A study of bone mineral density and physical growth in very low birth-weight infants after their discharge from hospital

The lumbar spinal bone mineral density (BMD) of very low birth-weight (VLBW) infants was determined by dual-energy X-ray absorptiometry, and its relationship with physical growth was analyzed. The influence of birth-related factors on changes in BMD after discharge from the hospital were also investigated. The BMD increased rapidly until the age of 2 years, in association with improvements in nutritional status and the increase in physical growth after discharge. The Z score, which indicates the degree of attainment of the age-specific standard BMD, was determined to be almost 90% at the age of around 2 years and older. Significant increases in the BMD continued to be noted after the age of 2 years, along with increases in body weight and body height. At the age-adjusted value at less than 1 year, however, the Z score tended to be low when the birth weight (<1000 g) and birth height (<36 cm) were low (P < 0.02, respectively), suggesting that the birth weight and birth height influence the BMD at this age. These results indicate that the condition at birth and nutritional management during hospitalization affect the BMD soon after discharge, but that the nutritional condition after discharge becomes a more important factor at the age of 2 years and older.