Studies on the relationship between vitamin D3 status and urinary excretion of calcium in healthy subjects: Effects of increased levels of 25-hydroxyvitamin D₃

Abstract

The metabolic consequences of a rapid increase in vitamin D status in healthy subjects were investigated. Circulating levels of 25-hydroxyvitamin D₃ were increased by 224% in 12 healthy men by giving oral vitamin D₃ for 7 weeks and by 200% in 15 healthy women by UVB irradiation for 7 weeks. No statistically significant effects on the serum levels of calcium, phosphate, creatinine, urate, albumin, PTH, basal urinary excretion of calcium, fasting urinary excretion of cAMP, or urinary excretion of calcium after calcium load tests were observed with the unpaired t-test. With the paired t-test the small stimulatory effects (about 25%) on basal urinary excretion of calcium became statistically significant in both experiments. The ratio between calcium and creatinine in fasting urine was significantly elevated following UVB irradiation (from 0.11 ± 0.02 to 0.21 ± 0.04, p<0.025 unpaired t-test, p<0.02 paired t-test) but not after oral intake of vitamin D₃. The level of 1,25-dihydroxyvitamin D in serum was not affected to a statistically significant degree by oral vitamin D₃, whereas there was a slight decrease from 48 ± 3 to 39 ± 3 pmol/1 following UVB irradiation. It is concluded that an increase in the concentration of 25-hydroxyvitamin D₃ up to about 125 nmol/l has small and negligible effects on calcium homeostasis in healthy subjects. This finding is discussed in relation to our previous finding that hypercalciuric renal stone formers have elevated serum levels of 25-hydroxyvitamin D₃ as compared with normocalciuric stone formers and healthy subjects.

Key Words:

• calcium load test
• 25-hydroxyvitamin D₃
• idiopathic hypercalciuria
• parathyroid hormone
• UV irradiation
• vitamin D status