What we know as Vitamin D (cholecalciferol D3 and ergocalciferol D2) can be ingested from diet and/or supplements and can be synthesized from cholesterol when sunshine is adequate. While called a vitamin, I believe that in the future it will be reclassified as a hormone like substance. It is currently the subject of much new research on infectious disease, auto immune disorders, and cancer - and of course bone health. Toxicity is extremely rare and has been documented only in does in excess of 50,000 IU daily over several months resulting in serum levels over 150ng/mL.
Currently for those living north of Denver, Colorado, we recommend 5000 IU daily during grey months and every other day in the summer.
When testing serum levels, keep in mind your highest levels will be at the end of the summer and early fall. Your lowest levels will be in the late winter and early spring. If there is any question, I recommend testing in August and February.
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