Incidence of vitamin D levels greater than 50 ng/mL has increased significantly during the past decade; however, acute vitamin D toxicity remains rare, according to research in Mayo Clinic Proceedings.
In a retrospective, population-based study of adults in Olmsted County, Minnesota, researchers found an increased use of vitamin D supplements over 10 years but only one case of clinical toxicity associated with the highest observed vitamin D value of 364 ng/mL.
“We found that even in those with high levels of vitamin D over 50 ng/mL, there was not an increased risk of hypercalcemia, or elevated serum calcium, with increasing levels of vitamin D,” Thomas D. Thacher, MD, a family medicine professor at Mayo Clinic, said in a press release.
Thomas D. Thacher
Thacher and colleagues analyzed data from patients in the Rochester Epidemiology Project in Olmsted County, the home of the Mayo Clinic, between 2002 and 2011. The NIH-funded medical records pool, which includes 95% of the Olmsted County population, is one of the few places worldwide where scientists can study an entire geographic population to identify health trends, according to researchers. Researchers reviewed medical records and laboratory data of participants with vitamin D levels greater than 50 ng/mL and hypercalcemia, within 3 months of the index date, to determine whether the hypercalcemia could be attributed to vitamin D or another cause.
Of 20,308 vitamin D tests performed between 2002 and 2011, 1,714 (8%) participants had a vitamin D value greater than 50 ng/mL and had authorized record abstraction. Of that group, only four had vitamin D concentrations associated temporally with hypercalcemia — all adult women taking 50,000 IU of vitamin D at least weekly for a minimum of 1 month. Of the four, there was one case of clinical vitamin D toxicity; a 51-year-old woman with a vitamin D level of 364 ng/mL and serum total calcium of 17.5 mg/dL. The woman was taking 50,000 IU vitamin D daily and 3,000 mg calcium at least once daily for 3 months, according to researchers, and presented with stage 3 chronic kidney disease and symptoms of severe hypercalcemia, including fatigue, anorexia, weight loss, nausea, vomiting and confusion.
Researchers also found that women older than 65 years were at the highest risk of having vitamin D levels greater than 50 ng/mL. In addition, incidence of high vitamin D levels greater than 50 ng/mL increased during the 10-year period of the study, from nine per 100,000 people at the start of the study up to 233 per 100,000 by the end.
The Institute of Medicine-recommended upper limit of vitamin D supplementation for people with low or deficient levels is 4,000 IU a day.
“It’s important for doctors to ask their patients about the doses of vitamin D supplements that they are using because capsules containing as much as 50,000 IU of vitamin D are available without prescription,” Thacher told Endocrine Today. “If taken on a daily basis, that amount could lead to toxicity.”
More research is needed regarding the long-term adverse effects of vitamin D levels greater than 50 ng/mL, Thacher said.
“Some observational studies found that increased risk of fractures, some cancers and death have been associated with vitamin D levels over 50 ng/mL, but these associations do not indicate that vitamin D caused these outcomes,” Thacher said. – by Regina Schaffer
Disclosure: The researchers report no relevant financial disclosures.