Changes in the kappa-casein and beta-casein concentrations in human milk during lactation

A microparticle-enhanced nephelometric immunoassay was developed for kappa- casein quantification in human milk. Together with a previously reported be ta-casein comparable immunoassay, it was applied to 862 samples milk, colle cted from 82 mothers, to investigate the changes in casein concentrations i n human milk during the first twelve weeks of lactation. kappa-casein immun oassay is sensitive (detection limit in the reaction mixture, 0.02 mg/L) an d can be performed in diluted milk, excluding any interference or sample pr etreatment. It allowed the quantitation of kappa-casein over a large range of concentrations (0.14-4.56 g/L) with accuracy and precision (coefficients of variation from 3 to 10%). beta- and kappa-casein concentrations and per centages among milk total proteins increase between colostrum (2.6 g/L, 14. 3% and 1.2 g/L, 6.5%, respectively) and transitional milk (4.4 g/L, 33.2% a nd 1.3 g/L, 9.5%), decrease at different rates from the third to the eighth week, then remain stable at least up to the end of the third month of lact ation (2.7 g/L, 25.3% and 0.9 g/L, 8.5%). The beta-casein/kappa-casein rati o is higher in colostrum (0.61) than in transitional and mature milk (0.30) and could be related to a better digestibility of colostrum casean micelle s by the neonate during the first days of life. J. Clin. Lab. Anal. 13:213- 218, 1999. (C) 1999 Wiley-Liss, Inc.