European Journal of Applied Physiology

, 108:383

Fat oxidation rate during and after a low- or high-intensity exercise in severely obese Caucasian adolescents

Authors

  • Stefano Lazzer
    • Istituto Auxologico Italiano, IRCCSLaboratorio Sperimentale di Ricerche Auxo-Endocrinologiche
    • Dipartimento di Scienze e Tecnologie BiomedicheUniversità degli Studi di Udine
  • Claudio Lafortuna
    • Istituto di Bioimmagini e Fisiologia Molecolare del Consiglio Nazionale delle Ricerche
  • Carlo Busti
    • Istituto Auxologico Italiano, IRCCSLaboratorio Sperimentale di Ricerche Auxo-Endocrinologiche
  • Raffaela Galli
    • Istituto Auxologico Italiano, IRCCSLaboratorio Sperimentale di Ricerche Auxo-Endocrinologiche
  • Tiziana Tinozzi
    • Istituto Auxologico Italiano, IRCCSServizio di Dietologia
  • Fiorenza Agosti
    • Istituto Auxologico Italiano, IRCCSLaboratorio Sperimentale di Ricerche Auxo-Endocrinologiche
    • Istituto Auxologico Italiano, IRCCSLaboratorio Sperimentale di Ricerche Auxo-Endocrinologiche
    • Istituto Auxologico Italiano, IRCCSDivisione di Auxologia
Original Article

DOI: 10.1007/s00421-009-1234-z

Cite this article as:
Lazzer, S., Lafortuna, C., Busti, C. et al. Eur J Appl Physiol (2010) 108: 383. doi:10.1007/s00421-009-1234-z
  • 562 Views

Abstract

The objective is to study the effects of low-intensity (LI) or high-intensity (HI) equicaloric exercises on energy expenditure (EE) and substrate oxidation rate during and after the exercises in severely obese Caucasian adolescents. Twenty obese boys (BMI-SDS 3.04 ± 0.52, %Fat Mass 38.2 ± 2.1%) aged 14–16 years (pubertal stage >3) participated in this study. Maximal oxygen uptake (V′O2max) and maximal fat oxidation rate were determined with indirect calorimetry using a graded exercise test on a treadmill. EE and substrate oxidation rate during equicaloric low-intensity (LI, 42% V′O2max for 45 min) and high-intensity (HI, 67% V′O2max for 30 min) exercises on a treadmill and during post-exercise recovery period (60 min) were determined with indirect calorimetry. Maximal fat oxidation rate was observed at 42 ± 6% V′O2max (62 ± 5% HRmax) and fat oxidation rate was 0.45 ± 0.07 g/min. The total amounts of EE, during the LI and HI exercises, and the post-exercise recovery periods were not significantly different (1,884 ± 250 vs. 1,973 ± 201 kJ, p = 0.453), but the total amount of fat oxidised was significantly higher (+9.9 g, +55.7%, p < 0.001) during the LI exercise than during the HI exercise. However, fat oxidation rates during the post-exercise recovery periods were not significantly different following LI and HI exercises. Total fat oxidised was significantly higher during the LI than during the HI exercise in obese adolescents. However, the equicaloric exercise intensity did not influence EE, fat and carbohydrate oxidation rate during the recovery period.

Keywords

Fat metabolismObesityExercise intensityEnergy metabolismBody composition

Introduction

Obesity is a major health problem in many industrialised countries of the world because of its association with cardiovascular disease, hypertension, and diabetes mellitus (Lobstein et al. 2004). Physical activity attenuates these risks (Wei et al. 1999) and is recommended as an adjunct to an energy-restricted diet for the treatment of obesity. Negative energy balance and, more importantly, negative fat balance are essential for reducing body fat stores (Swinburn and Ravussin 1993). According to the nutrient balance theory (Flatt 1993), the body’s stores of fat, carbohydrate, and protein are regulated independently of one another; therefore, breaking down more fat than is being consumed is essential for reducing adipose tissue. Physical activity increases energy expenditure (Lazzer et al. 2005), promotes fat use by skeletal muscles (van Aggel-Leijssen et al. 2001), and exercise training clearly enhances fatty acid oxidation during and after a physical activity period (Brandou et al. 2003) in obese subjects.

