1. The term, Vitamin, is misunderstood and abused. It is essentially a legal term, dictated un-Constitutionally by the FDA. Scientifically, a vitamin is an organic substance that the body must have, cannot make, and without which it will develop a deficiency disease.
2. The Essential Fatty Acids (2: LA/w6 and ALA/w3) are functionally vitamins, but the FDA refused to recognize them as such (unofficially called Vitamin F in old literature). In contrast, Vitamin D3 [Cholecalciferol] is not a vitamin; it is a cortico-steroid hormone made in the skin in response to full-spectrum sunlight.
3. Human beings in modern society now cover most of their skin when outdoors, while living indoors, with dim, partial-spectrum lighting. Thus, most human beings in urban environments now are by definition D-deficient.
4. Endocrinologists have been prescribing up to 50,000 IU of D3 since at least the early 1990s, and have achieved excellent results, especially in mitigating, even reversing, symptoms of Diabetes.
5. The main proviso is to avoid any Calcium supplementation when taking 40,000 IU or more of D on a regular basis, since the body becomes hyper-efficient in utilizing the macro-mineral.
Thanks for that.