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Vitamin B12 levels in human milk during the first nine months of lactation.

Vitamin B12 concentration was measured by competitive binding radioassay in 48 samples of human milk from healthy mothers eating unrestricted diets. Specimens were collected 1-35 weeks after full-term delivery and were subjected to proteolytic digestion before radioassay in order to destroy binding proteins. The distribution of the results was skewed, but the distribution of the logged values was not significantly different from normal. The geometric mean vitamin B12 level remained almost unchanged during the first 12 weeks postpartum (261-297 pmol/l) and then declined to a low of 139 pmol/l at 27-35 weeks. A significant (P = 0.033) decline in vitamin B12 concentration between 6-12 weeks and 19-25 weeks postpartum was observed.