Prescribing physical activity for body weight control needs a specific recommendation for the type, intensity and duration of the physical activity sessions if energy expenditure and fat oxidation are to be optimised. In obese (Lafortuna et al. 2008) and non-obese adults (Achten et al. 2003), walking was more convenient than cycling because the target energy expenditure was attained at lower heart rate (HR), or in a shorter time, with lower blood lactate concentration and greater fat oxidation. The exercise intensity is related to energy expenditure and is of course higher per time unit in high-intensity (HI) than low-intensity (LI) exercise. However, the effects of exercise intensity on 24-h energy expenditure are unclear. Some studies, in trained subjects, have shown that HI exercise has a more pronounced effect on post-exercise energy expenditure than LI exercise (Gore and Withers 1990; Smith and Mc Naughton 1993), whereas others found no difference (Sedlock et al. 1989). In addition, Saris and Schrauwen (2004) found no significant effect of exercise intensity on 24-h post-exercise energy expenditure in obese adults. Thus, determining the optimal exercise intensity needed to increase energy expenditure during and after exercise should be considered in the treatment of obese adolescents.

Previous cross-sectional studies in obese adolescents reported that the exercise intensity corresponding to maximal fat oxidation corresponded to about 40% of V′O2max (Maffeis et al. 2005; Brandou et al. 2006; Lazzer et al. 2007). In addition, substrate utilisation during the post-exercise recovery period could have an important effect on body fat stores. It has been shown that HI exercise, compared to LI exercise, resulted in greater (Bahr et al. 1991; Broeder et al. 1991; Schrauwen et al. 1997) or similar (Saris and Schrauwen 2004; Kuo et al. 2005; Henderson et al. 2007) fat oxidation during the post-exercise recovery period. More information is, therefore, needed with regard to energy expenditure and fat oxidised during and after low- or high-intensity exercise, particularly in severely obese adolescents.

Thus, the objective of the present study was to investigate the acute effects of both low- and high-intensity equicaloric exercises on energy expenditure and substrate oxidation during the exercise and the post-exercise recovery period in severely obese Caucasian adolescents.

Research design and methods

Subjects

Twenty severely obese boys aged 14–16 years with a pubertal stage >3, according to the Tanner system of pubertal staging (Tanner 1962), participated in this study. The subjects were recruited as in-patients from the Division of Auxology, Italian Institute for Auxology, IRCCS, Piancavallo (VB), Italy. Obese subjects who had previously participated in weight management programs or who had overt metabolic and/or endocrine diseases and those taking medications regularly or using any drugs known to influence energy metabolism were excluded. The experimental protocol was approved by the Ethics Committee of the Italian Institute for Auxology (Milan). Before the study began, the purpose and objectives were carefully explained to each subject and to his or her parents. Written informed consent was obtained from all adolescents and their parents.

Physical characteristics and body composition

At hospital admission, the medical history was taken and a physical examination was performed. Body mass (BM) was measured to the nearest 0.1 kg with an electronic scale (Selus, Italy) with the subject dressed only in light underwear. Stature was measured to the nearest 0.5 cm on a standardised Harpenden stadiometer (Holtain Ltd, UK). The body mass index (BMI) was calculated as weight (kg) divided by stature (m) squared. The standard deviation (SD) score for BMI was determined by the following equation:
$$ {\text{BMI-SDS}} = [ (Q /M )^{L} - 1]/(L\, \times S ) $$
where Q is the observed BMI. The M and S values correspond to the median and coefficient of variation of BMI at each age, respectively, and the L values allow for the substantial age-dependent skewness in the distribution of BMI. The values for L, M, and S have been tabulated for a series of ages (Cacciari et al. 2006).

