RESUMO
BACKGROUND: This study was designed to describe the longitudinal changes in the casein and whey fractions and the total and nonprotein nitrogen contents of preterm human milk for the first 10 days postpartum. METHODS: Eleven mothers delivering at 30 to 34 weeks of gestation were studied, six throughout the first 10 days and five on the first postpartum day. Four milk samples were collected every day by expression of one breast with an electric breast pump. Casein and whey protein were separated from the defatted milk by isoelectric precipitation in calcium chloride and by subsequent ultracentrifugation. The milk nitrogen content was determined before and after acid precipitation. Whey protein and casein were characterized by electrophoresis in polyacrylamide gel. RESULTS: Total and protein nitrogen showed a sharp decrease during the first 3 postpartum days, whereas the nonprotein nitrogen did not change markedly (mean concentration, 0.58 mg.ml-1). Casein content expressed as nitrogen in preterm human milk was 0.35 mg.ml-1 between days 3 and 5 and 0.60 mg.ml-1 between days 6 and 10. The whey protein-casein ratio before day 2, was 100:0, at day 3, 86:14, at day 5, 76:24, and at day 10, 70:30. Three days after delivery, casein levels rose progressively, increasing markedly after day 6. CONCLUSIONS: These findings suggest that delivering before term affects neither casein production nor its chemical characteristics.