Is there an impact of birth weight and early life nutrition on bone mineral content in preterm born infants and children?

Low birth weight and early life nutrition have a strong impact on the skeleton of infants appropriate for gestational age (AGA), as shown by single photon absorptiometry (1,2) and radiography (3 ) . The mineralization defect of bone matrix depends mostly on the mineral and vitamin D formulation of the diet (4). While the immediate effects of low birth weight and of early life nutrition on bone mineralization are accepted widely, there is poor agreement on their late effects. According to Helin et al. (3, preterm infants continue to show lower bone mineral content (BMC) than their agematched controls, up to the ages of 4 to 16 years; on the contrary, Horsman et al. (6) claim that the mineralization defect of the preterm AGA infant is quickly overcome by a phase of rapid mineral accretion, occurring between 40 and 60 weeks’ postconception. Moreover, at the postconceptional age of 56 weeks, AGA infants fed their own mother’s milk were found to have similar growth patterns but a different bone mineralization rate compared with preterm infants fed a standard cow’s milk-based formula (7). The aim of the present study was to assess if low birth weight and early life nutrition in AGA infants have late effects on BMC before adolescence. Eighty-two preterm-born infants and children aged 2 months to 12 years were admitted to the study after oral informed parental consent. The children were recruited from an urban community of Caucasian, middle-class, well educated and well nourished people. For each subject, date of birth, birth weight and feeding history from birth to weaning were retrieved from the clinical records and by mothers’ recall. BMC and bone width (BW) at the distal third of the right radius were measured by single photon absorptiometry (Norland bone mineral analyser 2780) during a clinic health visit. On the same occasion, body weight, height and head circumference were recorded also. In order to set up a reference group, BMC and BW values were also measured in a small group of 24 termborn infants and children aged 5 months to 15 years, belonging to the same population as the study group and attending the Orthopaedic Department for problems not related to bone mineral metabolism. They were recruited after oral informed parental consent. BMC and BW values measured in this small group of termborn babies were compared with those reported from larger series in the literature (8-1 1). As we found that the measured BMC and BW values were within the variance limits ( f 2 SD) of the larger series published previously, the latter were accepted as reference values. The 82 subjects (42 males and 40 females) were selected according to the following characteristics: gestational age at birth between 26 and 37 weeks, appropriate weight for gestational age at birth, birth weight from 900 to 2640 g. Half of the subjects (n = 40) had been fed their mother’s milk supplemented with human milk protein (1 g/dl/day), calcium (30 mg/kg/ day) and phosphorus (20 mg/kgg/day) from birth to time of discharge from the hospital at a body weight of 2200 g. After discharge, the infants were exclusively breast fed until weaning was started at the age of four months. The second half of the subjects (n=34) received a wheypredominant cow’s milk formula designed specifically for low-birth-weight infants containing 2.0 g of protein/ dl and providing 75 kcal/dl (Humana 0, Humana Milchwerke Westfalen-Herford, Germany) from birth to discharge and thereafter a whey-predominant cow’s milk formula designed for term infants containing 1.4 g of protein/dl and providing 69 kcal/dl (Humana 1, Humana Milchwerke Westfalen-Herford, Germany). For 8 infants, feeding history was not available at the time of study. When all infants were grouped by age at which the BMC measurements were performed, it was found that birth weights were different among the classes (Table 1). The collected data were therefore analysed by covariance analysis, adjusting for the covariates (age at which the BMC measurement was performed and birth