Vitamin D3

Vitamin D3 is cholecalciferol, one of two main vitamin D forms sold in foods and supplements. The NIH Office of Dietary Supplements (ODS) fact sheet states that vitamin D from sun, food, and supplements is biologically inert until the liver makes 25-hydroxyvitamin D and the kidney makes 1,25-dihydroxyvitamin D. Serum 25(OH)D is the main status marker ODS uses.

What is vitamin D3?

ODS says vitamin D in foods and supplements has two main forms: D2 (ergocalciferol) and D3 (cholecalciferol). They differ only in side-chain structure. Both are absorbed in the small intestine. D3 in supplements is typically made by irradiating 7-dehydrocholesterol from lanolin; an animal-free D3 from lichen also exists.

Skin exposed to UVB converts 7-dehydrocholesterol to previtamin D3, which becomes vitamin D3. ODS notes that glass blocks UVB, so indoor sun through a window does not make vitamin D.

What is vitamin D3 used for in the body?

ODS frames the RDA around bone health and normal calcium metabolism in healthy people. Vitamin D increases calcium absorption in the gut. This page reports those ODS statements. It does not add disease claims.

How much vitamin D3 do adults need?

ODS lists adult RDAs of 15 mcg (600 IU) a day from age 1 through 70, and 20 mcg (800 IU) after age 70. One microgram equals 40 IU. Those RDAs assume minimal sun exposure.

The adult Tolerable Upper Intake Level is 100 mcg (4,000 IU) a day from age 9 upward. ODS says toxicity is almost always from excessive supplement intakes and can cause high blood calcium.

NHANES figures on the same fact sheet show most people get far less than the Estimated Average Requirement from food alone, and that 3.2 percent of U.S. adults in 2013–2014 took 100 mcg (4,000 IU) or more.

How does vitamin D3 differ from vitamin D2?

ODS says both raise serum 25(OH)D and both are well absorbed. Most evidence it cites indicates D3 increases 25(OH)D to a greater extent and maintains the higher level longer than D2.

When is vitamin D3 low?

ODS, citing the Food and Nutrition Board, says people are at risk of deficiency at serum 25(OH)D below 30 nmol/L (12 ng/mL). Levels of 50 nmol/L (20 ng/mL) or more are sufficient for most people on that committee’s definition. Some people are potentially at risk of inadequacy between 30 and 50 nmol/L (12 to 20 ng/mL). Circulating 1,25(OH)2D is a poor status marker because it is tightly regulated.

Those are laboratory thresholds from the fact sheet, not a self-test protocol.