Body composition was measured using a tetrapolar impedancemeter (BIA, Human-IM Scan, DS-Medigroup, Milan, Italy). In order to reduce errors of measurement, attention was paid to the standardisation of the variables that affect measurement validity, reproducibility, and precision. Measurements were performed according to the method of Lukaski (1987) after 20 min resting in a supine position with arms and legs relaxed and not in contact with other body parts. Fat-free mass (FFM) was calculated using the prediction equation developed by Lazzer et al. (2008), and fat mass (FM) was derived as the difference between BM and FFM.

Basal metabolic rate

Basal metabolic rate (BMR) was determined in the morning (measurements starting between 0800 and 1000 a.m.), after an overnight fast by means of open circuit, indirect computerised calorimetry (Vmax 29, Sensor Medics, Yorba Linda, Ca, USA) with a rigid, transparent, ventilated canopy. Before each test, the gas analysers were calibrated using a reference gas mixture (95.00% O2 and 5.00% CO2). BMR of subjects was measured for 45 min. Oxygen consumption (V′O2) and carbon dioxide production (V′CO2), standardised for temperature, barometric pressure and humidity, were recorded at 1-min intervals. Results from the first 5–10 min, which corresponded to adjustment to the procedural environment, were excluded from the analysis. Energy expenditure was derived from the measured oxygen uptake and carbon dioxide output according to the formula of Weir (1949), and averaged over the whole measurement period.

Physical capacities and maximal fat oxidation rate

Peak oxygen uptake and maximal fat oxidation rate were determined using a graded exercise test on a motorised treadmill (TechnoGym, Gambettola, Italy), under medical supervision. Subjects were asked to avoid strenuous exercise the day before the test and came to the laboratory after a 12-h overnight fast. Three-day physical activity and dietary records were completed before the test to monitor the normal physical activity and composition of diet in each subject. Analysis of food records was performed using the GENI IV program (Micro 6, Villers les Nancy, France).

Before the beginning of the study, subjects were familiarised with the equipment and the procedures. Each test was undertaken in the morning (exercise starting between 0800 and 1000 a.m.), and comprised a 10-min rest period followed by walking in stages of 4-min duration. The rates (m/s) and incline (%) followed a sequence: 0.6 (incline 0%), 1.0 (0%), 1.0 (3%), 1.3 (3%), 1.4 (6%), 1.4 (9%) and 1.4 (12%). The workload was progressively increased until a HR of approximately 180 beats/min was reached, at which point exercise was concluded in order to avoid any cardiovascular complications associated with maximal effort which would be particularly risky in this kind of population. During the experiment, ventilatory and gas exchange responses were measured continuously by indirect calorimetry (CPX Express, Medical Graphics Corp, MN, USA). The flowmeter and gas analysers of the system were calibrated using, respectively, a 3-L calibration syringe and calibration gas (16.00% O2; 4.00% CO2). During the exercise test, an electrocardiogram was recorded continuously and displayed on line for visual monitoring, and heart rate (HR) was measured with a dedicated monitor device (Polar, Finland).

The maximal oxygen uptake (V′O2max) was estimated for each subject using the regression equations of V′O2 as a function of HR measured at the last minute of each step and extrapolated to the maximal theoretical HR (HRmax = 220 − age).

The substrate oxidation rate was calculated from V′O2 and V′CO2 (CPX Express, Medical Graphics Corp, MN, USA) during the last minute of each workload level, according to the protocol of Achten et al. (2002) and using the following equations (Frayn 1983):

Fat oxidation rate (g/min) = 1.67 × V′O2 (L/min) − 1.67 × V′CO2 (L/min) − 0.307 × Pox

Carbohydrate oxidation rate (g/min) = 4.55 × V′CO2 (L/min) − 3.21 V′O2 (L/min) − 0.459 × Pox

where Pox is the protein oxidation rate. The protein oxidation rate was estimated by assuming that protein oxidation contributed approximately 12% of resting energy expenditure (Frayn 1983):

Protein oxidation rate (g/min) = [energy expenditure (kJ/min) × 0.12]/16.74 (kJ/g).

