Bone mineral density of total body, spine, and femoral neck in children and young adults: a cross-sectional and longitudinal study.

Bone mineral density (BMD) of total body (TBMD), lumbar spine (L2-4), and femoral neck was measured in 266 normal subjects (136 males) aged 4-27 years (mean 13 years) using dual-energy x-ray absorptiometry (DXA). BMD of all sites increased significantly with age until 17.5 years in males and 15.8 years in females, except for femoral neck BMD in females, which peaked at age 14.1 years. Males had higher peak TBMD, which was attributed to greater weight and lean tissue mass. In contrast, despite a later timing, peak L2-4 BMD in males was not different from that in females. Before peak BMD, weight was the best predictor of TBMD and L2-4 BMD in both sexes (r2 ranged from 0.77 to 0.88), whereas femoral neck BMD was predicted equally by height and weight. Longitudinal information collected from 53 (25 boys) of these children, aged 4-16.9 years, showed that the average annualized gain in TBMD was 0.047 g/cm2 for boys and 0.039 g/cm2 for girls. No significant difference in the association between age and BMD (slopes) was found between cross-sectional and longitudinal data for either sex. We conclude that the timing for peak BMD was consistent for total body, lumbar spine, and femoral neck for each sex. The earlier peak BMD in females is most likely related to earlier puberty. The cross-sectional normative data of this study are useful in serving as a standard for serial assessment in health and disease states.

Body-composition assessment by dual-energy x-ray absorptiometry in subjects aged 4-26 y.

This cross-sectional study describes the body composition of 265 normal subjects (137 males and 128 females) aged 4-26 y determined by dual-energy x-ray absorptiometry (DXA). Lean tissue mass (LTM) and bone mineral content (BMC) increased with age in females until 13.4 and 15.7 y, respectively, and in males until 16.6 and 17.4 y, respectively. A strong relation between LTM and BMC was found for each sex (r = 0.98, P = 0.0001 for males; r = 0.98, P = 0.0001 for females). DXA percent body fat (%BFDXA) increased with age in females (r = 0.52, P < 0.001) but not in males and was higher in females than in males at all ages. Trunk to leg fat ratio (TLFR) was calculated as DXA trunk fat/leg fat. In post-pubertal age the TLFR was higher in males than in females (1.01 +/- 0.23 and 0.75 +/- 0.16, P = 0.001), but there was no sex difference in younger children. DXA weight underestimated scale weight by a mean of 0.83 kg. %BFDXA correlated with %BF by skinfold thickness measurement with good agreement for males but overestimated %BF by skinfold thickness for females. These normative data for body composition demonstrate significant sex differences in all body compartments after the pubertal years.

Decreased bone mineral density in children with phenylketonuria.

Previous studies have suggested that children with phenylketonuria (PKU) have a reduction in bone mineralization compared with control subjects. To investigate this, bone mineral density (BMD) of the total body (TBMD) was measured in 32 prepubertal children with PKU and in 95 age-matched control subjects. Spine bone mineral density (SBMD) was also recorded in a subset, 24 with PKU and 55 control subjects. The effect of dietary intake on bone mass was assessed in 30 of the children with PKU and in 12 control subjects. In the children with PKU, TBMD and SBMD were significantly lower than in the control subjects after adjustment for height and weight (P = 0.03 and P = 0.003, respectively). The children with PKU had a higher intake of calcium (P < 0.0001), phosphorus (P = -0.0002), and magnesium (P < 0.0001), suggesting that their lower BMD occurred despite an adequate diet based on current recommendations. Further study is needed to establish the cause of this deficit in bone mass and the benefit of additional nutritional support to reverse this problem.

Body protein in prepubertal children with phenylketonuria.

OBJECTIVE: To assess body protein and protein deposition in prepubertal children with phenylketonuria (PKU). DESIGN: Cross-sectional study with nested longitudinal cohort. SETTING: A tertiary referral paediatric hospital. SUBJECTS: 37 PKU patients (3.9-11.0 years) and 27 unselected healthy controls (4.0-11.5 years) of whom 29 PKU patients and 17 controls were followed longitudinally. INTERVENTIONS: All had measurements of height, weight, body fat and total body nitrogen (TBN) by neutron capture analysis; PKU patients and their unaffected siblings (n = 16) also had measurements of four day weighed food record and plasma amino acids by HPLC. RESULTS: The children with PKU compared with the controls were significantly shorter (height SD score -0.42 +/- 0.89 vs 0.17 +/- 0.94, respectively, P < 0.02) and had a lower TBN (575 +/- 200 vs 710 +/- 215g, respectively, P < 0.02). TBN in the controls was significantly correlated with lean body mass (LBM), weight, height and age (r = 0.97, 0.95, 0.95, 0.88, respectively, P < 0.001). The children with PKU had significantly lower TBN when predicted from LBM, weight and age (93%, 92%, 92% of predicted, respectively), but normal TBN predicted from height (102% of expected). The annual accretion of nitrogen was similar for the PKU and controls (86 +/- 45 and 77 +/- 58 g/y, respectively). There was no difference between the two groups in protein intake or plasma amino acids except for phenylalanine. CONCLUSION: The children with PKU had a deficit in height and body protein despite a normal to higher accretion of protein. If the deficit occurs early in life, amino acid supplementation and other nutritional practices used at this time need to be reviewed.

Hydrogen background in total body nitrogen estimations.

Total body nitrogen (TBN) is measured by in vivo prompt gamma neutron capture analysis. Usually the background under the nitrogen peak is subtracted before calculating TBN from the ratio of nitrogen counts (at 10.8 MeV) to hydrogen counts (at 2.2 MeV). The hydrogen acts as an internal standard. The background under the hydrogen peak is usually ignored. The current study was undertaken to measure the variation of the hydrogen background (HB) in patients of different sizes and to determine whether the accuracy of TBN measurements is improved when an HB correction is incorporated. Heavy-water (D2O) phantoms were used to quantify patient HB. D2O simulates patient neutron scattering without contributing to the hydrogen peak. Equations were developed to predict HB in patient measurements. HB was found to vary from 9% of total counts for a 99 kg subject to 19% for a 7 kg infant. It was demonstrated that the accuracy of TBN measurements is improved for child-sized anthropometric phantoms of known composition when an HB correction is incorporated.

Volumetric bone mineral density--a potential role in paediatrics.

The use of areal bone mineral density (aBMD) in paediatric populations has aroused some concern, as it fails to take the age-related increase in bone thickness into account. We have developed a measure of true bone density, volumetric bone mineral density (vBMD), which is independent of age and height. In order to examine the relationship between growth parameters, aBMD and vBMD, we studied patients with phenylketonuria (PKU, n = 40), chronic renal failure (CRF, n = 27) and chronic asthma (n = 19). aBMD of the femoral neck and the mid-femoral shaft was measured using dual energy X-ray absorptiometry (DXA), vBMD was calculated on the basis of values of bone mineral content and bone dimension provided by DXA, with the assumption that both sites are cylinders. aBMD and vBMD were then compared with the normal reference, expressed as a standard deviation score (SDS). aBMD and vBMD were normal in the femoral neck region of the PKU group, but aBMD, either standardized for age or for height, was low in the femoral shaft region (p < 0.01). In the CRF group, profound growth retardation was seen (mean height SDS, -3.2) and aBMD and vBMD were both low in the femoral shaft region but not in the femoral neck. In the asthma group, aBMD for age was low at both sites, but vBMD did not differ from that seen in normal individuals. We conclude that the true vBMD provides a different interpretation of bone density compared with aBMD and requires further evaluation in paediatrics because of its age and height independence.