COBALAMIN AND COBALAMIN-BINDING CAPACITY IN HUMAN-MILK

Information on variability of cobalamin and its binding protein concen trations in human milk is scarce. We investigated their relationship a nd 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. Coba lamin and its binding protein, cobalophin, expressed as cobalamin-bind ing capacity, presented higher correlations in colostrum and transitio n milk (r=0.621 and 0.842, respectively) than in other periods of lact ation (r<0.500). Cobalamin was significantly higher (p<0.01) in colost rum (mean, all subjects: 0.35 nmol/l) than in the third month of lacta tion (0.25 nmol/l), whereas cobalamin-binding capacity was significant ly lower (p<0.01) in colostrum (29 nmol/l) than in the third month (43 nmol/l). However, the patterns of individual longitudinal variation w ere different among women. There was a high intraindividual diurnal va riability, with coefficients of variation (CV) in different individual s ranging from 15 to 42% for cobalamin, and from 16 to 52% for the bin ding protein, reflecting the variations between breasts and between di fferent moments of collection within a day. Interindividual CV for sam ples obtained under identical conditions ranged from 32% (colostrum, m orning, left breast) to 85% (1st month, afternoon, right breast) for c obalamin, and from 20% (2nd month, morning, right breast) to 90% (1st month, evening, right breast) for cobalamin-binding capacity. Comparin g 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 fo r cobalamin analysis in population studies can be obtained within a da y of a determined lactation period irrespective of the time of day, br east or moment within a feed.