For each subject, the results of the graded exercise test were used to compute the relationship between fat oxidation rate as a function of exercise intensity expressed as %V′O2max and %HRmax. The best fit was obtained with a polynomial relationship of the second order. The relationship between fat oxidation rate and V′O2 was used to determine the exercise intensity which corresponded to the highest rate of fat oxidation.

Energy expenditure and substrate oxidation rate during submaximal exercise and post-exercise recovery

After determining the physical capacities of subjects and the exercise intensity which corresponded to the highest rate of fat oxidation, the 20 subjects were randomly split into two groups: 10 subjects participated in a LI exercise test and 10 subjects in a HI exercise test.

The submaximal tests took place 3 days after the V′O2peak test. These tests were designed in such a way that equal amounts of energy were expended during LI and HI exercises on a motorised treadmill (TechnoGym, Gambettola, Italy). Subjects were asked to avoid strenuous exercise the day before the test and came to the laboratory after a 12-h overnight fast.

The LI exercise test comprised a 10-min rest period in standing position on a treadmill, followed by 45-min walking at maximal fat oxidation rate intensity previously determined for that subject, and then a 60-min post-exercise recovery period. The HI exercise test comprised a 10-min rest period in standing position on a treadmill, followed by 30-min walking at about 70% of V′O2max, and 60-min post-exercise recovery. The HI exercise intensity corresponded to the maximum exercise intensity that these subjects could maintain for 30 min.

The V′O2 and V′CO2 were measured continuously (CPX Express, Medical Graphics Corp, MN, USA) during the rest, exercise and post-exercise recovery periods. Substrate oxidation rate was calculated over consecutive 5-min periods using the equations of Frayn (1983) as described above. Energy supply (kJ/min) during exercise and during post-exercise recovery was calculated as the sum of each substrate oxidation rate (g/min) multiplied by the appropriate conversion factor (carbohydrate and protein = 16.74 kJ/g; fat = 37.66 kJ/g). During the exercise tests and the post-exercise recovery, an electrocardiographic record was undertaken continuously and displayed on line for visual monitoring, and HR was measured with a dedicated monitor device (Polar, Finland).

Statistical analyses

Statistical analyses were performed using Statistica for Windows (Kernel version 5.5 A, StatSoft, Maisons-Alfort, France) with significance set at p < 0.05. All results were expressed as mean and standard deviation (SD). Any associations of the grouping (LI vs. HI) with physical characteristics, body composition, BMR and substrate oxidation were tested using ANOVA analysis. When significant differences were found, a Bonferroni post hoc test was used to determine the exact location of the difference.

Results

Physical characteristics of subjects

The physical characteristics of obese adolescents are shown in Table 1. No significant differences were shown in anthropometric and body composition characteristics between the LI and HI groups. Thus, mean BMI, BMI-SD, FFM and %FM were 37.5 ± 4.5 kg/m2, 3.04 ± 0.52, 68.5 ± 8.1 kg and 38.2 ± 2.1%, respectively.
Table 1

Physical characteristics of subjects

 

Low-intensity group (n, 10)

High-intensity group (n, 10)

p

Age (year)

15.9 ± 1.4 

(15.0, 16.9)

16.3 ± 1.4

(15.2, 17.4)

0.597

Stature (m)

1.73 ± 0.06

(1.69, 1.78)

1.71 ± 0.09

(1.64, 1.78)

0.462

Body mass (kg)

110.1 ± 13.3

(100.9, 119.4)

112.0 ± 15.8

(99.7, 124.2)

0.786

BMI (kg/m2)

36.6 ± 3.7

(34.0, 39.1)

38.4 ± 5.4

(34.2, 42.6)

0.371

BMI SDS

2.94 ± 0.48

(2.63, 3.25)

3.15 ± 0.50

(2.69, 3.61)

0.378

Waist circumference (m)

1.16 ± 0.06

(1.12, 1.20)

