Threonine kinetics in preterm infants fed their mothers' milk or formula with various ratios of whey to casein

Background: Plasma threonine concentrations are elevated in infants fed for mula containing a whey-to-casein protein ratio of 60:40 compared with conce ntrations in infants fed formula containing a ratio of 20:80 or human milk (60:40). Objective: We studied whether degradation of excess threonine was lower in formula-fed infants than in infants fed their mothers' milk. Design: Threonine kinetics were examined in 17 preterm infants (gestational age: 31 +/- 2 wk; birth weight: 1720 +/- 330 g) by using an 18-h oral infu sion of [1-C-13]threonine at a postnatal age of 21 +/- 11 d and weight of 1 971 +/- 270 g. Five infants received breast milk. Formula-fed infants (n = 12) were randomly assigned to receive 1 of 3 formulas (5.3 g protein/MJ) th at differed only in the whey-to-casein ratio (20:80, 40:60, and 60:40). Results: Threonine intake increased significantly in formula-fed infants wi th increasing whey content of the formula (48.5, 56.4, and 63.2 mu mol.kg(- 1).h(-1), respectively; pooled SD: 2.2; P = 0.0001), as did plasma threonin e concentrations (228, 344, and 419 mu mol/L, respectively; pooled SD: 75: P = 0.03), Despite a generous threonine intake by infants fed breast milk ( 58.0 +/- 16.0 mu mol.kg(-1).h(-1)), plasma threonine concentrations remaine d low (208 +/- 41 mu mol/L). Fecal threonine excretion and net threonine ti ssue gain, estimated by nitrogen balance, did not differ significantly amon g groups. Threonine oxidation did not differ significantly among formula-fe d infants but was significantly lower in formula-fed infants fed than in in fants fed breast milk (17.1% compared with 24.3% of threonine intake, respe ctively). Conclusion: Formula-fed infants have a lower capacity to oxidize threonine than do infants fed breast milk.