Relationship between urinary calcium and calcium intake during calcitriol administration

Area of Science:

• Endocrinology
• Nephrology
• Nutritional Science

Background:

• Hypercalciuria is a known side effect of 1,25(OH)2D3 (calcitriol) administration in individuals with normal renal function.
• This effect is thought to be linked to increased dietary calcium absorption and potentially enhanced bone resorption.

Purpose of the Study:

• To investigate the relationship between urinary calcium excretion and dietary calcium intake during calcitriol treatment.
• To assess the impact of calcitriol on bone resorption markers and serum calcium levels.

Main Methods:

• Twelve metabolic balance studies were conducted in healthy volunteers.
• Participants received calcitriol (0.5 microgram every 12 hours) after an initial 10-day period on either 350 mg/day or 1550 mg/day of dietary calcium.
• Dietary calcium was subsequently altered in 5-day periods to 650, 950, and 1550 mg/day (Group A) or decreased from 1550 to 950, 650, and 350 mg/day (Group B).

Main Results:

• Urinary calcium was less than calcium intake on the lowest dietary calcium level (350 mg/day) during calcitriol treatment.
• Urinary calcium correlated with dietary calcium (r = 0.60) up to 950 mg/day during calcitriol administration.
• Calcitriol reduced serum iPTH by 18-25% and urinary hydroxyproline by 11-19%, indicating decreased bone resorption. Postabsorptive serum calcium increased by 0.35 mg/dl.

Conclusions:

• Urinary calcium excretion during calcitriol treatment is significantly influenced by dietary calcium intake in individuals with normal renal function.
• Calcitriol appears to suppress bone resorption, as evidenced by reduced iPTH and urinary hydroxyproline levels.
• The study highlights the interplay between vitamin D, dietary calcium, and renal calcium handling.