Substrate oxidation in overweight boys at rest, during exercise and acute post-exercise recovery.

Abstract

To compare substrate oxidation between normal weight (n = 10) and overweight (n = 10) boys at rest, during exercise at 50% VO(2peak) and during the first 30 minutes of recovery post-exercise.
Twenty boys (8-11 years) were tested over two separate occasions. At the first session, body composition was measured by Dual-Energy X-ray Absorptiometry and peak aerobic capacity (VO(2peak)) was assessed using an incremental treadmill protocol. At least one week later, substrate oxidation was determined via indirect calorimetry in the fasted state at rest, during 10 minutes of exercise at 50% VO(2peak) and during the first 30 minutes of acute recovery post-exercise.
There were no significant differences in substrate oxidation between the two groups at rest or during exercise. However, during early recovery, respiratory exchange ratio (RER) transiently increased in the overweight boys (p = 0.034) but not in the normal weight boys (p = 0.796), with a shift towards greater carbohydrate oxidation in the former group at 15-20 (p = 0.044) and 25-30 (p = 0.052) minutes post-exercise. In contrast, absolute post-exercise fat oxidation was similar between the two groups.
Overweight boys may oxidise fat less efficiently than normal weight boys during recovery post-exercise, deriving a greater proportion of energy from carbohydrate oxidation.

Full text

Page 1

Address for correspondence: Nicole A. Crisp, School of Sport Science, Exercise and Health, The University of Western Australia, 35 Stirling Highway, Crawley,
WA 6009, Australia. Fax: ? 61 8 6488 1039. E-mail: crispn01@student.uwa.edu.au
(Received 16 June 2010; fi nal version received 21 November 2010)
ORIGINAL ARTICLE
Substrate oxidation in overweight boys at rest, during exercise
and acute post-exercise recovery
NICOLE A. CRISP , KYM J. GUELFI , REBECCA BRAHAM , MELISSA LICARI
School of Sport Science, Exercise and Health, The University of Western Australia
Abstract
Objective. To compare substrate oxidation between normal weight (n ? 10) and overweight (n ? 10) boys at rest, during
exercise at 50% V
were tested over two separate occasions. At the fi rst session, body composition was measured by Dual-Energy X-ray
Absorptiometry and peak aerobic capacity ( V
week later, substrate oxidation was determined via indirect calorimetry in the fasted state at rest, during 10 minutes of
exercise at 50% V
differences in substrate oxidation between the two groups at rest or during exercise. However, during early recovery, res-
piratory exchange ratio (RER) transiently increased in the overweight boys ( p ? 0.034) but not in the normal weight boys
( p ? 0.796), with a shift towards greater carbohydrate oxidation in the former group at 15 – 20 ( p ? 0.044) and 25 – 30
( p ? 0.052) minutes post-exercise. In contrast, absolute post-exercise fat oxidation was similar between the two groups.
Conclusion. Overweight boys may oxidise fat less effi ciently than normal weight boys during recovery post-exercise, deriving
a greater proportion of energy from carbohydrate oxidation.
.O 2peak and during the fi rst 30 minutes of recovery post-exercise. Study design. Twenty boys (8 – 11 years)
.O 2peak ) was assessed using an incremental treadmill protocol. At least one
.O 2peak and during the fi rst 30 minutes of acute recovery post-exercise. Results. There were no signifi cant
Key words: Normal weight , fat , carbohydrate , metabolism , obesity
Introduction
Childhood obesity, along with its associated detrimental
health consequences, is fast becoming one of the
most concerning issues for society. As a result, there
has been an increasing amount of research investigat-
ing the causes of excess fat deposition in children.
Substrate oxidation, a metabolic factor of particular
interest, relates to the relative contribution of fat
and carbohydrate oxidation to energy production.
Researchers have observed that this metabolic bal-
ance may have an important contribution to the
development and maintenance of obesity, given that
an impaired ability to oxidise fat may promote
increased fat storage (1–3). However, the majority of
studies have found that obese children have higher
rates of fat oxidation at rest compared with children
with a normal body composition (4–6).
