Bone and collagen markers in preterm infants: relationship with growth and bone mineral content over the first 10 weeks of life.

In a longitudinal study of 25 preterm infants, we have examined the relationship of bone-specific alkaline phosphatase (ALP), C-terminal propeptide of type I collagen (PICP), N-terminal propeptide of type III procollagen (P3NP), C-terminal telopeptide of type I collagen, urinary pyridinoline (Pyd) and deoxypyridinoline (Dpd), with rates of gain in weight, length, and lower leg length and with bone mineral content (BMC), all measured at weekly intervals over the first 10 wk of life. Concentrations of all collagen markers were 10-fold higher than in older children. Each marker showed a distinctive pattern of postnatal change, with early increases in PICP and P3NP and decreases in ICTP reflecting postnatal growth. Once markers had reached a plateau during weeks 4-10, P3NP was positively correlated, whereas Pyd and Dpd were negatively correlated with rate of weight gain (r = +0.44, -0.46, and -0.40, respectively, p < 0.05). P3NP was also positively correlated with overall linear growth (r = +0.44, p < 0.05). PICP was strongly correlated with mean BMC (r = +0.63,p < 0.01) and with total BMC attained by the end of the study period (r = +0.81, p < 0.001). Bone ALP was positively correlated with the rate of bone mineral accretion (r = +0.55, p = 0.01). We conclude that the marker of soft-tissue collagen formation, P3NP, is a good marker for overall ponderal and linear growth in preterm infants, whereas the markers of collagen breakdown, Pyd and Dpd, have inverse relationships with weight gain. The osteoblast markers, PICP and bone ALP, seem to be good surrogate markers for bone mineralization in preterm infants. Markers may provide information on whole-body turnover of bone and collagen that is complementary to traditional physical measures of growth and bone mineralization.

Calcium and phosphorus balance in extremely low birthweight infants in the first six weeks of life.

The absorption and retention of calcium and phosphorus in infants of 28 weeks' gestation, or less, who were fed either breast milk or a preterm formula, were determined in 22 balance studies. Breast milk contained significantly lower amounts of calcium and phosphorus than the formula milk. The percentage absorption of calcium was similar in the two groups, but the infants fed breast milk had greater urinary calcium losses and significantly lower calcium retention than the group fed formula milk. The percentage phosphate absorption was also similar in the two groups but because of the higher phosphate intake in the formula milk this group retained a significantly greater amount of phosphate. The extremely low birthweight infants fed on breast milk were phosphorus depleted, with low plasma phosphate concentrations, and seemed to retain as much phosphate as possible. The hypophosphataemia may result in reduced deposition of calcium in bone and explain the calciuria in the breast fed infants. Substrate deficiency may be an important factor in the aetiology of rickets of prematurity and, particularly if breast milk is used, preterm infants may require calcium and phosphate supplements from an early age.

Hypercalcaemia in extremely low birthweight infants.

We report 10 cases of hypercalcaemia associated with hypophosphataemia in the first two weeks of life in extremely low birthweight infants (birthweight less than 1000 g). At the time of diagnosis, the infants were fed mainly with expressed breast milk but they had also received intravenous nutrition. After treatment with additional phosphate plasma calcium concentrations returned to normal. There was a high incidence of rickets of prematurity in these infants.