Can you get too much vitamin D?
Too much vitamin D is genuinely harmful, but that risk only arises at blood levels far above the recommended values. Stick to the dosage on the packaging, and unsure about your level? Have it measured by your GP.
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Yes, too much vitamin D can be harmful, but only at truly high blood levels. Toxicity occurs when the vitamin D level in the blood rises above 750 nmol/L. The danger at that point is too much calcium in the blood (hypercalcaemia), which in serious cases can cause life-threatening complications. As a precaution, researchers use a safe upper limit of 250 nmol/L.
10,000 IU (international units, or 250 micrograms) per day is likely safe for virtually all adults over the long term. That sounds like a lot, but it is comparable to what your skin produces during full sun exposure. This is, however, an absolute upper limit, not a recommended daily amount.
The fact that more and more people are running into problems with vitamin D is mainly due to the over-the-counter availability of supplements. Without dosage guidance, people sometimes take far too high amounts. The number of reported poisonings worldwide has risen sharply as a result. Symptoms range from no complaints at all to an acute, life-threatening condition.
Animal research points to another possible side effect: prolonged moderately high intake may promote arterial calcification. Whether this risk also applies to humans has not yet been demonstrated, but in practice it is barely factored into risk assessments.
On the other side of the equation, a vitamin D deficiency affects more than one billion people worldwide, and such a deficiency is associated with cardiovascular disease, infectious diseases, autoimmune diseases and certain cancers. Note: these are associations, not always proven cause and effect. The message is therefore not that you should avoid vitamin D, but that more is always better does not hold true either.
Based on five claims with PMIDs 18689406, 32255467, 18290725, 34518055 and 28516265. The toxicity threshold and safe upper limit are based on clinical research; the atherosclerosis risk comes from animal research only; the rise in toxicity cases is based on case reports and observational research.