exercise are carbohydrate derived from muscle
glycogen or blood glucose from hepatic glycogen
stores, and fatty acids which may come from the
adipose triglyceride via plasma free fatty acids
(FFA) or from the intramuscular triglyceride
stores. The relative contributions of these fuels
during exercise is intensity dependent, with
the contribution of carbohydrate increasing as
exercise intensity increases. As documented by
Rowland (1985), effective aerobic training for the
child athlete requires a relatively high volume of
exercise at high intensity, thus placing large
demands on the body’s limited carbohydrate
stores (Coyle 1992).
In order to appreciate the nutritional conse-
quences of carbohydrate and fat manipulation
in the diet of the young athlete, it is necessary
to understand the underpinning physiological
bases of the child athlete. Erikksson et al. (1973),
Keul (1982) and Kindermann et al. (1978) have all
demonstrated lower levels of muscle phospho-
fructokinase activity and a reduced glycolytic
potential in children aged 11–13 years than in
adults. Conversely, Haralambie (1979) demon-
strated higher tricarboxylic-acid cycle enzyme
activity and increased lactate dehydrogenase
activity in 11–14-year-old girls than in adult
women and men.
Children performing prolonged exercise indi-
cate a preference for fat rather than carbohydrate
metabolism (Bar-Or & Unnithan 1994). Berg et al.
(1980) and Macek and Vavra (1981) demon-
strated significant increases in glycerol levels in
blood with prolonged (30–120 min) activity in
children. In addition, Martinez and Haymes
(1992) concluded that prepubertal girls relied
more on fat than on carbohydrate utilization
during exercise of moderate to heavy intensity.
Not only have higher glycerol (0.425 vs. 0.407
mmol · l–1) and FFA levels (1.97 vs. 1.82 mmol · l–1)
been noted in young children (10–12 years) vs.
adolescents (15–17 years) during exercise, but the
increase in glycerol (five times resting values)
occurred at an earlier time than seen in adults
(Berg et al. 1980). FFA uptake expressed per
minute per litre of O 2 uptake has been found to
be greater in children than in adults during pro-
434 special considerations
longed submaximal exercise. It is theorized that
a large immediate increase in noradrenaline
and a greater utilization of FFA is used by chil-
dren to offset hypoglycaemia during prolonged
exercise at the same relative exercise intensity
(Delamarche et al. 1992). A confounding factor in
the interpretation of the above observations is
the fact that it is assumed that a true maximal
oxygen uptake has been achieved by the individ-
uals under investigation. However, it has been
shown that only a minority of children and ado-
lescents attain a true maximal oxygen uptake
(Armstrong & Welsman 1994).
Respiratory exchange ratio (RER) data also
suggest a preference for fat utilization in chil-
dren. Asano and Hirakoba (1984), Macek and
Vavra (1981) and Martinez and Haymes (1992)
demonstrated lower RER values for children
than for adults during prolonged exercise.
Again, interpretation of these results is con-
founded by the fact that a failure to achieve a true
maximal oxygen uptake will result in over-
estimated submaximal work loads in children.
Therefore, comparisons between children’s and
adults’ data may not be appropriate. Macek and
Vavra (1981) demonstrated significant reduc-
tions in RER over 60 min of submaximal exercise,
in conjunction with increases in glycerol levels in
blood. Magnetic resonance spectroscopy work
by Zanconato et al. (1993) also demonstrated that
children were less able than adults to effect
adenosine triphosphate rephosphorylation by
anaerobic metabolic pathways during high-
intensity exercise. In conclusion, muscle enzyme,
RER and magnetic resonance spectroscopy data
suggest that children, as compared with adults,
seem better suited for aerobic than anaerobic
energy metabolism (Bar-Or & Unnithan 1994).
While it is acknowledged that children may
use fat rather than carbohydrate as the major fuel
during exercise, the ability to sustain this exercise
over a number of months and in high-intensity
exercise bouts would still depend upon
adequate carbohydrate stores being present.
Hence, appropriate knowledge and guidance
regarding carbohydrate intake are critical. Loosli
and Benson (1990) showed that in the absence of