Delphi a

Dual X-ray absorptiometry (DXA) scans of posterioranterior (PA) spine, proximal femur, forearm, and whole body were obtained using a fan-beam scanner (Delphi A, software version 12.3, Hologic, Waltham, MA, USA). BMD was determined for all available regions of interest. Whole body lean mass (LM) and fat mass (FM) were determined. All DXA scans were performed by technologists certified by the International Society for Clinical Densitometry, following standard quality control procedures, including daily phantom scanning. Coefficients of variation (CV) were 0.9% for total body BMD, 1.2% for PA spine (L1–L4) BMD, 0.9% for total hip BMD, 0.4% for forearm (ultradistal radius) BMD, 0.9% for whole body BMD, 1.2% for whole body FM and 0.5% for whole body lean mass.

Whole-body, posteroanterior lumbar spine, proximal femur, and forearm (distal one-third radius and ultradistal radius) dual-energy x-ray absorptiometry (DXA) scans were acquired on bone densitometers (QDR4500A, QDR4500W, and Delphi A; Hologic) for determination of BMC and areal bone mineral density (aBMD). All scans were analyzed centrally by the DXA Core Laboratory at the University of California, San Francisco, using Hologic (release 12.3) software.^{10 (link)} Both WB and aLST (excluding bone) were determined and corrected for intermachine differences and longitudinal drift. Whole-body and site-specific BMC and aBMD z scores, adjusted for age- and sex-specific height z scores, were calculated.^{10 (link)}

Total body bone mineral content (BMC, g) was measured via DXA (Delphi-A, Hologic Inc, Waltham, MA, USA). The same researcher performed and analyzed all DXA scans using instrument-specific software and procedures. In 10 females ages 5 to 8 years who were scanned twice over a 7-day period, intraclass correlation coefficients were calculated (all > 0.98).

Height and weight were measured using standard techniques [20 ], BMI derived as kg/m², and age- and sex-specific Z-scores (HAZ, WAZ, BMIZ) were calculated.[21 (link)] Using mid-upper arm circumference and triceps skinfold thickness, upper arm muscle and fat areas were derived, and Z scores (UAMAZ, UAFAZ) were computed.[22 (link)] Total fat-free mass (FFM), fat mass (FM), and percent body fat were obtained by whole body dual energy X-ray absorptiometry (DXA, Delphi A, Hologic, Inc., Bedford, MA). Pubertal status was determined using Tanner stages with a validated self-assessment questionnaire.[23 (link)] Resting energy expenditure (REE) was measured by open circuit indirect calorimetry using a computerized metabolic cart (SensorMedics Spectra, Yorba Linda, CA) as previously described.[15 ] REE was calculated from the modified Weir equation.[24 (link)] REE (kcals/day) as a percent of predicted values (%REE) was derived from the Schofield equations that adjust for age, sex, weight, and height.[25 (link)] Pulmonary function was assessed and predicted percentage FEV₁ calculated.[26 (link),27 (link)]

Dual-energy x-ray absorptiometry (DXA) scans were performed using a dedicated Hologic, Inc. (Bedford, MA) bone densitometer (QDR4500A, QDR4500W, Apex, and Delphi A models) at each clinical center. Scans were performed for each participant according to the manufacturer’s guidelines to obtain spine BMC and aBMD. Calibration stability across sites and over time was maintained using traveling and site-specific phantoms as previously described.^{15 (link)} All scans were analyzed by the DXA Core Laboratory at the University of California, San Francisco using Hologic software release 12.3 to obtain BMC and aBMD values for each participant. Data from the lumbar spine scans were used in the present analysis as they have shown associations with calcium intake in the BMDCS cohort²⁴ , and the lumbar spine is a recommended site for pediatric bone health assessment.^{25 (link)}