1.19 ± 0.09

(1.12, 1.26)

0.367

Hip circumference (m)

1.24 ± 0.10

(1.17, 1.30)

1.23 ± 0.10

(1.15, 1.30)

0.892

FFM (kg)

68.0 ± 7.6

(62.7, 73.3)

69.1 ± 8.7

(62.4, 75.8)

0.773

FM (kg)

42.2 ± 6.3

(37.8, 46.5)

42.9 ± 7.7

(36.9, 48.8)

0.829

FM (%)

38.2 ± 1.9

(36.9, 39.5)

38.2 ± 2.4

(36.4, 40.0)

0.982

BMR (MJ/day)

8.63 ± 0.90

(8.01, 9.26)

9.05 ± 0.68

(8.53, 9.58)

0.321

BMR [MJ/(kg FFM day)]

0.13 ± 0.01

(0.12, 0.14)

0.13 ± 0.02

(0.12, 0.15)

0.527

V′O2max (L/min)

3.48 ± 0.48

(3.15, 3.81)

3.42 ± 0.34

(3.16, 3.69)

0.783

V′O2max [mL/(kg FFM min)]

51.3 ± 5.0

(47.8, 54.8)

50.0 ± 8.5

(43.4, 56.6)

0.665

All values are mean and standard deviation (SD). 95% confidence intervals in brackets

BMI body mass index, BMI-SDS body mass index standard deviation score, FFM fat-free mass, FM fat mass, BMR basal metabolic rate, V′O2max maximal oxygen uptake

p represents significance by ANOVA test

BMR and predicted V′O2max, expressed in absolute and in relative values, were not significantly different between the groups (on average: BMR: 8.84 ± 0.79 MJ/day and 0.13 ± 0.02 MJ/(kg FFM day); V′O2max: 3.45 ± 0.41 L/min and 50.6 ± 6.8 mL/(kg FFM min), respectively).

Substrate oxidation rate during the incremental test

Figure 1 shows the relationship between fat and carbohydrate oxidation rates (g/min) and exercise intensity expressed as a percentage of predicted V′O2max. At the different steps of exercise intensity, no significant differences were found in fat and carbohydrate oxidation rate between the LI and HI groups. With increasing exercise intensity, fat oxidation rate increased to a maximum of 0.45 ± 0.07 g/min at 42 ± 6% V′O2max, which corresponded to 62 ± 5% HRmax (Fig. 1a). The 95% confidence interval for maximal fat oxidation rate was 0.37–0.47 g/min and no significant correlations were found between fat oxidation rate and body mass, or FFM or FM or V′O2max of the subjects. At exercise intensities above 50% V′O2max, fat oxidation rate decreased markedly. The contribution of fat oxidation to energy supply became negligible above 80 ± 7% V′O2max (86 ± 6% HRmax). Conversely, with increasing exercise intensity, carbohydrate oxidation rate increased markedly (Fig. 1a) and above 50% V′O2max carbohydrates became the predominant source of energy for oxidation (Fig. 1b).
https://static-content.springer.com/image/art%3A10.1007%2Fs00421-009-1234-z/MediaObjects/421_2009_1234_Fig1_HTML.gif
Fig. 1

Fat (filled circle) and carbohydrate (open circle) oxidation rates expressed in absolute values (g/min, a) and as percent of total energy expenditure (%EE, b) as a function of exercise intensity expressed as percent of maximal oxygen uptake (V′O2max)

In addition, ANOVA repeated test shows that the fat oxidation rates observed at the 4th minute of the graded exercise tests at 42 and 67% of V′O2max were not significantly different than those measured for the same exercise intensities throughout the steady-state tests (at 4–5–6–10–15–20–25–30–35–40 min and 4–5–6–10–15–20–25 min for the low- and high-intensity exercises, respectively).

