Longitudinal study of urinary hydroxy-pyridinium cross-links and growth in healthy infants: higher values with breastfeeding and after daytime sleep.

Urinary pyridinoline and deoxypyridinoline crosslinks (crosslink) are excreted when bone is resorbed. The aims of this study in healthy infants were to determine whether crosslinks a) could predict growth velocity, b) are variable due to circadian rhythm, and c) differ in infants who were either breast-fed or formula-fed. In 78 healthy infants (48 male; 30 female) urine samples were collected and anthropometric measurements were taken at 2, 3, 4, 5, 6, 8, 10 and 12 months of age. In addition, a total of 25 samples were collected during the day (0700-2000) in 5 of the infants to determine circadian rhythm of crosslink excretion. Crosslink excretion decreased (p < 0.001) with age between 2 and 12 months. Pyridinoline excretion showed a significant, but weak correlation (r > or = 0.21; p < 0.05) with linear growth velocity and weight velocity in the subsequent month until 6 months of age, and no correlation thereafter. Infants studied for circadian rhythm showed a 63% greater (p < 0.05) rate of pyridinoline excretion after a nap as compared to the 13-hour mean value. In a subset of infants whose energy intake was exclusively from breast milk (BF, n = 23) or formula (FF, n = 10), crosslink excretion was greater in BF infants at 3 months of age (p < 0.05). The correlations between crosslink excretion and growth parameters indicate that crosslinks may be useful as a marker of growth in infant populations. However sources of variation in crosslink excretion, such as circadian rhythm and diet may limit their utility to predict growth in an individual infant. These factors should be considered in future studies examining markers of bone turnover in infants.