Aspects of folate metabolism in lactating women studied after ingestion of 14C-methylfolate.

Radioactive methylfolate (14C-5CH3H4PteGlu) (10-14 microgram/kg) was fed to four lactating women presenting with breast abscesses necessitating cessation of lactation. The appearance of radiofolate in milk, plasma and urine over the next 24 hours was investigated. In spite of a minimal postabsorption rise of plasma biofolate, plasma radiofolate (including a dialysis-resistant (bound) fraction) increased steadily to 1.26 to 5.11 microgram/l at 24 hours. Urinary radiofolate excretion was considerable. Total milk biofolate rose significantly by 15 to 28 microgram/l, in contrast with a much smaller radiofolate fraction (1.95-3.88 microgram/l) which at 24 hours was comparable with that of plasma. Milk radiofolate included a dialysis-resistant fraction rising to 0.75 to 1.15 microgram/l at 24 hours. On chromatography (Sephadex-DEAE-A50) plasma, urine and milk showed a nonbound radiofolate peak suggestive of 10-CHO.H4PteGlu. This folate may originate predominantly from the apocrine mammary glands. The in vitro labelled radiofolate milk binder could not be identified chromatographically, but it was shown that the in vitro milk binders of PteGlu and 5CH3H4PteGlu could be separated chromatographically.