Evidence that calcium modulates circulating 25-hydroxyvitamin D in man

Area of Science:

• Endocrinology
• Nutritional Science
• Human Physiology

Background:

• 1,25-dihydroxyvitamin D3 (1,25(OH)2D) previously shown to prevent vitamin D-induced increases in serum 25-hydroxyvitamin D (25-OHD) in normal subjects.
• Understanding the interplay between vitamin D, calcium, and their metabolites is crucial for maintaining mineral homeostasis and bone health.

Purpose of the Study:

• To investigate whether increased calcium intake alters the serum 25-hydroxyvitamin D (25-OHD) response to vitamin D challenge in healthy individuals.
• To explore the impact of combined vitamin D and calcium administration on serum mineral levels and vitamin D metabolites.

Main Methods:

• Eight healthy subjects participated in a controlled study.
• Vitamin D (100,000 U/d for 4 days) was administered alone (control) and in combination with calcium (2,000 mg/d for 4 days).
• Serum levels of 25-OHD, calcium, ionized calcium, phosphorus, creatinine, and 1,25(OH)2D were measured.

Main Results:

• Vitamin D alone significantly increased serum 25-OHD by 133% (24 ng/ml).
• Combined vitamin D and calcium administration resulted in a significantly smaller increase in serum 25-OHD (63%, 12 ng/ml) compared to vitamin D alone (p < 0.02).
• Concurrent administration of vitamin D and calcium led to significant increases in serum calcium and ionized calcium, and a significant decrease in serum 1,25(OH)2D.

Conclusions:

• Increased calcium intake significantly attenuates the rise in serum 25-hydroxyvitamin D following vitamin D supplementation.
• This finding suggests that calcium plays a role in modulating the vitamin D endocrine system's response.
• Further research is warranted to elucidate the precise mechanisms underlying this interaction and its clinical implications.