QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Full Text Full Text (PDF) Purchase Article View Shopping Cart Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Darling, P. B. Articles by Pencharz, P. B. Search for Related Content PubMed PubMed Citation Articles by Darling, P. B. Articles by Pencharz, P. B.

---

Human Nutrition and Metabolism

Phenylalanine Kinetics Differ between Formula-Fed and Human Milk–Fed Preterm Infants¹

Pauline B. Darling^2,*,, Michael Dunn^**,, G. Sarwar Gilani, Ronald O. Ball^, and Paul B. Pencharz^*,,**

^* Research Institute, The Hospital for Sick Children, Toronto, Ontario M5G 1X8, Canada; Department of Nutritional Sciences, University of Toronto, Toronto, Ontario M5S 1A1, Canada; ^** Department of Paediatrics, University of Toronto, Toronto, Ontario M5S 1A1, Canada; Perinatal Unit, Women’s College Hospital, Toronto, Ontario M5S 1B2, Canada; Bureau of Nutritional Sciences, Health Protection Branch, Ottawa, Ontario K1A 0L2, Canada; and Department of Agricultural, Food, and Nutritional Sciences, University of Alberta, Edmonton, Alberta T6G 2P5, Canada

²To whom correspondence should be addressed. E-mail: darlingp{at}smh.toronto.on.ca.

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 µmol · 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 µmol · L^–1). Using [1-¹³C]phenylalanine tracer and ¹³CO₂ production in breath we found that although there was a significant positive relation between phenylalanine oxidation and phenylalanine intake in formula-fed infants (r² = 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.

KEY WORDS: • phenylalanine kinetics • tyrosine • preterm infants • infant formula • breast milk