We have previously shown that obese children with prior fracture have a lower bone mass for body size. We further hypothesised that obese children would fail to gain bone mass in relation to body size over a one-year period compared to lean controls.
Methodology: Out of 103 children recruited into obese (BMI>98th percentile) and control (BMI<98th percentile) groups at baseline, 66 children aged 6.518 years returned at one year. Total body and regional bone and fat mass were measured by DXA. Lumbar volumetric density was calculated by Carters method (BMAD). Baseline and one-year height and weight corrected Z-scores for bone parameters were calculated based upon values for the non-obese, non-fracturing controls. Comparison of one-year changes in body size adjusted bone z-scores between obese and control children were determined by analysis of covariance correcting for baseline measurements. Gains in fat and bone mass were evaluated by paired t-tests.
Results: The significant reduction in Z-scores for total body BMC, radial (ultradistal and 33% radius) BMC and BMD and lumbar spine BMC, BA, areal BMD and BMAD in obese children observed at baseline persisted at follow-up. Total body fat mass (g) increased over one year in obese (1902±4345, P=0.01) and control children (1863±2961, P=0.003). Compared to Z-scores derived from control children without a prior fracture calculated at baseline and year 1, obese children failed to make significant gain in bone Z-scores for lumbar BMC (−0.28±1.24, P<0.0001), lumbar BA (−2.75±2.61, P<0.0001), ultradistal radius BMC (−2.42±1.99, P<0.0001) and BMD (−2.60±2.62, P=0.0001) and 33% radius BMC (−0.53±1.11, P=0.001) and BMD (−1.57±1.39, P=0.0005). On dividing groups according to a prior history of fracture, obese children with prior fracture had a significantly lower gain in lumbar BMAD (F-ratio=3.6, P=0.02) over one year.
Summary: Despite significant increases in total body fat mass, obese children failed to achieve adequate gain in regional body size adjusted bone mass and density. This was pronounced at the lumbar spine in obese children with a prior history of fracture. Obesity limits temporal gain in regional bone mass during a critical period of bone accrual. This may have a detrimental effect on achieving peak bone mass during adolescence.