In contrast, a recent study has reported that fat
and carbohydrate oxidation rates remain similar
between obese and nonobese adolescents during
exercise at a wide range of intensities (2). However,
Maffeis et al. (3) reported similar rates of fat oxida-
tion but signifi cantly higher rates of carbohydrate
oxidation in severely obese prepubertal boys, com-
pared with overweight or obese boys, during wal-
king at three different intensities. This suggests that
severely obese children and adolescents may have
an impaired capacity to oxidise fat during exercise,
deriving a greater proportion of energy from carbohydrate
oxidation.
No studies have explored the relationship between
body composition and substrate oxidation in chil-
dren during recovery post-exercise. This is of par-
ticular interest given that for a period of time
following an acute bout of exercise, oxygen consump-
tion remains elevated (7,8) and fat is preferentially
oxidised by the body to spare further depletion of
glycogen stores (9,10). Alterations in substrate oxida-
tion during this post-exercise period may have a sig-
nifi cant impact on fuel storage in the long term. In
International Journal of Pediatric Obesity, 2010; Early Online, 1–7
ISSN Print 1747-7166 ISSN Online 1747-7174 © 2010 Informa Healthcare
DOI: 10.3109/17477166.2010.543684
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2 N. A. Crisp et al.
adults, the only study that has explored the effect of
body composition on substrate oxidation post-exercise
found that the respiratory exchange ratio (RER)
remained signifi cantly higher for a longer duration
in obese men, when compared with lean men (11).
Furthermore, the duration of excess post-exercise
oxygen consumption was signifi cantly longer in the
latter. This suggests that obese individuals may utilise
fat less effi ciently for a longer duration post-exercise
and derive a greater proportion of energy from car-
bohydrate oxidation. However, it is unknown whether
a similar relationship exists in children. Therefore,
the purpose of this study was to examine substrate
oxidation at rest, during moderate intensity exercise
and during acute recovery post-exercise in normal
and overweight boys.
Methods
Participants
Twenty male children aged 8 – 11 years were recruited
from both the Childhood Obesity Clinic at Princess
Margaret Hospital (PMH) and the general commu-
nity based initially on the body mass index (BMI)
classifi cations established by Cole et al. (12). How-
ever, total percentage of body fat (% FM) deter-
mined using Dual Energy X-ray Absorptiometry
(DEXA; Lunar Prodigy, encore 2004, GE Medical
Systems, Madison, Wis., USA) enabled formal clas-
sifi cation into normal weight (n ? 10) or overweight
(overfat and obese; n ? 10) categories according to
McCarthy and colleagues ’ classifi cation index (13).
Exclusion criteria included taking any prescribed
medication or diabetes (Type I or II). After obtaining
approval from the Human Research Ethics Commit-
tee at both The University of Western Australia
and PMH, written consent was obtained from each
participant and their parent/guardian prior to their
involvement.
Assessment of peak aerobic capacity
Peak aerobic capacity ( V
an incremental treadmill protocol (14). The treadmill
was set at a constant speed of 5.6 km/h while the
gradient was increased by 2 – 4% every 3 minutes,
depending on the subject ’ s physical fi tness. Each par-
ticipant exercised continuously until their individual
end point of fatigue, with strong encouragement
from the experimenter. Throughout the duration of
the test, participants breathed into a mouthpiece
connected to a computerised gas analysis system. All
participants were well familiarised with the mouth-
piece to ensure that they were comfortable and
relaxed throughout the duration of testing. Ventilation
.O 2peak ) was determined using
was recorded using a turbine ventilometer (Morgan,
225A, Kent, England), which was calibrated before
the test using a 1 litre syringe. Oxygen (O 2 ) and car-
bon dioxide (CO 2 ) concentration in the expired air
was continuously analysed using Ametek gas analy-
sers (Applied Electrochemistry, SOV S-3A11 and
COV CD-3A, Pittsburgh, PA, USA), which were also
calibrated immediately before and verifi ed after each
test using a certifi ed beta-grade gravimetric gas mix-
ture of known concentration (BOC Gases, Chatswood,
Australia). Both the ventilometer and gas analysers
were connected to a PC that measured and displayed
variables every 15 seconds. The sum of the four high-
est consecutive volumes of O 2 consumption was
recorded as the participants V
.O2peak .
