Urinary Hydroxyproline

Abstract

There is little information on urinary hydroxyproline (UHP) excretion in premature infants. We hypothesized that UHP excretion would positively correlate with growth in premature infants, and that there would be correlations between UHP excretion and serum alkaline phosphatase concentration as well as bone mineral content (BMC). Twenty‐six premature infants (birth weight, < 1,300 g; gestational age, ≤ 32 weeks) received one of four oral feedings. Seven received mother's own milk (HM), and eight received mothers own milk fortified with 0.85 g/dl of bovine whey, 90 mg/dl of Ca, and 45 mg/dl of P. Six and five infants received Similac, 20 cal/oz (SIM) and Similac Special Care, 20 cal/oz, respectively. Measurements of UHP, serum alkaline phosphatase, BMC (photon absorptiometry), and growth were made during the 1st 7 weeks of life. The lowest UHP excretion was in the HM group. For all infants, there was a significant correlation between UHP excretion (mg/day) and absolute weight (r = 0.64, p < 0.001), as well as rate of weight gain (r = 0.50, p < 0.01). The UHP excretion (milligrams per day) also correlated with absolute length (r = 0.41, p < 0.01) and rate of gain in length (centimeters per week) (r = 0.70, p < 0.001). The UHP excretion did not correlate significantly with BMC or serum alkaline phosphatase concentration. We conclude that UHP excretion is increased in the growing premature infant compared to older infants and adults and is a good marker for somatic growth in this population. Total UHP excretion is not a useful marker for bone turnover and ongoing bone mineralization in these infants.