Effects of low-calcium diet on urine calcium excretion, parathyroid function and serum 1,25(OH)2D3 levels in patients with idiopathic hypercalciuria and in normal subjects.

We have used a low-calcium diet providing only 2 mg/kg (body weight) per 24 hours of calcium to distinguish between "renal" and "absorptive" idiopathic hypercalciuria. Sixteen of 27 hypercalciuric subjects excreted calcium in excess of intake during days seven, eight and nine of he diet, suggesting some element of renal hypercalciuria; however, all patients had low or normal serum PTH and urine cAMP levels. In general, fasting urine calcium was elevated in these 16 subjects and normal in the remaining 11, who conserved calcium more normally. SErum 1,25(OH)2D3 levels were the same in patients and normal subjects, even though PTH levels of the patients were below those of he normal subjects. Urine magnesium excretion and phosphorus excretion were both increased in the patients who excreted calcium in excess of intake. Our findings suggest that renal and absorptive hypercalciuria may not be distinct entities but rather the two extremes of a continuum of behavior. A uniform elevation of intestinal calcium absorption and a variable defect of renal calcium reabsorption could explain our results far better than the hypothesis of distinct absorptive and renal forms of hypercalciuria.