Effect Of Exercise On Abdominal Visceral Fat And Body Composition

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Body Composition

Published data on the effect of exercise training intensity on body composition and regional body fat are mixed.[4,15,17,27,36,41] With regard to total body fat loss, total caloric expenditure seems to be the key factor.[4,15,17,37] Slentz et al.[37] reported that low-amount/moderate-intensity and low-amount/vigorous-intensity endurance training (i.e., activity equivalent to ~12 miles·wk-1 of walking or jogging) were equally effective in reducing percent body fat, fat mass, waist circumference, and abdominal circumference in previously sedentary, overweight, middle-aged adults. They also reported that high-amount/vigorous-intensity endurance training (activity equivalent to ~20 miles·wk-1 of jogging) was more effective in reducing percent body fat and fat mass compared with the two low-amount training groups.[37] Although the exercise intensity was not equated across training volumes, the authors did effectively demonstrate a dose-response relationship between training volume and amount of weight change using a pooled analysis.[37]

Our results suggest that HIET may be an effective stimulus for inducing favorable changes in body composition. Specifically, HIET significantly reduced body weight, BMI, percent body fat, fat mass, and waist circumference ( Table 1 ). Our results are consistent with those of Tremblay et al.[42] who reported that high-intensity intermittent exercise training induced greater subcutaneous fat loss compared with moderate-intensity exercise training under isocaloric training conditions. Similarly, Tremblay et al.[41] also reported results from the Canadian Fitness Survey that indicated that vigorous physical activity was associated with lower subcutaneous skinfold thickness, which continued to remain significant after adjusting for total energy expenditure. It should be realized that HIET was likely associated with slightly greater exercise energy expenditure and total energy expenditure than LIET. The kilocalorie pertraining session was based on total energy expenditure (e.g., 300, 350, or 400 kcal per session), and the resting metabolism was part of the total. Therefore, on the high-intensity exercise days where duration was ~6 min shorter ( Table 4 ), resting metabolism would contribute to a lower fraction of the total energy expenditure. This resulted in an approximately 400-kcal difference in exercise energy expenditure between the high- and the low-intensity groups over the 16-wk time frame (~25 kcal·wk-1). In addition, it is likely that postexercise oxygen consumption was higher on the HIET days.

In view of the previous work and the present findings, an interaction between exercise intensity and training volume may exist with respect to changes in body composition. Further investigations are warranted to examine the interaction between training volume and intensity on changes in body composition.

Regional Body Fat

Exercise training, even in the absence of weight loss, is associated with a significant reduction in AVF.[27] Whether intensity of exercise is an important training variable for inducing reductions in AVF is not clear, although data on responses to acute exercise suggest that higher-intensity exercise may be more effective than low- to moderate-intensity exercise for mobilizing AVF by inducing secretion of lipolytic hormones, facilitating greater postexercise energy expenditure and fat oxidation, and by favoring a greater negative energy balance.[21,32,33] Our results indicate that HIET is an effective exercise abdominal subcutaneous fat (-47 vs -11cm2, adjusted for baseline) and AVF (-24 vs -7 cm2, adjusted for baseline). Data from Slentz et al.,[36] however, suggest that low-amount/moderate-intensity orlow-amount/vigorous-intensity exercise training was equally effective in preventing significant increases in AVF associated with continued physical inactivity in sedentary, overweight, middle-aged adults under isocaloric conditions. These authors also reported a significant reduction in AVF in subjects who completed 8 months of high-amount/vigorous-intensity exercise training (activity equivalent to ~20 miles·wk-1 of jogging), indicating that training volume may play a critical factor in exercise-induced AVF loss.[36] However, by not including a high-amount/moderate-intensity exercise training group, the authors eliminated the opportunity to determine whether an interaction between training volume and training intensity exists for AVF loss. The training volume in the present study was equated across training conditions and was similar to the training volume in the high-amount/vigorous-intensity training condition reported by Slentz et al..[36] Taken together, these data suggest that an interaction between training volume and training intensity may exist for AVF loss.

BMR, Physical Activity, and Diet

Reported total physical activity and BMR remained unchanged. Unfortunately, due to incomplete dietary data, we were unable to adequately analyze changes in caloric intake and composition. Although several studies suggest that some women gain weight (and body fat) in response to exercise training, most of these studies have used low- to moderate-intensity exercises.[8,11] The present data indicate that exercise training above the LT (i.e., HIET) may be an effective exercise intensity for inducing weight loss in obese women. Although not measured, it is also likely that HIET resulted in increased postexercise energy expenditure, which in turn was related to lower body fat deposition.[44]

Exercise Adherence

The present results demonstrate that endurance training intensity does not significantly impact exercise adherence. The primary reasons given for missing exercise sessions in the present cohort were related to time conflicts and personal travel. As the mean (and median) exercise adherence was ~80%, 4 d of structured endurance training (at ~1600 kcal·wk-1) seems to be a more realistic goal in this cohort of obese women with the metabolic syndrome. All participants were encouraged to make up their missed training sessions when possible, and the participants in the HIET condition were encouraged to complete all three HIET sessions per week. Four days per week (at ~1600 kcal·wk-1) of endurance training would still remain within the current recommendations.[19] It is also important to realize that the HIET was a blend of LIET (2 d·wk-1) and HIET (3 d·wk-1) and that participants were allowed to initially complete the HIET sessions in an "interval/intermittent"-type fashion. For example, for the first few laps of each training session, some subjects would perform one lap at an RPE of 16-17 and the subsequent lap at an RPE of 13-14, with the majority of the laps performed at an RPE ? 15. Moreover, the overall mean RPE for each HIET session was ?15. The present results also demonstrate that even very sedentary, unfit, obese women (people) can adhere to a supervised program incorporating HIET.

