Many of the patients had already been taking vitamin D supplements; they had baseline concentrations of 100 nmol/L of 25(OH)D. The patients also received about 1,000 mg/day of calcium phosphate.
“So if you've got a cohort that's going to be susceptible to vitamin D toxicity in a phase I study, this is it,” he said.
No events of hypercalcemia and no change in urinary calcium levels have occurred. The investigators have received funding to extend the study.
With the results of his study and after a review of the literature, Dr. Vieth concluded that about 1 mg/day or 40,000 IU/day of vitamin D3 might be the threshold at which toxicity begins.
But the actual UL for vitamin D should be about 10,000 IU/day, or 250 mcg/day, Dr. Vieth suggested. This is not an RDA, but it is a level not likely to cause harm in most individuals.
In support of a UL of 10,000 IU/day, Dr. Vieth noted that a colleague has used 1,250 mcg/day vitamin D3 “for some time,” and induced 25(OH)D levels of up to 643 nmol/L without hypercalcemia. Others also have used vitamin D several times higher than the current upper limit of 2,000 IU/day. Sunshine also can safely provide a dose of 10,000 IU/day to an adult, he said.