Vitamin D toxicity

Calcium metabolism is very tightly controlled by several hormonal systems responding to blood calcium levels, as to much or too little have severe effects. The active form of vitamin D is a hormone the 1 and 25 di hydroxy form D3 is formed in the kidney from VitD2. This conversion is tightly regulated by calcium levels, so that excess D2 is not converted to VitD3. This protects from excess D2 and excess calcium in the diet by not stimulating Calcium absorption from the bowel.
This protective mechanism is bypassed by taking Vit D3, which is not found in food.
So large and even massive doses of D2 are not toxic, but excess D3 can be. It is confusing, and even in this paper on VitD, it is often unclear what forms of Vit D are being discussed.

It is obviously important to know what form of VitD is in your supplements!

This discovery of Dudenkov et al1is logical because the body tightly controls the conversion of 25(OH)D2 to 1,25-dihydroxyvitamin D3, which is responsible for regulating calcium metabolism by enhancing intestinal calcium absorption and mobilizing calcium from the skeleton. A study of healthy adults who received 1000 IU of vitaminD3 daily for 11 weeks and who raised their blood levels of 25(OH)D from 22.213 to 33.67.5ng/mL reported that blood levels of 1,25-dihydroxyvitamin D did not change: baseline,35.413.0 pg/mL; 11 weeks later, 34.313.8pg/mL.1

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