Energy expenditure and substrate oxidation rate during the submaximal exercise and during the post-exercise recovery period

The mean exercise intensities completed by subjects of the LI and HI groups were 42.0 ± 5.7 and 67.4 ± 6.9% of V′O2max, (61.2 ± 3.4 and 77.4 ± 2.8% HRmax), respectively (Fig. 2). During the post-exercise recovery period, V′O2 decreased to the basal values in 10 and 5 min, averaging 23.9 ± 8.6 and 20.6 ± 6.6% V′O2max (p = 0.634) after the LI and HI exercises, respectively (Fig. 2a, b). In addition, HR was significantly higher than the basal values only during the first 5 min of the LI post-exercise recovery period (Fig. 2c), whereas it remained significantly higher than the basal values during the 60-min post-exercise recovery period of the HI exercise (Fig. 2d).
https://static-content.springer.com/image/art%3A10.1007%2Fs00421-009-1234-z/MediaObjects/421_2009_1234_Fig2_HTML.gif
Fig. 2

Variations of oxygen uptake, expressed in percent of maximal oxygen uptake (%V′O2max, a, b), and heart rate, expressed in percent of maximal heart rate (%HRmax, c, d), during the exercise and the post-exercise recovery period in low-intensity (a, c) and high-intensity (b, d) groups (filled circle). *p < 0.05 when compared to corresponding values of basal metabolic rate (dashed line)

Energy expenditures during the LI and HI exercises were not significantly different (1,454 ± 150 vs. 1,530 ± 181 kJ, respectively, p = 0.629). Similarly, energy expenditures during the post-exercise recovery periods were not significantly different (431 ± 121 vs. 442 ± 141 kJ, respectively, p = 0.843). Furthermore, total EEs during the exercise and post-exercise recovery were not significantly different between the LI and HI groups (1,884 ± 250 vs. 1,973 ± 201 kJ, p = 0.453).

In contrast, during the exercise and recovery periods, energy from fat and the amount of fat oxidised were significantly higher (+372 kJ and + 9.9 g; +55.7%, p < 0.001), in the LI group than in the HI group, whereas energy from carbohydrates and the amount of carbohydrates oxidised were significantly lower in the LI group (−449 kJ and −26.8 g; −41.5%, p < 0.001) than in the HI group (Fig. 3a, b). Finally, the estimated energy from protein and the amount of protein oxidised during the exercise and recovery periods were not significantly different between the LI and HI groups (Fig. 3a, b).
https://static-content.springer.com/image/art%3A10.1007%2Fs00421-009-1234-z/MediaObjects/421_2009_1234_Fig3_HTML.gif
Fig. 3

Comparison of energy expenditure (a) and substrate oxidation rate (b) during low-intensity (filled square) and high-intensity (open square) exercises and post-exercises recovery period. Triple asterisks denote significantly different between low- and high-intensity exercises (p < 0.001)

Interestingly, during the recovery period, no significant differences were shown between the LI and HI groups, either in fat oxidation (264.5 ± 74.9 vs. 262.1 ± 90.5 kJ and 7.02 ± 1.99 vs. 6.96 ± 2.40 g, p = 0.951, respectively), carbohydrate oxidation (118.4 ± 39.8 vs. 144.0 ± 60.8 kJ and 7.07 ± 2.38 vs. 8.60 ± 3.63 g, p = 0.261) or protein oxidation (47.8 ± 13.3 vs. 49.5 ± 15.7 kJ and 2.86 ± 0.79 vs. 2.95 ± 0.94 g, p = 0.811).

Figure 4 shows the kinetics of fat and carbohydrate oxidation rates during the LI and HI exercises and during the recovery periods. At the onset of the LI exercise, fat oxidation rate increased significantly in 5 min, then increased slowly but not significantly until the end of exercise (mean fat oxidation rate: 0.44 ± 0.05 g/min) (Fig. 4a). During the recovery period, fat oxidation rate fell progressively but did not reach basal values until after 10 min. In addition, carbohydrate oxidation rate increased during the first 10 min of the LI exercise and then achieved the maximal value (0.76 ± 0.13 g/min, Fig. 4c). After that, it decreased significantly, almost linearly, to 0.59 ± 0.12 g/min (p < 0.05) until the end of the exercise. During the recovery period, carbohydrate oxidation rate remained higher than the basal values for the first 5 min, then decreased to the basal values.
https://static-content.springer.com/image/art%3A10.1007%2Fs00421-009-1234-z/MediaObjects/421_2009_1234_Fig4_HTML.gif
Fig. 4

