Phenylalanine kinetics differ between formula-fed and human milk-fed preterm infants.
Infants fed casein-dominant formulas have higher plasma phenylalanine and tyrosine concentrations than those fed mother's milk. Conversely, elevated plasma threonine concentrations are observed in infants fed whey-dominant formulas. We recently showed that formula-fed preterm infants have a lower capacity to degrade threonine than do preterm infants fed mother's milk. We hypothesized that these same infants (n = 18) would differ in their catabolism of phenylalanine in response to phenylalanine loads provided by formulas with increasing casein content of formulas (whey:casein 60:40, 40:60, and 20:80) compared with preterm infants fed mother's milk. Plasma phenylalanine concentrations significantly rose (49, 46, 79 micromol . L(-1) for whey:casein 60:40, 40:60, and 20:80, respectively, pooled SD 8, P < 0.05); and plasma phenylalanine concentrations in infants fed mother's milk were low (40 +/- 4 micromol . L(-1)). Using [1-(13)C]phenylalanine tracer and (13)CO(2) production in breath we found that although there was a significant positive relation between phenylalanine oxidation and phenylalanine intake in formula-fed infants (r(2) = 0.43, P = 0.03), these infants were not able to increase their oxidation of phenylalanine enough to prevent a significant rise in plasma phenylalanine when fed the 20:80 formula. Compared to infants fed mother's milk, formula-fed infants had significantly lower phenylalanine oxidation (39.1 vs. 30.7% of phenylalanine intake, respectively, P < 0.05). We conclude that one of the mechanisms for the differences in plasma amino acid concentration between formula-fed and mother's milk-fed preterm infants may be in vivo down-regulated catabolism of 2 important essential amino acids (phenylalanine in addition to threonine) in formula-fed preterm infants.
Darling
Pauline B
PB
Research Institute, The Hospital for Sick Children, Toronto, Ontario M5G 1X8, Canada. darlingp@smh.toronto.on.ca
Dunn
Michael
M
Gilani
G Sarwar
GS
Ball
Ronald O
RO
Pencharz
Paul B
PB
D005260:Female; D006207:Half-Life; D006801:Humans; D007223:Infant; D007225:Infant Food; D007231:Infant, Newborn; D008297:Male; D008895:Milk, Human; D010649:Phenylalanine / Q000097:blood / Q000378:metabolism* / Q000493:pharmacokinetics; D014443:Tyrosine / Q000097:blood / Q000378:metabolism* / Q000493:pharmacokinetics
D014443:Tyrosine; D010649:Phenylalanine