discussion). Therefore, as dehydration levels and more importantly the risk of
developing a heat-related illness seem to be similar between adults and younger athletes, then the
recommendation regarding fluid replacement is
likely to be similar too.
Young athletes underestimate the amount of
fluid they need to consume during prolonged exercise in order to stay hydrated, especially in hot
and humid conditions and in particular when the
fluid available to them is water. As thirst is often
reported to be a poor indicator of fluid needs, it is
important to encourage drinking before, during
and after exercise to prevent dehydration. Previous
research has shown that involuntary hypohydration can reach up to 1–2% in both unacclimatised,
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link with performance is far less clear. Although
it is often recommended that 25–30% of energy
should come from dietary fat, such recommendations for total fat intake are highly dependent
on the energy expenditure. As in adults first priorities are adequate protein and carbohydrate intake and fat can make up for the remaining energy. Restricting fat intake in non-obese children
has been suggested to impair growth and development, although it is not clear whether this
is a direct effect of low fat intake or low energy
[42]. If weight loss is required in children who
are involved in relatively hard physical training,
it seems sensible to reduce the fat intake rather
than protein or carbohydrate (see also the section
on weight management below).
54
in the heat state that a child who weighs 40 kg
should drink 150 ml of cold water or flavoured
salted beverage every 20 min and an adolescent
who weighs 60 kg should drink 250 ml, even if the
child does not feel thirsty. Such guidelines, however, are very general and do not take into account
environmental conditions, exercise intensity, acclimatisation and individual differences. With a
lack of studies showing the effects of dehydration in children and the impact on performance,
it is very difficult to give balanced and objective
guidelines. At an elite level, it seems sensible to
develop an individual strategy that aims to reduce
fluid losses in excess of 3% body mass. This can be
done by measuring body weights before and after
training and correcting them for fluid intake to
obtain some measure of sweat rates. This would
eventually allow the prediction of sweat rates in
similar conditions.
Nutrition Supplements
Supplement use amongst junior athletes is common. In a study of 32 track and field junior athletes selected for Team Great Britain at the World
Junior Championships, it was found that 62% of
the sample used supplements [55]. Females (75%)
were found to use more supplements than males
(55%), although this difference was not statistically significant. This trend may be attributed to a
greater awareness among females, greater genuine
need for supplementation (e.g. menstrual loss) or
advertising campaigns having a greater influence
on females [55]. The most commonly used supplements were multivitamins, followed by vitamin C and iron [55]. In a review, McDowall [56]
concluded that the prevalence of supplement use
was between 22 and 71% in young athletes (age
ranges from 13 to 19 years). The most frequently
cited reasons for using a supplement were: health
benefits, illness prevention, enhancing performance, taste, rectifying a perceived poor diet and
increasing energy [57].
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untrained [48] as well as acclimatised, trained
[49] boys. Whilst education for parents, coaches,
teachers and indeed young athletes themselves
can improve fluid intake in elite young athletes,
research has also shown that there are other ways
to enhance thirst and therefore stimulate drinking. One of these is through the addition of small
amounts of sodium chloride to water, as this sensitises the thirst mechanism through the maintenance of plasma osmolality and reduces the diuretic effect of water [13, 49, 50], whilst also replacing
lost electrolytes. Another way is through the addition of carbohydrate to the drink, as this increases
the palatability of the solution [51]. Finally, the
addition of flavour is another important way to
promote increased re-hydration. In fact, RiveraBrown and colleagues [49] reported that in heat
acclimatised, trained boys the addition of flavour
to a carbohydrate-electrolyte drink helped to reduce voluntary dehydration by 32%, which was
enough to maintain euhydration over a 3-hour
period of intermittent cycling exercise (at 60%
V̇O2 max) in 30°C heat (53–62% relative humidity). Further, Meyer et al. [52] compared water and
different flavoured drinks in exercising Canadian
children and found that children preferred grapeand orange-flavoured drinks to apple-flavoured
drinks and water. It is likely, however, that these
findings are heavily influenced by cultural factors
and that different preferences would occur in different areas and countries.
Current recommendations for fluid replacement in children are scant. The 2007 American
College of Sports Medicine (ACSM) position
statement on ‘Exercise and Fluid Replacement’
barely refers to children’s needs, only referring to
the fact that pre-pubescent children have a lower
sweat rate than adults [53], whilst the 2009 ACSM
position statement on ‘Nutrition and Athletic
Performance’ does not comment upon children’s
or adolescents’ needs at all. In contrast, the policy statement re-issued in 2000 by the American
Academy of Pediatrics [54], regarding the fluid replacement guidelines for children during exercise
Nutrition
Regardless of whether supplements have the effects they are claimed to have, it is clear that young
elite athletes have a perceived need for nutritional
supplementation. However, there must be reservations to even the most commonly used supplements regarding long term use, combinations and
appropriate dosages in an elite young athlete. These
reservations concern: (1) an increased health risk
to an otherwise healthy population, and (2) the
possibility of positive doping tests caused by supplements containing banned substances [61]. To
minimise potential health risks arising from potentially inappropriate supplement use, an increased involvement of health professionals, with
appropriate training, is desirable.