Variations of fat (a, b) and carbohydrate (c, d) oxidation rate during the exercise and the post-exercise recovery period in low-intensity (a, c) and high-intensity (b, d) groups (filled circle). *p < 0.05 when compared to corresponding values of basal metabolic rate (dashed line)

During the HI exercise, fat oxidation rate increased gradually for 20 min, then remained constant for 10 min (mean fat oxidation rate 0.32 ± 0.08 g/min) (Fig. 4b). During the first 10 min of the post-exercise recovery period, fat oxidation rate was significantly higher (on average 0.27 ± 0.06 g/min) than the basal values; then it fell to basal values. Finally, at the onset of the HI exercise, carbohydrate oxidation rate increased for 10 min and achieved the maximal value (2.11 ± 0.32 g/min) (Fig. 4d). Then, it decreased significantly to 1.56 ± 0.35 g/min (p < 0.05) until the end of the exercise. During the recovery period, carbohydrate oxidation rate remained higher than the basal values for the first 5 min, then decreased to the basal values.

Discussion

The results of the present study performed in severely obese adolescents show that (1) fat oxidation rate, during the walking graded exercise test, reached a maximum value at an exercise intensity corresponding to 42 ± 6% V′O2max or 62 ± 5% HRmax and no significant correlations were found between maximal fat oxidation rate and V′O2max, or BM, or FFM, or FM of subjects; (2) for the same amount of total energy expenditure, compared to HI exercise, the LI exercise resulted in 90.9% more fat oxidised and 45.2% less in the amount of carbohydrate oxidised; (3) the intensity of the submaximal equicaloric exercises did not influence energy expenditure, fat or carbohydrate oxidation rate during the post-exercise recovery period.

According to the theory of Flatt (1993), oxidising more fat than is consumed is essential if adipose tissue is to be reduced. Exercise induces a decrease in insulin (Wolfe et al. 1986) and an increase in epinephrine plasma levels (Galster et al. 1981) which are crucial for fat mobilisation during exercise. Increased sympathetic nervous system activity also participates in the increase in fat oxidation (Jeukendrup et al. 1998), and increases in plasma GH and cortisol levels could contribute to fat mobilisation (Moller et al. 1992). Furthermore, the percentage of energy derived from fat oxidation decreases with increasing exercise intensity, whereas the relative contribution of carbohydrate oxidation to total energy expenditure increases (Brooks and Mercier 1994). In the present study, substrate oxidation rates obtained at the 4th min of graded exercise test on a treadmill are not significantly different than those observed throughout the steady state of the same exercise intensities (42 and 67% of V′O2max). This suggests that information obtained during the 4th min graded test can be used to determine changes in substrate oxidation rates as a function of the exercise intensity in obese subjects (Venables and Jeukendrup 2008). As well, maximal fat oxidation rate observed during the graded exercise test on a treadmill was reached at a low intensity (42% V′O2max). This result is in agreement with those of previous studies which considered obese pre-pubertal children walking on a treadmill (Maffeis et al. 2005) and adolescents pedalling on a cycloergometer (Lazzer et al. 2007).

The fat oxidation kinetics during the LI and HI equicaloric exercises (Fig. 4a, b) clearly show that the LI exercise was convenient for maximising fat oxidation rate. In fact, at the onset of the LI exercise, fat oxidation rate increased rapidly and reached a plateau after 5 min. In contrast, the HI exercise induced a slow increase in fat oxidation rate, which reached the maximal value after 20 min. In addition, during the LI exercise, the mean fat oxidation rate was 37.5% higher, and the amount of fat oxidised was 90.9% higher than during the HI equicaloric exercise. In contrast, carbohydrate oxidised was significantly lower during the LI than during the HI exercise (−45.2%, p < 0.001). Since the considerable increase in carbohydrate oxidation could contribute to earlier exhaustion, increased appetite, and compensatory food consumption, walking at LI seems more appropriate than HI exercises for promoting fat mass loss in severely obese adolescents.

