How safe are high dose Vitamin D3 (vD3) supplements?
In this study on over 3,600 study participants ~25% of the participants did not take any vD3 supplements, while 47% took up to 2000IU per day, 1.8% took over 10,000IU and the majority took up to 5000IU per day. Despite taking as much as 10,000IU Vitamin D3 per day, no participant in the study demonstrated vD3 levels in the "toxic" range of >200ng/ml. For those severely deficient in vD3, each 1000IU of increased supplementation resulted in an increase of 10ng/ml in vD3 blood levels. Those with existing blood levels above 30ng/ml demonstrated an 8ng/ml increase in blood level for every 1000IU given, and those with existing levels above 50ng/ml showed an increase of only 5ng/ml for every 1000IU of Vitamin D3. This study demonstrates how difficult it would be to take enough vD3 to reach toxic levels. As blood levels increase, absorption decreases.
Perhaps the most surprising finding was the dose of vD3 necessary to ensure that 95.7% of the population achieved levels of >40ng/ml was 9,600IU. Although such doses are an order of magnitude higher than those currently recommended, these calculated daily intakes are of the same magnitude as produced by a single dose of UV-B radiation achieved during a few minutes of solar UVB exposure near noon in midsummer, assuming nearly complete skin exposure.
Garland et al. ANTICANCER RESEARCH 31: 607-612 (2011)
"These results demonstrate the safety of taking 5,000U or 10,0000U of vD3 per day and the importance of measuring blood levels of vD3 to ensure that the dose taken is adequate to achieve optimal blood levels."