The plasma clearance rates of endogenous serum creatinine and folate were studied in 11 non-pregnant and 37 pregnant healthy women. The mean clearance of creatinine by the kidney was about 50 per cent higher and the mean folate clearance over 100 per cent higher in the pregnant women. Creatinine and folate clearance increased independently and it is argued that the greater urinary loss of folate was probably the result of altered renal tubular function, not increased glomerular filtration rate.
The urinary loss of folate was not so great as to make a significant contribution to the high folate requirements of pregnancy, but could be a major cause of depletion of the unbound fraction of serum folate and the lowering of the total serum folate activity from early pregnancy. Low serum folate activity should not be interpreted as evidence of a pregnant patient being in negative folate balance in the absence of signs of tissue folate depletion.