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To determine the effects of physical training intensity on the cardiovascular fitness, percentage of body fat, and visceral adipose tissue of obese adolescents.

 

The primary hypothesis of this study was that, when compared with obese adolescents given lifestyle education (LSE) alone, obese adolescents who participated in physical training would show favorable changes in cardiovascular fitness, %BF and VAT.  A secondary hypothesis was that high-intensity physical training would be more effective than moderate-intensity physical training.

Subjects needed to have a triceps skinfold thickness greater than 85^th percentile for sex, ethnicity, and age; to not be involved in any other weight control or exercise program and to not be restricted as to physical activity.

Youths underwent baseline testing and were randomly assigned, within sex and ethnicity groups, to 1 of 3 experimental groups. It was emphasized that, although only 2 of the 3 groups would participate in controlled physical activity at our facility, all groups would receive potentially valuable diet and physical activity information and that we were unsure which type of intervention would be most effective. One group was assigned to engage in biweekly lifestyle education (LSE) classes alone, the second group was assigned to LSE + moderate-intensity physical training, and the third group was assigned to LSE + high-intensity physical training. 

The entire project was carried out in 2 waves, such that one-half of the subjects went through the project during its first year (cohort 1) and one-half during the second year (cohort 2); this allowed us to have class sizes small enough to carefully supervise the physical training.  Most youths were transported to and from the physical training and LSE classes by our vans.  Some youths lived outside the geographic area covered by our vans and agreed to provide their own transportation to participate in the study.

Full testing sessions were conducted at baseline and after 8 mo of experimental period.  Minitests of anthropometric indexes and free-living diet and physical activity were also conducted midway between the baseline and 8-mo test sessions.  The youths were remunerated and encouraged to return for test sessions regardless of their degree of participation in the interventions so that they could be included in effectiveness (i.e., intention-to-treat) analyses.

Physical Activity Interventions

The physical training was offered 5 days per week, except during the weeks when the group was scheduled for LSE on 1 day.  Largely on the basis of our experiences in an earlier physical training study in which obese 7-11-y-olds expended an average of 971 kJ  (232 kcal)/session during 4 mo of physical training (22), we decided hold estimated EE constant at »1045 kJ (250 kcal)/session regardless of physical training group assignment.  An exercise prescription was then developed for each subject on the basis of the data collected from the baseline treadmill test.  First, the peak VO₂ achieved by each youth on the treadmill test was identified, and the EE associated with 55-60% of peak VO₂ (moderate-intensity physical training) or 75-80% of peak VO₂ (high-intensity physical training) was determined.  Because the high-intensity physical training group used more energy per minute, they were scheduled to exercise for fewer minutes per session than the moderate-intensity group.  The number of minutes of exercise needed to expend 1045 kJ was estimated for each subject.  Then, the heart rate and EE associated with that VO₂ were determined for each youth.

During every physical training session, each youth wore a heart rate monitor (Polar Vantage XL; Polar).  After each session, the minute-by-minute heart rate were downloaded into a computer and displayed to the youth.  The youths were encouraged to maintain their heart rates within 5 beats/min of their target heart rates.

Each exercise session provided some flexibility for the youths to select activities of their preference.  Activities included exercise on machines (e.g., treadmills, cycles, rowers, and steppers), aerobics, basketball, badminton, kickball, and aerobic slide.  The instructors helped the youths to modify the exercise intensities to stay within their prescribed target heart rate zones.  As an incentive, each youth was awarded points for maintenance of target heart rates that were redeemed for prizes.  To encourage attendance, each subject was paid $1 for each physical training class attended.

Estimates of the achieved physical training EE were calculated separately for the first and second 4-mo components of the intervention.  To estimate physical training EE during the first 4 mo of physical training, the average physical training heart rate for the first 4 mo was entered into the regression equation relating EE to heart rate derived from the pre-intervention treadmill test; the EE obtained was multiplied by the number of minutes that each youth actually exercised per session to derive the estimated physical training EE per session for the first 4 mo.  This procedure was then repeated for the second 4-mo period with the use of the regression equation obtained on the post-intervention treadmill test; the average of the first and second 4-mo periods of physical training was then calculated to derive an estimate of the physical training EE over the entire 8-mo period of physical training.

