Biotin in human milk: methods, location, and chemical form.

For infants, no Recommended Dietary Allowance for biotin has been published; the estimated safe and adequate intake seems to be based on measurements of human milk. However, published estimates of the biotin content disagree substantially. We sought to address several of the potential sources of disagreement by defining the conditions for the collection, storage, and subcompartment distribution of biotin in human milk using the [125I]avidin assay. Composition of the collection vessel (glass vs. common plastics) had no effect on biotin content of human milk. The biotin content of milk did not change during storage at room temperature for at least 1 wk, at 5 degrees C for at least 1 mo, or at -20 degrees C or -70 degrees C for at least 1.5 y. Biotin in the cell pellet and fat fraction accounted for less than 5% of that in the skim fraction. Of the biotin in the skim fraction, none (less than 3%) was reversibly bound to macromolecules, and less than 5% was covalently bound to macromolecules. We conclude that assay of free biotin will detect almost all of the biotin present in a sample of mature human milk.