Cardiorespiratory Fitness

Epidemiological data indicate that elevations in cardiorespiratory fitness (i.e., V?O2peak) are associated with an attenuation cardiometabolic risk among individuals with the metabolic syndrome.[25] It is well established that endurance training intensity is a primary determinant for exercise-induced improvements in cardiorespiratory fitness.[3] HIET increased V?O2peak more than LIET did (~14% vs ~9%), and this difference approached statistical significance after 16 wk ( Table 1 ). It is possible that larger-intensity-related differences in V?O2peak enhancement may take longer than 4 months in previously sedentary adults. We previously reported that training-induced elevations in V?O2peak and V?O2 at the LT in response to training at or above the LT were similar across the first 4 months of training in previously sedentary women.[43] However, training above the LT was more effective than training at the LT beyond 4 months.[43]

Metabolic Syndrome Parameters

Despite significant improvements in body composition, including significant reductions in waist circumference and AVF, within the HIET condition, the improvement in some cardiometabolic risk factors associated with the metabolic syndrome (e.g., increased HDL-C, decreased TG) did not reach statistical significance and did not seem to be related to exercise training intensity. However, the present study was powered for changes in body composition and not for cardiometabolic risk factors. As expected, exercise training induced reductions in resting blood pressure. Typically, training-induced reductions in resting blood pressure are reported to be independent of training intensity.[13] The significantly greater reduction in systolic blood pressure after LIET may have been due in part to the higher initial value, because baseline blood pressure seems to be an important factor in the blood pressure response to exercise.[13] However, covarying for the baseline systolic blood pressure did not attenuate the effect of LIET on systolic blood pressure.

Spearman Correlation Analyses

Weight loss was associated with reductions in triglycerides and systolic blood pressure, whereas fat loss was only associated with reductions in triglycerides. Although not statistically significant, AVF loss was associated with the expected reductions in triglycerides. The nonsignificant associations between AVF loss and metabolic syndrome components (i.e., fasting glucose, HDL-C, triglycerides, and systolic and diastolic blood pressures) indicate that it might take a greater AVF loss in our abdominally obese cohort to observe significant improvements in these parameters. For example, recent data from Thamer et al.[39] indicated that subjects with high AVF and high liver fat have a reduced chance in profiting from lifestyle intervention and suggested that they may require intensified lifestyle intervention strategies and/or pharmacological approaches to improve the metabolic profile. Moreover, it has been previously reported that individuals with excessive AVF (e.g., >130 cm2) often develop these cardiometabolic risk factors.[9,10] Because the mean baseline AVF cross-sectional area for each exercise condition was substantially elevated (>153 cm2) and, although reduced as a result of training, was still well above 130 cm2 (>146 cm2), it is possible that greater reductions in AVF may be required to observe favorable changes in these metabolic syndrome parameters.

Limitations

We recognize that a potential limitation of the present study is that the subjects in the HIET group tended to have slightly higher levels of AVF at the onset of the study. However, adjusting for baseline levels of AVF did not significantly attenuate the impact that HIET had on AVF. It has been previously reported that the use of single-slice images to measure changes in AVF is less precise than multislice images[40] and therefore may also be a limitation of the present study. However, a more precise measurement of the change in AVF likely would have resulted in narrower 95% confidence intervals and significant between-group differences with respect to the change in AVF. Although the present study was initially powered to detect significant changes in the AVF (~30 cm2) with 12 participants per group, the present study did not achieve this level of recruitment because the number of dropouts exceeded our original estimation. However, despite this limitation, we did observe a statistically significant improvement in body composition (including AVF) within the HIET condition. Finally, due to incomplete dietary data, we were not able to adequately analyze the impact of reduced caloric intake on the changes in body composition. It has also been suggested that the use of RPE for exercise prescription may be a limitation. For example, when subjects know they are supposed to exercise at an RPE of 15-17 but they do not want to exercise that vigorously, this is a circumstance that may be prone to inflating a given RPE. However, the training program used did result in differentiated training effects for V?O2peak and peak treadmill velocity. Finally, due to the issues related to statistical power, it is possible that some variables would have reached the level of statistical significance if more subjects had been studied. Thus, the nonsignificant results presented need to be interpreted with caution.

Summary

The results of the present investigation support our primary hypothesis that HIET would be more effective than LIET for altering body composition in obese women with the metabolic syndrome. Further investigations are warranted to determine the impact of training duration, gender, race, age, and menopausal status on modulating theeffect that exercise training intensity has on AVF and associated cardiometabolic risk factors.


Submitted by DMorgan on Sun, 01/18/2009 - 9:06pm.