The Lack of Effect of Chronic Metabolic Acidosis on 25-OH-Vitamin D Metabolism and Serum Parathyroid Hormone in Humans*†

Abstract

We evaluated the turnover of the plasma 25-OH-vitamin D pool, acid, and mineral balances in paired balance studies of 6 normal subjects during normal acid base conditions and during stable chronic metabolic acidosis induced by NH₄Cl. Positive acid balances and negative Ca balances due to hypercalciuria were observed as previously reported. Plasma 25-OH-D pool turnover averaged 6.1 ± 0.4 nmol/day during control and did not change during acidosis (6.5 ± 0.5 nmol/day) nor were any significant increments in net intestinal absorption of Ca, PO₄ or Mg, the physiological expression of vitamin D action, observed during acidosis. In 3 other subjects, repetitive measurements of serum iPTH during 7 control days and 24 days of stable NH₄Cl acidosis showed no changes. We interpret the data to support the hypothesis that neither PTH nor vitamin D and its metabolites mediates the increase in net bone resorption that must accompany chronic metabolic acidosis.