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 beta-casein comparable immunoassay, it was applied to 862 samples milk, collected from 82 mothers, to investigate the changes in casein concentrations in human milk during the first twelve weeks of lactation. kappa-casein immunoassay is sensitive (detection limit in the reaction mixture, 0.02 mg/L) and can be performed in diluted milk, excluding any interference or sample pretreatment. 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 percentages 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% and 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 lactation (2.7 g/L, 25.3% and 0.9 g/L, 8.5%). The beta-casein/kappa-casein ratio is higher in colostrum (0.61) than in transitional and mature milk (0.30) and could be related to a better digestibility of colostrum casein micelles by the neonate during the first days of life.