Toxic levels of vitamin D were rarely seen in people taking supplements, according to Mayo Clinic researchers. Findings from the study are published in the Mayo Clinic Proceedings.

Scientists from the Mayo Clinic evaluated data between 2002–2011 from patients in the Rochester Epidemiology Project. They set out to learn more about the health of those with high vitamin D levels (>50ng/mL) in light of increased use of supplements. Of the 20,308 total measurements, 8% had levels >50ng/mL, and <1% had levels >100ng/mL. Even among those with high levels of vitamin D, there was no evidence of increased risk of hypercalcemia. In addition, the team found that women aged >65 years had the highest risk of having vitamin D levels >50ng/mL. During the 10-year study period, the incidence of high vitamin D levels (>50ng/mL) increased from 9 per 100,000 at the start of the study to 233 per 100,000 by the end.

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Overall, only one case was identified as true acute vitamin D toxicity at a level of 364ng/mL. The individual had been taking 50,000 international units (IU) of vitamin D supplements every day for more than three months, as well as calcium supplements. 

Physicians should communicate with their patients about which vitamin D supplements they are taking because even over-the-counter capsules can contain as much as 50,000 IU. Researchers concluded that vitamin D toxicity is a rare occurrence and is usually due to intentional or inadvertent intake of very high doses.

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