^aDepartment of Physical Education, College of Physical Education, Dong-A University, Korea

^bDepartment of Health and Physical Education, The University of Tokyo, Japan

　

The purpose of this study was to examine the effects of aerobic exercise on visceral fat and cardiac function. Furthermore we investigated the relationship between fat distribution and LV structure and function in obese adolescents. The subjects were randomly assigned to either the aerobic exercise training group (n=7) or served as controls (n=7). Aerobic training subjects exercised at 50-60% HRmax (1-4 weeks), 60-70% HRmax (5-24 weeks), for 60 minutes per day (duration), and 6 days per week (frequency). Abdominal fat volume measures consisted of subcutaneous fat volume (SFV), visceral fat volume (VFV), and VFV/SFV by CT (computed tomography). Subjects also underwent M-mode and doppler echocardiography to assess left ventricular size, mass and function. Paired t-tests were used to evaluate the difference in variables of interest at baseline and following 24 weeks of aerobic exercise. The 0.05 level of significance was the critical level for this study. Weight and body fat percentage (BF%) significantly decreased and lean body mass significantly increased in the aerobic exercise group. However, weight and BF% increased and lean body mass did not change in the control group. Abdominal fat tended to increase in the control group, while it significantly decreased in the aerobic exercise group. Insulin concentration was significantly decreased, but did not change in the control group. Most variables with regard to LV function were unchanged in the control group. However, whereas LV mass was not changed, FS and EF were significantly increased, and VO₂max (ml/kg/min) was significantly increased in the aerobic exercise group. In addition, the 24-week aerobic exercise subjects reduced both visceral and subcutaneous fat. However, although the cardiac function of obese adolescents was also significantly improved, there was no change in LV mass.

　

Key Words : Aerobic Exercise, Visceral Fat, Subcutaneous Fat, Left Ventricular Function, Adolescent

　

●Correspondence address : Department of Physical Education, Dong-A University. Sang-kab PARK, Hadan-dong, Saha-ku, Busan, Korea, TEL 82-51-200-7843　FAX 82-51-202-4808

　

Adolescent obesity is strongly associated with adult obesity and may lead to type 2 diabetes mellitus, hypertension, or stroke (Daniels, 2001). Excessive overweight in adolescent is associated with higher morbidity and mortality than would have been expected by chance in adult life (Carl-Erik Flodmark, 1998). Positive treatment of obesity has important implications since obesity also has influence on self-confidence, exercise performance, mental, emotional and social development as well as the risk of cardiovascular disease (Strauss, 1999). Studies have included cross-sectional observations of athletes and other subjects who are characterized as trained or untrained, active or inactive, and sedentary, or engaged in short-term training protocols. Cross-sectional designs are limited in that subject selection may influence results, and designations of activity status do not ordinarily include a quantified estimate of physical activity.

Furthermore, cardiac dimensions are associated with age and body surface area (BSA), and these relationships are especially important in children and adolescents (Henry et al., 1980; Pelliccia, 1996). The effects of physical activity or training on cardiac dimensions during childhood and adolescence appear to be equivocal. Although the results of some studies indicate that cardiac dimensions are greater in young athletes than in non-athletes (Allen et al., 1977; Medved et al., 1986), others have not found such differences (Rowland et al., 1994; Telford et al., 1988). The results of short-term (2-8 months) endurance training programs have demonstrated either increases (Geenan et al., 1982) or no significant changes (Ricci et al., 1982) in cardiac dimensions in younger subjects. It has also been found that when age and/or body size are considered in the analyses of the relationship between cardiac dimensions and aerobic capacity, cardiac dimensions were of minimal importance in determining aerobic capacity in children and adolescents (Blimkie et al., 1980; Janz et al., 1996). Already in childhood, unfavorable levels of LV structure and function are associated with excess adiposity (Gutin et al., 1998). There is some evidence that central fat deposition may be especially deleterious (Daniels et al., 1999; Mensah et al., 1999). However, these studies used methods that could not distinguish subcutaneous adipose tissue from visceral adipose tissue (VAT), which is especially likely to be correlated with various CV disease risk factors in both adults (Despres, 1988) and juveniles (Owens et al., 1993, 2000). Also, little is known about the relationship of VAT to LV structure and function. Therefore, this study examines the effects of aerobic exercise training on visceral fat and cardiac function as well as the relationship of fat distribution to LV structure and function in obese adolescents.

