The Effects of Oral CaCO3Loading and Dietary Calcium Deprivation on Plasma 1,25-Dihydroxyvitamin D Concentrations in Healthy Adults*

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Abstract
Plasma 1,25-dihydroxyvitamin D [1,25(OH)2D] concentrations were observed to be inversely correlated with the urinary Ca/creatinine (r = –0.56) in 43 healthy adults eating ad libitum, suggesting that dietary Ca is one of the factors regulating the plasma concentration of this hormone. Increased Ca intake in 32 volunteers achieved by oral CaCO3 (39–130 mmol/day) for 4 days resulted in a rise in serum Ca within 6 h, hypercalciuria, a prompt fall in serum parathyroid hormone (PTH) and urinary cAMP/creatinine, followed by a sustained fall in plasma 1,25(OH)2D within 18–24 h from control levels of 84 ± 19 to 46 ± 28 pmol/liter (P < 0.001). A reduction in dietary Ca intake in six volunteers to only 1–3 mmol/day for 4 days resulted in a prompt fall in serum and urinary Ca, a rise in serum PTH an urinary cAMP/creatinine, and a sustained rise within 48 h of plasma 1,25(OH)2D concentrations from 63 ± 32 to 92 ± 28 pmol/liter (P < 0.05). For both groups, the changes in plasma 1,25(OH)2D were inversely correlated to both the changes in dietary Ca intake per kg BW/day (r = –0.76) and those in urinary Ca/creatinine (r = –0.75) and were directly correlated to both the changes in serum PTH (r = 0.65) and those in urinary cAMP/creatinine (r = 0.69). Thus, dietary Ca intake, mediated either by changes in serum Ca concentration or, more likely, via changes in serum PTH, is a determinant of the plasma concentration of the renal hormone 1,25(OH)2D in healthy adults.