Although we do not want to engage in detailed discussions of a long list of supplements,
one supplement has received substantial attention recently. Caffeine is one of the most widely
used drugs and energy drinks containing caffeine are now marketed specifically to young
adults and children. It is therefore important to
understand the effects of caffeine in this population. Energy drinks containing high concentrations of sugars and caffeine represent the fastest
growing segment of the beverage industry. Very
few studies have examined the physiological and
psychological effects of caffeine and therefore it
is difficult to give sound advice on caffeine use
for young athletes. However, there is evidence
that children and adolescents, although receiving similar benefits, may be particularly vulnerable to the negative effects of caffeine. Therefore,
caffeine should be used with caution. In general,
supplements for younger athletes are not recommended [22].
Weight Management and Dangers
Perhaps one of the greatest threats to child health
is inappropriate weight control in young athletes.
If a reduction in body mass is desired, this should
be done gradually and not more than 1.5% of
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In a recent study amongst elite young UK athletes (mean age 17.7 years), supplement use was
investigated [58]. Of the 1,674 questionnaires distributed, 412 were returned and 403 were within the age range required. Single supplement use
was reported by 48.1%. The most popular supplements were sports drinks consumed by 41.7%
of all athletes and 86.6% of the supplement users. Other popular supplements included vitamin
C (22.8%), multivitamins (22.8%), whey protein
(21.3%) and creatinine (13.4%). Echinacea (7.7%),
caffeine (5.7%), iron (4.7%), ginseng (1.7%) and
melatonin (1.0%) were amongst the most often
reported supplements. Among the desired outcomes resulting from supplement use, maintaining strength was the most frequently cited reason among all athletes in the sample (34.7%) and
supplement users (72.2%), followed by avoiding
sickness (56.1% of users) and endurance enhancement (55.2% of users). One-third of the supplement user athletes listed the ability to train longer
(30.4%) and helping to recover (32.5%) among the
reasons, whereas 23.2% take supplements to remedy imbalanced diet. A difference noted between
this study and a similar survey in elite adult athletes [59, 60] was that the reasons for supplement
use were mostly performance based. The younger
elite athletes seemed to appear less ‘health conscious’ and more ‘performance focused’ than their
adult counterparts.
The source of the advice varied and was different for different supplements. Interestingly,
many young athletes appeared to decide on their
nutritional supplementation themselves without
advice. There was, however, considerable overlap between self-managed supplementation and
medical advice. The coaches’ role in advising athletes on supplements, especially in taking energy
drinks and protein was also evidenced. Among
health professionals, athletes indicated that advice was sought from nutritionists and/or physiotherapists. The only exception was iron supplementation, which was taken following the general
practitioners’ advice.
body mass per week [62]. A more rapid weight
loss may result in muscle protein breakdown, and
this may interfere with growth and development.
To lose half a kg of fat in 1 week, one must expend 14,700 kJ (3,500 kcal) more than one consumes. It is often suggested that the preferred
way to do this is to consume 7,350 kJ (1,750 kcal)
fewer per week and expend 7,350 kJ (1,750 kcal)
more per week by exercising. When possible, the
athlete should be counselled by a registered dietician who has experience of working with athletes
and their families. A lean and light physique is
often desired in certain sports, especially endurance sports such as distance running and aesthetic sports such as gymnastics. Although in some
cases there are clear links to better performance
it is important to be aware that there are also risks
of energy deficiency, micronutrient deficiencies,
menstrual irregularity, poor bone health and eating disorders. These issues are reviewed in more
detail in an excellent review by Manore et al.
[63].
Conclusions
Children and adolescents need adequate energy
intake to ensure proper growth, development, and
maturation, but also to ensure optimal exercise
performance. There are several metabolic differences between young and adult athletes. Younger
athletes generally rely more on fat as a fuel, have
smaller glycogen stores and have a limited glycolytic capacity. Glycogen loading is not advised
and there is limited evidence that carbohydrate
intake during prolonged exercise can be beneficial. There are differences in thermoregulation,
but the impact on fluid requirements is not clear.
The limited evidence suggests that acute energy
and fluid imbalances can be detrimental to performance and there may be benefits of ingesting
carbohydrate and fluid during exercise, especially during more prolonged exercise. Rapid weight
loss and too much focus on weight in children can
pose a health risk. Supplements are not recommended for children.
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Prof. A.E. Jeukendrup
School of Sport and Exercise Sciences
University of Birmingham
Birmingham B15 2TT (UK)
Tel. +44 0 121 414 4124, Fax +44 0 121 414 4121, E-Mail A.E.Jeukendrup@bham.ac.uk
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