Abstract. Fasting duodenal bile acid concentrations and conjugation patterns were studied during the first 5 weeks of life in 65 low-birth-weight infants, 31 to 36 weeks of gestational age. One group was fed human milk. Approximately 55% of this milk was pooled, expressed, and pasteurized (62 Celsius for 30 minutes), 35% was similarly treated milk from the infant's own mother, and the remainder (10%) was provided by breast-feeding. The other infants, from 3 days of age, were fed one of three formulas: an adapted formula (Fl), F1 supplemented with taurine (F2), or F1 supplemented with taurine and cholesterol (F3). The fasting intraluminal concentration of conjugated bile acids was higher in the infants fed human milk than in the infants fed formulas (F = 30.03, p < .001) reflecting the higher concentrations of all individual bile acids. No significant increase over time was found in the concentration of total bile acids in any feeding group. Chenodeoxycholic acid concentrations, however, increased significantly over time in the infants fed human milk (r = .286, P < .05). Thus, in the infants fed human milk, the ratio of cholates to chenodeoxycholates changed from 2.03 to 1.29 (P < .001), whereas it remained stable (2.61) in the groups fed formula. Tauroconjugated bile acids predominated until at least 5 weeks of life in all the infants fed human milk, F2, or F3. In the infants fed Fl, the concentration of glycoconjugates increased and that of tauroconjugates remained stable. Consequently, the ratio of glycine- to taurine-conjugated bile acids increased twofold, from 0.64 to 1.40 at 5 weeks of age. No further differences due to supplementation of formula with taurine plus cholesterol were found. Feeding human milk results in higher intraluminal concentrations of bile acids than does formulas.... [ABSTRACT FROM AUTHOR]