Life Style Education

The 1-h LSE sessions were offered once every 2 wk for the 8 mo of the intervention; youths were paid $5 for each LSE class attended.  Subjects from the 2 physical training groups were combined and the subjects in the LSE alone group were seen in separate sessions in alternating weeks to minimize contamination across the physical training and no physical training groups.  The LSE included principles of learning and behavior modification, information about nutrition and physical activity, discussions of various aspects of the food consumption process, psychosocial factors related to obesity, and problem solving and coping skills.  The LSE sessions were taught by a licensed clinical psychologist (CRL) who specializes in the treatment of eating disorders and obesity and has experience in providing LSE to children, adolescents and adults.

Body Composition

Total body composition was measured by dual-energy X-ray absorptiometry (DXA) (QDR-1000, software version 6.0; Hologic, Inc., Waltham, MA) which segments the body into the 3 compartments of fat mass, bone mineral content, and fat-free soft tissue, the last 2 of which constitute fat-free mass. %BF and bone density are also derived.  Svendsen et al (13) showed that DXA values agree well with carcass analysis of pigs.  We have found DXA to be reliable in children (14) and to be sensitive to changes elicited by physical training.  The whole-body scan time was 10-12 min and the radiation dose of »15 mSv (»1.5 mRem) was about the amount received in a cross-country airplane flight; therefore, our Human Assurance Committee approved the use of DXA in these healthy adolescents.  Because nothing is known about the possible effects of even this small amount of radiation on a developing fetus, a urine sample was obtained from the girls to rule out pregnancy before the DXA measurement.

Visceral Adipose Tissue and Subcutaneous Abdominal Adipose Tissue

VAT and subcutaneous abdominal adipose tissue (SAAT) were determine in the Dept. of Radiology at the Medical College of Georgia with the use of a 1.5-T magnetic resonance imaging system (General Electric Medical Systems, Milwaukee) according to procedures previously described.  Spin-echo techniques were used to produce T1-weighted images showing good contrast between adipose and non-adipose tissues.  Details of the magnetic resonance imaging acquisition were as follows; repetition time, 450 ms; echo time, 12 ms; field of view, 400-480 mm; matrix, 192 x 256; and number of excitations.  1. Respiratory compensation was used to reduce artifacts caused by respiratory motion.  With subjects in the supine position a series of five 1-cm thick transverse images was acquired beginning at the inferior border of the fifth lumbar vertebra and proceeding toward the head.  A 2-mm gap between images was used to prevent crosstalk.  Tissues superior to and inferior to the 5 slices were saturated to prevent blood flow in the aorta or inferior vena cava from appearing as high-intensity artifacts in the images.  VAT and SAAT were quantified as adipose tissue within a region of interest bounded by the innermost aspect of the abdominal and oblique muscle walls and the posterior aspect of the vertebral body.

The multislice measurements are reported in the cm³, they were derived by multiplying the surface area for the individual images by the image width (1 cm) and then summing across the 5 images.  To reduce interbserver variability, all images were analyzed by the same experienced observer.  The intraclass correlation coefficients for VAT and SAAT from separate-day repeat analyses of the same scans exceeded 0.99.  Because some adults experience claustrophobia when placed in the magnetic resonance imaging system, the youths viewed a brief videotape of the magnetic resonance imaging procedure at the time of the scheduling test sessions.  The youths were then allowed to decline the magnetic resonance imaging measurement if they wished.  None declined being scanned at baseline and 2 declined the scan at the post-intervention assessment.

80 obese 13-16 year-olds.

The prescribed intensities and durations of exercise, the heart rates actually attained during the physical training, the corresponding estimates of energy expended during the physical training, and the attendance at the physical training sessions are shown in Table 3. The average attendance values of the 2 physical training groups were not significantly different.

