Body composition measured by dual-energy X-ray absorptiometry half-body scans in obese children

AIM

To perform a methods comparison of a left or right half-body scan versus whole-body scan for measuring body composition in a sample of obese children.

METHODS

A group of obese children (n = 58; ≥ 95th BMI percentile; 8-18 years) were required to undergo a dual-energy X-ray absorptiometry (DXA) body composition measurement as part of an ongoing cohort study; 34 fit within the imaging field of the DXA scanner and were eligible for inclusion in the present analysis. Percent fat, total mass, fat mass, lean mass and bone mineral content (BMC) were estimated from half-body scans and compared with the whole-body results. Assessment was completed using GE enCORE 11.40 software.

RESULTS

In comparing left- and right-side scans to whole-body scans, there was significant correlation for all body composition variables (p ≤ 0.005, R(2) = 0.996-1.0). Bland Altman analyses also showed high levels of agreement between half-body estimates and whole-body measurements.

CONCLUSION

This study supports using a half-body scan methodology for percent fat, total mass, fat mass, lean mass, and BMC as a valid alternative to full-body analysis in obese children and youth.

Variability in adherence to an unsupervised exercise prescription in obese women

OBJECTIVE

To measure adherence to a specific exercise prescription (1500 kcal week(-1)) by objectively quantifying unsupervised exercise energy expenditure (ExEE) in obese women.

DESIGN

The 16-week lifestyle intervention consisted of weekly meetings with research staff and promotion of increased ExEE (1500 kcal week(-1)) and a decreased dietary intake (-500 kcal day(-1)).

PARTICIPANTS

Twenty-nine obese females (body mass index=36.8+/-5.0 kg m(-2), body fat=49.6+/-3.7%) from a hospital-based lifestyle intervention were included in the analysis.

MEASUREMENTS

ExEE was estimated and monitored weekly using heart rate monitoring, and body composition was measured before and after the intervention by dual-energy X-ray absorptiometry.

RESULTS

Free-living adherence to the exercise prescription was variable and, on average, modest such that 14% achieved 1500 kcal week(-1), and the average weekly ExEE (768 kcal week(-1)) represented 51.2% of the total amount prescribed. ExEE was correlated with changes in body weight (r=0.65, P<0.001) and fat mass (r=0.65, P<0.001). Achievement of a 5% weight loss target was dependent on the achievement of an ExEE level of 1000 kcal week(-1) (P<0.001). Exercise 'adherers' (>1000 kcal week(-1)) lost more weight (-9.9 vs -4.1 kg), more fat mass (-6.8 vs -3.0 kg) and more waist circumference (-9.8 vs -5.6 cm) when compared to 'non-adherers' (<1000 kcal week(-1)).

DISCUSSION

Exercise is an integral component of lifestyle interventions aimed at reducing obesity and its complications. However, without accurate and objective measures of ExEE, it is difficult for relationships between exercise and health outcomes to be elucidated. The present study suggests an alternative to self-report to increase the confidence with which conclusions are drawn regarding the role of exercise within lifestyle interventions.

Implications of the variability in time to isotopic equilibrium in the deuterium dilution technique

OBJECTIVE

To investigate the variability in isotopic equilibrium time under field conditions, and the impact of this variability on estimates of total body water (TBW) and body composition.

DESIGN AND SETTING

Following collection of a fasting baseline urine sample, 10 women and 10 men were dosed with deuterium oxide (0.05 g/kg body weight). Urine samples were collected every hour for 8 h. The samples were analysed using isotope ratio mass spectrometry. Time to equilibration was determined using three commonly employed data analysis approaches.

RESULTS

Isotopic equilibrium was reached by 50, 80 and 100% of participants at 4, 6 and 8 h, respectively. The mean group equilibration determined using the three different plateau determination methods were 4.8+/-1.5, 3.8+/-0.8 and 4.9+/-1.4 h. Isotopic enrichment, TBW, and percent body fat estimates differed between early (3-5 h), but not later sampling times (5-8 h).

CONCLUSION

Although the three different plateau determination approaches resulted in differences in equilibration time, all suggest that sampling at 6 h or later will decrease the likelihood of error in body composition estimates resultant from incomplete isotopic equilibration in a small proportion of individuals.