COBALAMIN AND COBALAMIN-BINDING CAPACITY IN HUMAN-MILK

Information on variability of cobalamin and its binding protein concentrations in human milk is scarce. We investigated their relationship and longitudinal changes up to the third month of lactation, and their inter and intraindividual variability within the day, between breasts and within the feed in samples from nine healthy lactating women. Cobalamin and its binding protein, cobalophin, expressed as cobalamin-binding capacity, presented higher correlations in colostrum and transition milk (r=0.621 and 0.842, respectively) than in other periods of lactation (r<0.500). Cobalamin was significantly higher (p<0.01) in colostrum (mean, all subjects: 0.35 nmol/l) than in the third month of lactation (0.25 nmol/l), whereas cobalamin-binding capacity was significantly lower (p<0.01) in colostrum (29 nmol/l) than in the third month (43 nmol/l). However, the patterns of individual longitudinal variation were different among women. There was a high intraindividual diurnal variability, with coefficients of variation (CV) in different individuals ranging from 15 to 42% for cobalamin, and from 16 to 52% for the binding protein, reflecting the variations between breasts and between different moments of collection within a day. Interindividual CV for samples obtained under identical conditions ranged from 32% (colostrum, morning, left breast) to 85% (1st month, afternoon, right breast) for cobalamin, and from 20% (2nd month, morning, right breast) to 90% (1st month, evening, right breast) for cobalamin-binding capacity. Comparing the means of all subjects in each period of lactation there were no significant differences, or even trends, in cobalamin and its binding protein between breasts, fore and hindmilk, and morning, afternoon and evening samples. Therefore, a representative sample of breast milk for cobalamin analysis in population studies can be obtained within a day of a determined lactation period irrespective of the time of day, breast or moment within a feed.