Prescribed versus Attained Heart Rate

For the moderate-intensity group, the prescribed and attained heart rates were not significantly different (137 and 138 beats/min, respectively).  However, researchers had difficulty in keeping those assigned to the high-intensity group in their target zones, and the actual mean heart rate achieved (154 beats/min) was significantly lower than the prescribed mean heart rate (167 beats/min; P<0.001).  Nevertheless, the attained mean heart rate of the high-intensity group was significantly higher than that of the moderate-intensity group, whereas the estimated EES did not differ significantly.  Because the intensity distinction between groups was not as great as planned, our ability to test the physical training intensity hypothesis was correspondingly weakened.

Change in VO₂

The change in VO₂ – 170 from baseline to 8 mo in the 3 groups is shown in Figure 1.  We obtained a VO₂-170 value for only 57 of 61 subjects who returned for post-intervention testing because of technical problems. The mean changes in VO₂-170 for the 3 groups were significantly different (P<0.001).  The post hoc tests showed that

• the LSE + high-intensity physical training group improved significantly more than did the LSE alone group
• the moderate-intensity group was not significantly different from either of the other groups. 

Only 38 subjects achieved the physiologic criteria for VO₂ max on both the baseline and post-intervention tests, and the ANOVA did not indicate a significant difference between groups.

The intention-to-treat analyses of %BF and VAT showed up significant differences between the 3 groups (data not shown).  Thus, we proceeded to conduct efficacy analyses by using only subjects who met the criteria for exposure to the physical training.  First we tested the primary hypothesis that those youths who participated in LSE + physical training would show more favorable change in cardiovascular fitness and body composition than would those who engaged in LSE alone.  Thus, we formed a group comprising those in both physical training groups who attended >= 2 d/wk in both the first and second 4-mo periods of the intervention and compared this group with the LSE alone subjects who attended >= 40% of the LSE sessions.  The hypothesis was supported for the 3 primary outcome variables.

The LSE + physical training group improved more than did the LSE alone group in both VO₂-170 and VO₂max, regardless of whether these variables were expressed per unit of weight.  The P values for VO₂max were somewhat higher than those for VO₂-170, perhaps partly because fewer subjects were available for the VO₂max comparisons and the mean differences between groups were somewhat less than for VO₂-170.

VAT and SAAT

VAT declined significantly more in the LSE + physical training group.  Although the group difference in SAAT was in the hypothesized direction, it was not significant.  %BF in the LSE + physical training group declined, whereas it increased slightly in the LSE alone group; this group difference was significant.

Total Body Composition

The group difference in fat mass was nearly significant (P = 0.072); the changes in fat-free soft tissue were not significantly different between groups.  The increases in bone mineral content and bone density were significantly greater in the LSE + physical training group.  With respect to diet, total energy intake increased significantly more in the LSE + physical training group, whereas none of the group differences in changes in intakes of the individual macronutrients were significant.

To test the hypothesis that the high-intensity physical training would be more efficacious than the moderate-intensity physical training, we initially set 2 criteria for adequate exposure to the specific does of physical training: attendance of >= 40% and heart rate within 10 beats/min of the prescribed moderate-or high-intensity heart rate zones.  However, because of the difficulty we had in keeping subjects in their target zones, the number of subjects remaining in the physical training groups fell to 9 and 8, compromising our ability to test the intensity hypothesis.  No evidence was provided that the high-intensity physical training was more efficacious than the moderate-intensity physical training in enhancing cardiovascular fitness or body composition (data no shown).

In summary, obese adolescents who participated in an afternoon LSE + physical training program with > 40% attendance improved in cardiovascular fitness and declined in total body and visceral adiposity to a greater degree than did a group who engaged in the LSE alone.

The high-intensity physical training, but not the moderate-intensity physical training, elicited a significantly greater improvement in cardiovascular fitness than the LSE alone, whereas we found no evidence that the moderate- and high-intensity physical training differed in their effects on body composition. 

The correlational analyses suggested that youths who engaged in more free-living vigorous physical activity had lower baseline %BF and that those who experienced greater doses of physical training during the intervention declined more in %BF.  Taken together, these results suggest that for enhancement of cardiovascular fitness it is reasonable to advise obese youths to exercise as vigorously as they can sustain.  However, improvements in body composition can apparently be obtained by both moderate and vigorous exercise, with no clear effect of intensity.

Government:

NHLBI