　

The subjects consisted of 14 obese girls between 15 and 16 years of age and had a body composition consisting of >40% body fat. They were recruited via flyers sent to parents of children who attended area schools in Busan, Korea. A complete medical history evaluation was obtained for each subject. Exclusion criteria consisted of: medications known to affect body composition or physical activity, diagnosis with syndromes known to affect body composition and/or fat distribution, or any major illness since birth. The subjects were randomly assigned to the control group (n=7) or the aerobic training group (n=7). Parental informed consent and the subject's and their parent's assent were obtained before the study.

　

All anthropometric measurements, physical examinations, and blood sampling were performed before and after the 24-week aerobic exercise program. Standing height was measured using STDK-AD (Shintokyo Denshikizai, Japan). Weight was measured without shoes or coats using a calibrated electronic scale. Body mass index (BMI), defined as weight in kilograms divided by the square of height in meters (kg/m²) was calculated. Body fat percentage (BF%) and lean body mass (LBM) were measured by Inbody 3.0 (Biospace, Korea). Resting blood pressure (BP) was recorded in the sitting position obtained from the right arm by auscultation using a mercury sphygmomanometer.

　

Exercise capacity was measured both at the start and at the end of the training program. All participants were familiarized with the exercise testing protocol by having a preliminary exercise test with respiratory gas exchange measurement performed one to three days before the baseline exercise test. All participants performed a maximum treadmill exercise test according to the modified Bruce protocol. Maximal indications for stopping were those recommended by the American College of Sports Medicine (2000) and consisted of failure of heart rate to increase with further increases in exercise intensity and a rating of perceived exertion of more than 17 (6 -20 scale). Participants breathed through a mask with a turbine volume transducer that measured the volume of inspired and expired air. Expired gases were withdrawn through the mask for determination of VO₂ and VCO₂ and were analyzed breath-by-breath method (Quinton Metabolic Cart 4500, U.S.A). The gas analyzers as well as the volume transducer were calibrated before each test. Maximal oxygen consumption (VO₂max) was defined as the mean VO₂ of the last minute of the exercise test.

　

The aerobic exercise training program using the treadmill (Intertrack 6000, Taeha, Korea) and the bicycle ergometer (Interbike, Taeha, Korea) involved sixty minutes of exercise per day for six days per week for a period of 24 weeks at 50-60% HRmax for 1- 4 weeks and at 60-70% HRmax for 5 -24 weeks. Exercise was maintained at target heart rate intensity (THR= heart rate reserve × work intensity (%) + rest heart rate) by continuously monitoring heart rate with the Polar Analyzer (Polar Electro Oy, Finland). Subjects in the control group were specifically asked not to participate in any aerobic exercise training program.

　

In each subject, the computed tomogram taken immediately cranial to the iliac was chosen for further analysis. Tomography performed at this level usually traverses the body of the fourth lumbar vertebra and is proximal to the umbilicus in most subjects. By means of describing regions of interest (ROI) with a light pen cursor and assessing the number of pixels within the fat density range (-250 to -50 Hounsfield number) the cross-sectional areas of both visceral fat and subcutaneous fat were calculated.

　

All subjects underwent a standard echocardiographic examination (HDI 5000 System, Phillips, Netherlands). Two-dimensional echocardiography was performed at baseline and following 24 weeks of exercise training or the control condition. Echocardiograms were performed by experienced technician and repeated by the same technician, with care taken to obtain similar serial images. The images were videotaped at the end of the expiration phase of normal respiration and were analyzed by a cardiologist who was blinded regarding clinical data. A standard protocol was used based on apical four-and two-chamber views according to the recommendations of the American Society of Echocardiography. The following variables were measured or derived: Ejection fraction (EF) was calculated as the quotient (SV/LVDV) × 100. Fractional shortening (FS) was calculated as the quotient (LVDD-LVSD) × 100/LVDD. LVM was calculated with a formula that has been validated for children with normal hearts: LVM (g) = 0.80 [1.04 × [(IVSD + LVIDD + LVPWD) ³ - (LVEDD) ³] + 0.06] , where IVSD = intraventricular septal dimension, LVIDD = LV internal dimension during diastole, and LVPWD = LV posterior wall dimension.

　

Blood was drawn from the deep arm vein at baseline and following the 24 week protocol. After the subjects fasted for a minimum of 12 hours, blood samples of approximately 10ml were collected in the early morning by venipuncture from the antecubital vein. The samples were then centrifuged to obtain the serum, which was stored at 4 ℃. All analyses were completed within 48 hours of the collection of the blood samples. The concentration of insulin in the serum was measured using Radioimmunoassay.

　

Statistical analysis was performed using the SPSS statistical software (SPSS, Korea). Data were presented as means and standard deviations (SD). Paired t-tests were used to evaluate the difference between the pre and post-training time points. Correlation coefficients were obtained by Pearson correlation coefficients. Statistical significance was accepted as p<0.05.