4.3.6 Bone mass: Measurement of bone area, BMC and BMD of the whole body, lumbar vertebrae (L2-4) and femur at all visits will be conducted using DXA (QDR 4500A Discovery series, Hologic Inc.) at the Mary Emily Clinical Nutrition Research Unit. DXA has been validated for measuring infant whole body and regional BMC (49, 54, 55) and delivers minimal radiation (6 uSv) in comparison to standard pediatric x-rays (~60 uSv). Since positioning in infants is difficult to standardize, only BMC is used for whole body and femur. For lumbar spine, standardized positioning is feasible enabling assessment of bone area, BMC and BMD. Infants will not be sedated and will be scanned while sleeping and wrapped in a receiving blanket and standardized clothing without any metal zippers, clasps etc. Data will be expressed as absolute values for bone area, BMC and BMD and also as a rate of change for BMC. The measurement of whole body bone mass is ideal and recommended by ISCD (56). To enable complete data regarding bone, the next best measurement is lumbar spine, vertebra 1-4 (56). This measurement is easily attainable in infants to 1 year of age and only requires 30 to 60 seconds compared to whole body at 3 minutes. In addition, the femur can be scanned in 30 to 60 seconds and offers a novel assessment of a long bone. Whole body BMC will be corrected to body weight, length and lean mass and also expressed as change over the period of study. Correction to weight or length is endorsed by ISCD rather than more novel methods such as use of bone area and height (59) since normative data for infants is not currently available. Differences in whole body bone mineral accretion between inception, 3 and 6 months and between 6 and 12 months are greater than the error of measurement by DXA estimated at <4.5% (25, 49). Values for spine bone area, BMC and BMD and femur BMC will be compared among groups.