Fat and bone in children: differential effects of obesity on bone size and mass according to fracture history.

Fat mass predicts bone accrual in prepubertal children, but obese children have increased fracture risk. We hypothesised that bone size and mass would vary according to prior fracture in obese children. One hundred and three children (52 obese) underwent dual-energy X-ray absorptiometry (DXA) scanning of the lumbar spine, total body, and radial metaphysis and diaphysis. We derived body size-adjusted bone mineral density (BMD) estimates for each site using commonly employed procedures. Following adjustment for either age, age(2) and weight, or height and weight based on a reference group of nonobese controls without previous fracture, obese children with prior fracture showed a 0.8 to 1.2 SD reduction in total body areal BMD (aBMD), a 3.0 SD decrease in lumbar (L2-4) aBMD, and a 2.0 SD reduction in radial shaft aBMD. These changes were significant at p < .005. Lumbar volumetric BMD (vBMD) calculated by Carter and Kröger algorithms was significantly reduced in obese children with prior fracture (2.0 to 3.3 SD). Eighteen percent of obese children fulfilled the criteria for osteoporosis. Despite greater lean mass for height in obese children (p < .0001), total body bone mineral content (BMC) for lean mass was reduced (p = .002). Multiple regression models adjusting for height, weight, and gender demonstrated an inverse relationship between total body fat mass and total body, lumbar, and ultradistal radius BMC and aBMD. The data suggest that fat mass substantially inhibits bone accrual in children with prior fracture. These children may require targeted interventions to increase bone mass during adolescence to achieve optimal peak bone mass and reduce the risk of osteoporosis later in life.