Determination of substrate oxidation
At rest . On a subsequent visit, participants arrived at
the laboratory after an overnight fast for the deter-
mination of substrate oxidation at rest, during exer-
cise and recovery. No dietary restriction was imposed
in the days before testing; however, food records were
completed for three consecutive days prior to testing
to ensure relatively normal food consumption by
each of the participants. In addition, they were
instructed to avoid strenuous exercise for at least
24 hours prior to testing. Upon arrival, participants
were encouraged to lie quietly and relax in a quiet,
dark room for 30 minutes while maintaining normal
breathing patterns. During this time, each boy
breathed through a mouthpiece enabling the collec-
tion of expired air into a 120 litre Tissot gasometer
tank (Collins Inc, Braintree, Massachusetts). The
Tissot tank was fl ushed twice with the participants
own air in the initial 20 minutes of rest prior to
collecting an expired air sample over the fi nal fi ve
minutes for the determination of substrate oxidation
at rest. From this sample of expired air, the concen-
tration of O 2 and CO 2 were determined using the
previously described Ametek gas analysers.
Sub-maximal exercise protocol . Following the determi-
nation of substrate oxidation at rest, each participant
exercised on a treadmill for 10 minutes at 50%V
for the determination of substrate oxidation during
exercise. This exercise intensity was selected to ensure
that the lactate threshold was not exceeded to prevent
acidosis from causing displacement of CO 2 from the
body ’ s bicarbonate stores, which results in a higher
estimation of RER (15). Additional confi rmation that
the exercise was below the lactate threshold was
obtained by checking each participant ’ s ventilatory
threshold from the incremental treadmill test as the
lactate and ventilatory thresholds generally coincide
.O 2peak
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Post-exercise substrate oxidation in boys
3
(16,17). This exercise duration (10 min) was selected
as being achievable by all participants and a steady
state was attained within this time for each individual.
During the exercise bout, each boy breathed through a
mouthpiece connected to the same computerised gas
analysis system used for the determination of V
Exercise V
minute of the exercise bout.
.O 2peak .
.O 2 and RER were recorded in the penultimate
Post-exercise recovery . Immediately after exercise,
measurements of post-exercise oxygen consumption
were made with the participants resting quietly in a
supine position. To determine substrate oxidation,
expired air samples were collected during minutes
5 – 10, 15 – 20 and 25 – 30 post-exercise into a 120 litre
Tissot gasometer tank (Collins Inc, Braintree,
Massachusetts) using the same method described for
determination of substrate oxidation at rest.
Calculation of macronutrient oxidation rate . The oxida-
tion rate of fat and carbohydrate (g/min) was calcu-
lated using equations from Frayn (15) at rest, during
exercise and during recovery post-exercise. It was
assumed that protein oxidation contributed to
approximately 12% of resting metabolic rate based
on previous studies (2,3).
Statistical analysis
A one-way between-groups ANOVA was used to
compare subject characteristics and all substrate oxi-
dation measurements at rest, during exercise and
during recovery post-exercise between the normal
and overweight groups. Paired-sample t-tests were
used in the event of a signifi cant main effect to iden-
tify where any differences lay. In addition, Cohen ’ s
effect sizes ( d ) were calculated for relationships
approaching signifi cance.
Results
Participant characteristics
Characteristics of the normal and overweight groups
are presented in Table I. Age, height and fat-free mass
(FFM) were similar between the groups. However,
as expected, the overweight group had a signifi cantly
higher body mass ( p ? 0.001), body mass index
(BMI; p ? 0.001), fat mass (FM; p ? 0.001) and total
percentage of body fat ( p ? 0.001). Aerobic fi tness,
whether expressed in absolute values [ V
relative to FFM [ V
not signifi cantly different between the two groups
( p ? 0.667; p ? 0.084, respectively). However, when
expressed relative to total body mass [ V
kg/min)], the overweight boys had signifi cantly lower
.O 2peak (L/min)] or
.O 2peak (mL/kg FFM/min)], was
.O 2peak (mL/
aerobic fi tness than the normal weight boys
( p ? 0.001).
