You may be nervous about dosing over a few hundred IU of vitamin D. If so, you aren’t alone. Vitamin D toxicity worries many clinicians as people become more and more cavalier about taking it. However! The dose makes the poison, as Paracelsus warned us hundreds of years ago, and as Dr. Vieth (a vitamin D researcher in Toronto) paraphrases “if it has any effect on your body, it can become poisonous”. So the question should not be “is vitamin D toxic” but “at what dose does vitamin D become toxic”. This is difficult to study, for ethical reasons. But there is some good data nonetheless.

First, what does it mean to have vitamin D toxicity?
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Vitamin D, among hundreds of other functions, increases the absorption of calcium in the body. Calcium in too high levels in the blood stream is toxic, and can be fatal. This abnormally high level of calcium in the blood is “hypercalcemia” (hyper=high, calc = calcium, emia= blood) in medical (ie pretentious!) language. High levels of calcium leads to kidney failure, abdominal pain, (Vieth says many cases of vitamin D toxicity are originally though to be a stomach bug, ie “gastroenteritis”), confusion (you aren’t sure where you are, what’s going on, etc), increased thirst, increased urination, nausea, vomiting, constipation. Before your blood calcium increases, the amount of calcium in your urine increases. This is because your kidneys do not want you to have too much calcium in your blood (smart guys) so they increase the amount of calcium they excrete into your urine. This help keep the level of calcium in your blood down, until the level of calcium is so high the kidneys can’t handle it anymore and the level increases in your blood. So the “canary in the coal mine” as Vieth puts it, is hypercalciuria (hyper=high, calc = calcium, uria = urine).

So at what dose do you start to see high levels of calcium? In the blood, the dose is about 40,000 IU a day for weeks or months on end. Rarely has calcium levels in the urine been measured, but Vieth studied multiple sclerosis patients’ urine calcium levels while increasing their dose of vitamin D from 28,000 IU to 250,000 IU! He found only slight increases in urine calcium, but these doses were only continued for a few weeks, not for months (he also was very carefully monitoring urine and serum calcium, please do not do this at home!!). So “high” doses are clearly relatively safe.

Vieth in one paper outlined the mechanisms of vitamin D toxicity and elucidates that there is a protein that binds to excess vitamin D as way of preventing toxicity. This protein doesn’t get “used up” until high doses like 50,000 IU are given. From sun exposure, levels of the equivalent of 4,000 IU a day are seen. Therefore he considers a “physiologic” (ie “normal for humans”) dose to be between 4,000-10,000 IU for healthy people (a particularly long exposure or short exposure with a high level of UVB can create 10,000 IU, which I think is why 10,000 IU is included in his idea of a physiologic dose (1,2)).
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So 4,000 IU is generally considered the “physiologic” dose for most people. But this differs based on specific populations – skin color, pregnancy, etc. For details on "special" populations and more details on dosing for "average" adults, read my next post.