The effect of oral contraceptives on folate metabolism. III. Plasma clearance and urinary folate excretion.

The plasma clearance of folic acid (PGA) and urinary excretion of folates were determined in women on oral contraceptives and in control subjects. The initial phase of plasma clearance of PGA (first 5 minutes) was much faster in oral contraceptive users than in the controls, but the subsequent rate of clearance was the same in the two groups. There was a good correlation between the 12-hour urinary folate excretion and serum or red cell folate in both groups. The women on oral contraceptives excreted more folate in their urine for any given level of serum or red cell folate. The increased urinary folate excretion may partly explain the lower serum and red cell folate in oral contraceptive users.