PURPOSE: The purpose of this study was to investigate whether dietary calcium intake in primary hyperparathyroidism is associated with differences in bone mineral density and biochemical parameters, and to determine whether these observations are related to 1,25-dihydroxyvitamin D.

PATIENTS AND METHODS: Dietary calcium intake was determined from diet records in 71 unselected patients enrolled in an ongoing longitudinal study on the natural history of primary hyperparathyroidism. Subjects were placed into one of three dietary calcium groups based on their mean dietary calcium intake: very low (<300 mg/day; mean = 199 ± 14), low (300 to 800 mg/day; mean = 529 ± 21), and US RDA (>800 mg/day; mean = 1023 ± 73). Biochemical indices were indicative of patients with modern day primary hyperparathyroidism, showing mild hypercalcemia (2.79 ± 0.02 mmol/L), low normal serum phosphorus (0.90 ± 0.03 mmol/L), elevated parathyroid hormone levels by mid-molecule (764 ± 69 pg/mL) and immunoradiometric (118 ± 8 pg/mL) assays, and high normal 1,25-dihydroxyvitamin D (60 ± 3 pg/mL) and urinary calcium excretion (6.3 ± 0.4 mmol/day). Bone mineral density was measured by dual energy x-ray absorptiometry at the lumbar spine, right femoral neck and distal third of the nondominant radius for each subject.

RESULTS: Over the entire range, there was no significant effect of dietary calcium on serum parathyroid hormone levels, calcium, phosphorus, 25-hydroxyvitamin D, 1,25-dihydroxyvitamin D, urinary calcium excretion, or bone mineral density of the lumbar spine, femoral neck or distal one-third radius. Serum 1,25-dihydroxyvitamin D was elevated in 37 patients (52%). Despite similarly low dietary calcium intake among patients with normal and elevated levels of 1,25-dihydroxyvitamin D (477 ± 50 mg/day vs. 533 ± 40 mg/day), patients with elevated levels of 1,25-dihydroxyvitamin D had higher parathyroid hormone levels by immunoradiometric assay (136 ± 11 pg/mL vs. 97 ± 10 pg/mL; P <.05), and urinary calcium (7.4 ± 0.05 mmol/day vs. 5.1 ± 0.05 mmol/day; P <.05 or 0.82 ± 0.04 mmol/mmol creatinine vs. 0.56 ± 0.04 mmol/mmol creatinine; P <.01).

CONCLUSIONS: The data suggest that patients with normal levels of 1,25-dihydroxyvitamin D can liberalize their calcium intake without adverse consequences. However those with elevated levels of 1,25-dihydroxyvitamin D are advised to be more restrictive in order to prevent hypercalciuria.