Substrate oxidation
At rest . At rest, there was no signifi cant difference
in RER between the normal weight and overweight
groups (normal, 0.81 ? 0.03; overweight, 0.82 ? 0.04;
p ? 0.561; Figure 1). There was also no signifi cant
difference between the two groups in the amount of
fat and carbohydrate oxidised at rest when expressed
in absolute values ( p ? 0.513; p ? 0.331, respec-
tively) or relative to FFM ( p ? 0.866; p ? 0.722
respectively; Table II). As a result, energy expenditure
was not signifi cantly different between the normal
weight and overweight boys at rest when expressed
in absolute values (normal, 5665 ? 628 kJ/day; over-
weight, 6323 ? 956 kJ/day; p ? 0.097) or relative to
Table I. Participant characteristics.
Normal weight
(n ? 10)
Overweight
(n ? 10)
Age (years)
Height (cm)
Body mass (kg)
BMI (kg/m 2 )
Fat mass (kg)
Body fat (%)
Fat-free mass (kg)
V
V
V
9.7 ? 1.0
136.9 ? 6.8
32.2 ? 5.9
17.0 ? 1.8
4.6 ? 2.3
14.5 ? 5.0
25.8 ? 3.4
1.7 ? 0.3
53.1 ? 7.4
65.5 ? 7.3
9.5 ? 1.1
142.8 ? 8.6
48.5 ? 12.2 ∗
23.5 ? 4.3 ∗∗
17.1 ? 7.5 ∗∗
35.7 ? 6.9 ∗∗
29.2 ? 5.3
1.8 ? 0.4
36.2 ? 4.4 ∗∗
59.5 ? 7.4
. O 2peak (L/min)
. O 2peak (mL/kg/min)
. O 2peak (mL/kg FFM/min)
Values are mean ? standard deviation. BMI: body mass index.
Signifi cantly different from normal weight, ∗ p ? 0.05; ∗ ∗ p ? 0.001.

Figure 1. Comparison of respiratory exchange ratio (RER) at rest,
during exercise and during recovery post-exercise in normal
weight and overweight (including obese) boys classifi ed according
to percentage body fat (% FM). ∗ Signifi cantly different from rest
(both groups combined; p ? 0.05).
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4 N. A. Crisp et al.
FFM (normal, 222 ? 27 kJ/kg FFM/day; overweight,
229 ? 31 kJ/kg FFM/day; p ? 0.593).
Sub-maximal exercise . During the sub-maximal
exercise bout, the normal and overweight groups
both worked at the same relative intensity (50 ? 3;
50 ? 4% V
the amount of energy expended, based on steady
state oxygen consumption, was not signifi cantly dif-
ferent between the two groups whether expressed in
absolute values (normal, 17.1 ? 2.2 kJ/min; overweight,
18.3 ? 5.1 kJ/min; p ? 0.511) or relative to FFM
(normal, 0.67 ? 0.09 kJ/kg FFM/min; overweight,
0.62 ? 0.11 kJ/kg FFM/min; p ? 0.288). With the
commencement of exercise, there was an increase in
RER from resting values ( p ? 0.004); however, the
resulting RER was comparable between the normal
weight and overweight boys (0.84 ? 0.03; 0.84 ? 0.03;
respectively, p ? 0.872; Figure 1). Furthermore, there
was no signifi cant difference between the groups in
the amount of fat or carbohydrate oxidised whether
expressed in absolute values ( p ? 0.592; p ? 0.720,
respectively) or relative to FFM ( p ? 0.864;
p ? 0.442, respectively; Table II).
.O 2peak , respectively, p ? 0.896. As a result,
Post-exercise recovery . RER remained elevated from
resting levels during the fi rst 5 – 10 minutes post-
exercise ( p ? 0.001; Figure 1). Of interest, RER at
this time was higher than during exercise for the over-
weight boys ( p ? 0.034) but not the normal weight
boys ( p ? 0.796; Figure 1). Despite this, the difference
in RER between the two groups was not signifi cant
(normal, 0.84 ? 0.04; overweight, 0.87 ? 0.05;
p ? 0.142). Similarly, there were no signifi cant differ-
ences between the normal weight and overweight boys
in the amount of fat or carbohydrate oxidised
whether expressed in absolute terms ( p ? 0.992;
p ? 0.146, respectively) or relative to FFM
( p ? 0.813; p ? 0.452, respectively; Table II).