Exercise is known to increase energy expenditure and fat oxidation not only during the exercise period, but also during the post-exercise recovery period. Some studies have shown that HI exercises have a more pronounced effect on post-exercise energy expenditure than LI exercises (Smith and Mc Naughton 1993; Phelain et al. 1997), whereas other authors found no differences (Sedlock et al. 1989). These inconsistent results might be explained by the fact that subjects presented different training status or the LI and HI exercises had not been matched for energy expenditure. In our study, the LI and HI equicaloric exercises did not affect significantly energy expenditure during the post-exercise recovery period. Although the post-exercise period considered in our study was relatively short (60 min), our results are in agreement with previous studies which considered the effects of equicaloric LI or HI exercises on 24-h energy expenditure (Melanson et al. 2002; Saris and Schrauwen 2004).

Particularly relevant, for the treatment of obesity, is the effect of exercise intensity on substrate oxidation. We hypothesised that exercise intensity could differentially influence substrate oxidation during the post-exercise recovery period. However, in the present study, fat oxidation rate decreased rapidly during the LI and HI post-exercise recovery periods (Fig. 4a, b) and reached the basal values in about 10 min. Furthermore, the LI and HI exercises did not affect significantly fat oxidation during the 60-min post-exercise recovery period. These results are in agreement with those of previous studies in obese and non-obese subjects (Thompson et al. 1998; Melanson et al. 2002; Saris and Schrauwen 2004) which reported differences, which were not statistically significant, in fat oxidation during 6- or 24-h recovery periods following LI (33–40% V′O2max) and HI (66–80% V′O2max) equicaloric exercises. When equating energy expenditure during exercise at 75 and 50% V′O2max, Phelain et al. (1997) reported that fat oxidation was lower at some, but not at all time points during the 3-h recovery period following an HI than after a LI exercise in active females. Conversely, Treadway and Young (1990) reported that fat oxidation was higher after 45-min exercises at 55 and 75% V′O2max than after exercises at 35% V′O2max in active adults, but these authors did not equate exercises for total energy expenditure. Consequently, if the exercises are not matched for the energy expenditure, the amount of fat oxidised during the post-exercise recovery period may seem to depend on the exercise intensity and its duration.

In conclusion, our data showed that severely obese adolescents exhibited maximal fat oxidation rate during walking at 42% V′O2max (62% HRmax). Total fat oxidised during the LI exercise and the post-exercise recovery period was significantly higher (+9.9 g, +55.7%; p < 0.001) than during and after the HI exercise. However, the intensity of the equicaloric exercises did not influence energy expenditure, and fat and carbohydrate oxidation rate during the post-exercise recovery period. Consequently, for obese adolescents, it is wise to encourage LI physical activity, which favours fat oxidation rate and is more feasible and acceptable than intense exercise. In addition, as obese subjects become adapted to physical activity, a progressive increase in the amount of exercise, through increases in its duration, is beneficial to weight loss (Tremblay et al. 1994) .

Acknowledgments

We are grateful to the adolescents for their kind collaboration and the nursing staff of the Division of Auxology, Italian Institute for Auxology, IRCCS, for their qualified assistance during the clinical study, to Dr. PG. Marinone, Dr. R. Zennaro and Dr S. Marzorati for their help with measurements. We thank Dr M. Vermorel (INRA, France) for his valuable advice in analysing the data and for improving the manuscript, and Dr J. M. H. Buckler for the English revision. The study was supported by Progetti di Ricerca Corrente, Italian Institute for Auxology, Milan, Italy.

Copyright information

© Springer-Verlag 2009