Vitamin D: The Endocrine Society’s Clinical Practice Guideline

Evaluation, Treatment, and Prevention of Vitamin D Deficiency: an Endocrine Society Clinical Practice Guideline,The Journal of Clinical Endocrinology & Metabolism, Volume 96, Issue 7, 1 July 2011

Bits (text in red are my notations):

We suggest that the maintenance tolerable upper limits (UL) of vitamin D, which is not to be exceeded without medical supervision, should be … 4000 IU/d for everyone over 8 yr.

25(OH)D is the major circulating form of vitamin D, with a circulating half-life of 2–3 wk. The circulating half-life of 1,25(OH)2D is approximately 4 h. It circulates at 1000 times lower concentration than 25(OH)D, and the blood level is tightly regulated.
25(OH)D —> biologically inert, what lab tests measure
1,25(OH)2D —> active form of vitamin D. 4 hours is not long. This is a powerful molecule.

The major source of vitamin D for children and adults is exposure to natural sunlight. Very few foods naturally contain or are fortified with vitamin D. Thus, the major cause of vitamin D deficiency is inadequate exposure to sunlight.

Wearing a sunscreen with a sun protection factor of 30 reduces vitamin D synthesis in the skin by more than 95% (39).

People with a naturally dark skin tone have natural sun protection and require at least three to five times longer exposure to make the same amount of vitamin D as a person with a white skin tone.

There is an inverse association of serum 25(OH)D and body mass index (BMI) greater than 30 kg/m2, and thus, obesity is associated with vitamin D deficiency.

Vitamin D produced in the skin may last at least twice as long in the blood compared with ingested vitamin D

When an adult wearing a bathing suit is exposed to one minimal erythemal dose of UV radiation (a slight pinkness to the skin 24 h after exposure), the amount of vitamin D produced is equivalent to ingesting between 10,000 and 25,000 IU.

Above and below latitudes of approximately 33°, vitamin D3 synthesis in the skin is very low or absent during most of the winter.
33 degrees north is approximately at the midpoint between the equator (0 degrees) and the Arctic Circle (66.6 degrees).

Few foods naturally contain vitamin D2 or vitamin D3 (Table 1).

Because most tissues and cells in the body have a vitamin D receptor and 1,25(OH)2D influences the expression levels along with other factors of up to one third of the human genome, it is not at all unexpected that a numerous of studies has demonstrated an association of vitamin D deficiency with increased risk of more than a dozen cancers, including colon, prostate, breast, and pancreas; autoimmune diseases, including both type 1 and type 2 diabetes, rheumatoid arthritis, Crohn’s disease, and multiple sclerosis; infectious diseases; and cardiovascular disease.
Wow.