However, the overweight boys had a higher energy
expenditure at 5 – 10 minutes post-exercise when
expressed in absolute values (normal, 3.82 ? 0.42 kJ/min;
overweight, 4.62 ? 0.73 kJ/min; p ? 0.012) but
this was no longer the case when FFM was taken
into consideration (normal, 0.15 ? 0.01 kJ/kg
FFM/min; overweight, 0.16 ? 0.01 kJ/kg FFM/min;
p ? 0.366).
At 15 – 20 minutes post-exercise, RER was compa-
rable with resting levels ( p ? 0.305) and similar
between the two groups ( p ? 0.208; Figure 1). There
was also no signifi cant differences between the two
groups in the amount of fat oxidised whether expressed
in absolute values ( p ? 0.555) or relative to FFM
( p ? 0.506; Table II). However, the overweight boys
oxidised a greater absolute amount of carbohydrate
than the normal weight boys at this time ( p ? 0.044)
and the difference between the two groups approached
signifi cance when carbohydrate oxidation was
expressed per kilogram of FFM ( p ? 0.067; d ? 0.91;
Table II). Accordingly, the energy expenditure at
15 – 20 minutes post-exercise was signifi cantly greater
for the overweight group than the normal weight
group when expressed in absolute values (normal,
3.88 ? 0.56 kJ/min; overweight, 4.79 ? 0.74 kJ/min;
p ? 0.013) and per kilogram of FFM (normal,
0.15 ? 0.00 kJ/kg FFM/min; overweight, 0.17 ? 0.01 kJ/kg
FFM/min; p ? 0.013).
Finally, at 25 – 30 minutes post-exercise, RER
remained comparable with resting levels ( p ? 0.213)
and was similar between the two groups ( p ? 0.556;
Figure 1). The amount of fat oxidised whether
Table II. Absolute and relative amount of substrate oxidised (mean ? standard deviation) in the normal weight and overweight groups at
rest, during exercise and during recovery post-exercise.
Absolute (mg/min) Relative (mg/kg FFM/min)
Fat CarbohydrateFatCarbohydrate
Rest
Normal
Overweight
Exercise
Normal
Overweight
Post-exercise (5 – 10 minutes)
Normal
Overweight
Post-exercise (15 – 20 minutes)
Normal
Overweight
Post-exercise (25 – 30 minutes)
Normal
Overweight
49.8 ? 8.3
53.0 ? 11.8
87.8 ? 23.8
104.7 ? 46.6
1.94 ? 0.27
1.98 ? 0.66
3.45 ? 1.10
3.64 ? 1.05
187.8 ? 49.6
201.9 ? 64.7
460.4 ? 156.4
491.4 ? 218.9
7.21 ? 1.24
7.07 ? 2.22
18.34 ? 7.26
16.12 ? 5.20
39.8 ? 15.4
39.9 ? 23.8
107.6 ? 43.1
152.0 ? 75.9
1.58 ? 0.57
1.49 ? 0.97
4.39 ? 1.96
5.11 ? 1.96
56.2 ? 18.1
50.7 ? 19.2
68.9 ? 35.7
131.6 ? 72.9 ∗
2.13 ? 0.56
1.88 ? 0.91
2.72 ? 1.59
4.31 ? 1.72
54.4 ? 19.3
49.9 ? 19.5
77.2 ? 29.9
127.7 ? 66.7
2.08 ? 0.57
1.80 ? 0.81
3.12 ? 1.45
4.23 ? 1.49
∗ Signifi cantly different from normal weight group ( p ? 0.05).
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Post-exercise substrate oxidation in boys
5
expressed in absolute values ( p ? 0.614) or relative
to FFM ( p ? 0.406) was also similar between the
normal weight and overweight groups at this time
(Table II). However, a tendency remained for the
overweight boys to oxidise a greater absolute amount of
carbohydrate than the normal weight boys ( p ? 0.052;
d ? 0.92), although this was no longer the case when
carbohydrate oxidation was expressed per kilogram
of FFM ( p ? 0.118; Table II). As a result, the over-
weight boys also had a signifi cantly greater absolute
energy expenditure than the normal weight boys at
this time (normal, 3.93 ? 0.43 kJ/min; overweight,
4.66 ? 0.87 kJ/min; p ? 0.037). However, the amount
of energy expended was not signifi cantly different
between the two groups when expressed relative to
FFM (normal, 0.15 ? 0.00 kJ/kg FFM/min; over-
weight, 0.16 ? 0.01 kJ/kg FFM/min; p ? 0.340).
