Phosphate deprivation increases serum 1,25-(OH)2-vitamin D concentrations in healthy men

To re-evaluate whether phosphate-deprivation alters serum 1,25-(OH)2-D concentrations in men, we measured serum 1,25-(OH)2-D levels in seven healthy men while they ate constant diets providing 52.2 +/- 4.3 sd mmoles PO4/day for 12 days and then while they ate a diet providing 29.4 +/- 6.0 mmoles PO4/day and also were given Al(OH)3 for 18 days. Serum PO4 levels fell only transiently. Net intestinal PO4 absorption became indistinguishable from zero but, because of renal PO4 conservation, PO4 balances did not change. Nevertheless, serum 1,25-(OH)2-D concentrations increased from 80 +/- 21 pM during control to 104 +/- 26 pM; during PO4 deprivation; P less than 0.01. Net intestinal calcium (Ca) absorption tended to rise during PO4 deprivation, the individual increments being correlated to the increments in serum 1,25-(OH)2-D concentrations (r = 0.78; P less than 0.05). Urinary Ca excretion rose; P less than 0.001, the increments exceeding the increments in net intestinal Ca absorption so that Ca balances became more negative; P less than 0.001. Urinary hydroxyproline excretion did not change. Thus, as in women and animals, PO4 deprivation appears to cause enhanced net bone resorption apparently by reducing bone formation.