Physical activity and body-composition in children

Nutrition Research Newsletter, August, 2005

The prevalence of childhood obesity continues to increase, and established obesity is difficult to resolve. There is an urgent need to establish modifiable risk factors for obesity during childhood to mitigate the emerging epidemic. It is not clear which environmental factors, in particular which components of physical activity and diet, are the most important contributors to this risk.

While the increase in childhood obesity is frequently attributed to a decline in physical activity (PA), a remarkable lack of consistency exists in the relations between reported levels of PA and degrees of fatness. This inconsistency could be due to methodologic flaws in assessing activity, inactivity, and body composition.

A study was designed to establish the relation between PA and body composition in nonobese children by simultaneously using 2 independent and objective measures of PA, and by using an approach to optimally normalize body-composition variables for body size. Investigators sought to determine in healthy nonobese children, some of whom could be considered preobese on the basis of parental characteristics, whether different associations were apparent between PA, activity energy expenditures (AEE), and sensitive measures of body composition in prepubescence.

Subjects included 100 healthy children (60 boys, 40 girls). They were recruited into 2 groups according to their risk (high or low) of future obesity. High-risk children had >1 biological parent with a BMI >30, and low-risk children had 2 nonobese biological parents (BMI: <30). All measurements took place during the school term and were conducted over a 3-year period. Weight and height of both the subjects and parents were measured and BMI was calculated. Body fat mass in children was measured by isotope dilution during the measurements of energy expenditure by using the doubly labeled water (DLW) method.

Total energy expenditure (TEE) was measured over 10 days by the DLW method. Besides being used to calculate TEE, the intercepts of the isotope disappearance curves were used to provide estimates of total body water. FFM was calculated from total body water by dividing the water content of fat-free tissue with age-and sex-specific values. Predictive equations based on weight, height, sex, and age of the child were used to estimate basal metabolic rate (BMR). The equations, derived from collated DLW energy expenditure data, allow for 4 levels of activity--sedentary, low activity, active, and very active--and for a corresponding activity coefficient in the energy equations. During the DLW study period, patterns of free-living physical activity were also assessed by heart rate (HR) monitoring. The objective was to obtain 7-day HR data. The subjects were asked to wear the monitor at all times, except while swimming or showering.

High-risk children had significantly higher BMI, lean mass index (LM[), and fat mass index (FMI) than did low-risk children, but no group differences in PA were found. AEE and PA level were positively associated with LMI and, after adjustment for sex and FFM, negatively associated with FMI, but not with BMI. Boys who spent more than the median time in light-intensity activities had significantly higher FMI than did less sedentary boys. This difference was not observed in girls.

AEE and PA level were found to be negatively associated with body fat in nonobese children. Accurate measures of body composition are essential to appropriate assessment of relations between PA and obesity risk.

K. Rennie, M. Livinstone, J. Wells, et al. Association of Physical Activity with Body-Composition Indexes in Children Aged 6 years to 8 years at Varied Risk of Obesity. Am J Clin Nutr; 82:13-20 (July, 2005). [Correspondence: K Rennie, Northern Ireland Centre for Diet and Health. University of Ulster, Coleraine BT52 ISA, United Kingdom. E-mail: klr1000@cam.ac.uk]