Body composition in normal weight, overweight and obese children: matched case-control analyses of total and regional tissue masses, and body composition trends in relation to relative weight.

BACKGROUND: Childhood obesity is defined on the basis of weight and height, using body mass index (BMI). There is little detailed information on the body composition characteristic of overweight and obesity.
OBJECTIVE: To evaluate total and regional body composition in overweight, obese and control children aged 7-14 years.
DESIGN: Body composition was measured by the four-component model and dual X-ray absorptiometry in 38 age- and sex-matched pairs of obese and control children. Body composition trends were also evaluated by quintile of BMI standard deviation score (SDS) in these and 31 other children (n=107; BMI SDS range -1.0 to 4.3).
RESULTS: Obese children were taller than controls (Delta=0.6 SDS; P=0.01) and had greater hydration of fat-free mass (FFM) (Delta=1.8 %, P<0.0001). After adjusting for these variables, obese children had greater FFM, fat mass (FM) and mineral (P<0.0001). Regional analyses showed that these differences were apparent in the arm, leg and trunk, but the three tissues had different proportional distributions of the excess. Fat was primarily in the trunk, but mineral in the leg. FM, FFM, hydration and mineral mass all increased across BMI SDS quintiles (P<0.0001), but the trend for FM was much the steepest. DISCUSSION: The greater weight of obese children is due to excess FFM including mineral as well as excess fatness. Increasing weight has a strong continuous relationship with increasing FM across the whole spectrum of weight.