Discussion
Previous research has suggested that the relative
contribution of fat and carbohydrate oxidation to
energy production may play a role in the develop-
ment and maintenance of obesity. However, no stud-
ies have explored the relationship between body
composition and substrate oxidation in children
during recovery post-exercise. This is of interest
given that alterations in substrate oxidation at this
time may also infl uence fuel storage in the long term.
Therefore, the purpose of this study was to examine
substrate oxidation at rest, during exercise and dur-
ing acute recovery post-exercise in normal weight
and overweight boys classifi ed by % FM obtained
from DEXA. The proportion of fat and carbohy-
drate oxidation at rest and during exercise was sim-
ilar between the two groups. However, RER
transiently increased after exercise in the overweight
boys but not in the normal weight boys indicating a
shift towards greater carbohydrate oxidation in the
former. Consequently, the overweight boys oxidised
more carbohydrate during early recovery from exer-
cise while the rate of fat oxidation was similar
between the two groups. Whether these alterations
in substrate oxidation contribute to a positive fat
balance and increased fat storage in the long term
remains yet to be determined.
At rest, the fi nding of a similar RER between the
two groups indicated that the proportion of fat and
carbohydrate oxidised was comparable in the normal
weight and overweight boys in the basal state. Fur-
thermore, the absolute and relative amount of fat
oxidation was similar between the two groups. This
outcome is in contrast to previous studies conducted
in children (4,5,6), which observed that obese chil-
dren had a lower respiratory quotient at rest, or higher
rates of fat oxidation, when compared with normal
weight children. One explanation for the inconsistency
may be the methodology that was used to defi ne
obesity. For example, Maffeis et al. (5) classifi ed their
participants into normal weight and obese categories
based on BMI. However, this method fails to distin-
guish between fat mass, muscle or bone and may
incorrectly classify individuals with high muscularity
as overweight or obese (13). To demonstrate this
point, one participant in the present study was clas-
sifi ed as overweight by BMI, but normal weight when
% FM was used, as a result of their high muscularity.
In addition, two of the participants in the present
study were classifi ed as normal weight by BMI, but
obese by % FM, due to their high proportion of fat
mass relative to total body weight. As obesity is driven
by increased fat mass, it could be suggested that ideal
classifi cation tools directly address adiposity. Other
studies have utilised skinfold measurements (4,6);
however, this method tends to be less accurate in
obese individuals due to limitations in the span width
of the calipers (18). Therefore, an advantage of the
present study was that DEXA measurements for per-
centage fat mass (the laboratory ‘ gold ’ standard;
(19)) enabled classifi cation into normal weight and
overweight groups. To our knowledge, no other pub-
lished studies have used DEXA to classify children
when examining the relationship between body com-
position and substrate oxidation. Although in adults,
a recent study using DEXA (11) reported compa-
rable fi ndings at rest to the present study. That is, the
proportion of fat and carbohydrate oxidation at rest
was similar between the normal weight and over-
weight adults.
It is well established that carbohydrate oxidation
increases in response to exercise (2,3). However, the
extent of this proportionate increase in carbohydrate
oxidation and subsequent decline in fat oxidation
may be different between individuals with vary-
ing levels of adiposity. In the present study, RER
increased in response to exercise but remained simi-
lar between the normal weight and overweight groups.
The absolute rates of fat and carbohydrate oxidation
also remained comparable between the groups. Like-
wise, Lazzer et al. (2) reported no signifi cant differences
for fat and carbohydrate oxidation rates between
obese and nonobese adolescents during exercise at a
wide range of fi ve-minute steady-state exercise inten-
sities. It is important to note; however, that as over-
weight individuals are less likely to be able to push
through the pain, shortness of breath and fatigue
experienced during high intensity exercise (20),
together with the fact that additional criteria (e.g.,
oxygen plateau, RER, blood lactate, HR) may not be
appropriate to establish whether peak aerobic capac-
ity was attained (21), it is possible that the overweight
boys were exercising at a lower percentage of their
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6 N. A. Crisp et al.
aerobic capacity (rather than 50% V
steady-state exercise protocol than the normal weight
boys. This would infl uence their substrate oxidation
preferences and make it diffi cult to compare the
results between the two groups. It is also important
to note that in order to examine the infl uence of the
exercise bout alone on substrate oxidation during
and post-exercise, it was necessary for the partici-
pants to be fasted. The results may be different with
the ingestion of a meal prior to exercise. Finally, to
ensure that all participants could cope with the exer-
cise protocol utilised in the present study, a duration
of only 10 minutes was examined. Future research
should investigate whether exercise of a longer duration
causes differences to emerge between children of
varying body composition.
During recovery post-exercise, many studies have
observed an increased proportion of fat oxidation,
which is indicated by a decline in RER (9,10,11,22).
This preferential oxidation of fat after exercise has
been proposed to spare further depletion of glyco-
gen stores. To our knowledge, this is the fi rst study
to examine substrate oxidation in children during
the acute post-exercise recovery period. It was
observed that RER increased during the fi rst 5 – 10
minutes of recovery in the overweight boys but not
in the normal weight boys, indicating a lower
proportion of fat oxidation in the former group.
Furthermore, the overweight boys had a signifi cantly
greater absolute energy expenditure than the normal
weight boys at 5 – 10, 15 – 20 and 25 – 30 minutes post-
exercise. This increase in energy expenditure is likely
ascribable to their higher rates of carbohydrate
oxidation as opposed to fat oxidation rates, which
remained similar between the two groups across the
post-exercise period. These fi ndings, consistent with
those of Wong and Harber (11) in adults, suggest
that overweight boys may oxidise fat less effi ciently
during recovery post-exercise, deriving a greater
proportion of their total energy expenditure from
carbohydrate oxidation.
In the present study, a period of only 30 minutes
post-exercise was examined to ensure successful
completion by all participants (i.e., to maintain their
focus and minimise fi dgeting). It remains to be deter-
mined whether any differences may emerge over a
longer duration of recovery given that Saris and
Schrauwen (22) observed a compensatory change in
substrate oxidation over 24 hours in obese subjects.
For instance, although RER may tend to remain
higher in overweight individuals during the acute
post-exercise recovery period, whether these indi-
viduals maintain a lower RER than their normal
weight counterparts for the remainder of the day is
yet to be determined. Additional studies should be
conducted to further explore substrate oxidation
.O 2peak ) for the patterns during the post-exercise recovery period in
children with varying levels of adiposity.
In summary, there were no signifi cant differences
in all substrate oxidation measures at rest or during
exercise between the normal weight and overweight
boys. However, during the acute post-exercise recov-
ery period, RER was transiently elevated in the over-
weight boys but not in the normal weight boys,
indicating that they may oxidise a lower proportion
of fat at this time. Despite this, the absolute rate of
fat oxidation was similar between the two groups
across the post-exercise period examined in this
study, with the higher RER and energy expenditure
ascribed to higher rates of carbohydrate oxidation in
the overweight boys. The identifi cation of alterations
in substrate oxidation (including RER and the rates
of fat and carbohydrate oxidation) in overweight
children during exercise and recovery may provide
valuable information to assist in the implementation
of future exercise-based interventions to help manage
and combat childhood obesity. In particular, there
may be an optimal exercise intensity that maximises
long term fat loss, whether that be by maximising
energy expenditure and fat oxidation during the
exercise bout alone or in combination with its effect
on the post-exercise period. It may be benefi cial for
future research to concentrate on determining the
optimal exercise intensity, duration and type to
promote increased fat oxidation (taking into account
the post-exercise period). Consequently, it should
be determined whether the application of exercise
interventions aimed at maximising fat oxidation, in
comparison to other exercise training protocols,
are more successful for long term weight loss in
overweight children. Intervening in the early onset
of the condition (i.e., childhood) may prevent
overweight and obesity, along with the associated
detrimental health consequences, from persisting
into adulthood.
Acknowledgements
Thank you to the Childhood Obesity Clinic at PMH
for assistance in subject recruitment and the subjects
for their kind collaboration.
Declaration of interest: The authors report no
confl icts of interest. The authors alone are respon-
sible for the content and writing of the paper.
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