Vitamin D3

What Vitamin D3 does

Vitamin D is a fat-soluble vitamin that the body must convert through two hydroxylations into its active form; it is required to promote calcium absorption in the gut and to maintain adequate serum calcium and phosphate concentrations for normal bone mineralization, and it is needed for bone growth and remodeling. Without sufficient vitamin D, bones can become thin, brittle, or misshapen; vitamin D sufficiency prevents rickets in children and osteomalacia in adults, and together with calcium it helps protect older adults from osteoporosis. The NIH notes vitamin D has other roles in the body, including reduction of inflammation and modulation of processes such as cell growth, neuromuscular and immune function, and glucose metabolism, with many genes involved in cell proliferation, differentiation, and apoptosis modulated in part by vitamin D. The NIH also cautions that the extent to which serum 25(OH)D levels serve as a biomarker of effect on health outcomes is not clear and that optimal concentrations for general health have not been established. — per the NIH Office of Dietary Supplements; not medical advice.

Forms & absorption

In foods and dietary supplements, vitamin D has two main forms, D2 (ergocalciferol) and D3 (cholecalciferol), which differ chemically only in their side-chain structures; both are well absorbed in the small intestine, and the presence of dietary fat enhances but is not required for absorption. According to the NIH, both D2 and D3 raise serum 25(OH)D levels and appear equally able to cure rickets, but most evidence indicates vitamin D3 increases serum 25(OH)D to a greater extent and maintains those higher levels longer than D2. D2 is manufactured by UV irradiation of ergosterol in yeast and D3 typically from 7-dehydrocholesterol in sheep's-wool lanolin, with an animal-free D3 sourced from lichen also available; the NIH adds that the 25(OH)D3 form is about three to five times as potent per microgram as vitamin D3, though no 25(OH)D3 supplements appear to be on the U.S. market.

Food sources of Vitamin D3

Few foods naturally contain vitamin D: the flesh of fatty fish (such as trout, salmon, tuna, and mackerel) and fish liver oils are among the best sources, while beef liver, egg yolks, and cheese have small amounts, and mushrooms provide variable amounts of vitamin D2. In American diets, fortified foods (for example, milk, ready-to-eat breakfast cereals, and some orange juice, yogurt, and margarine) provide most of the vitamin D — per the NIH Office of Dietary Supplements.

Vitamin D3 deficiency — who's at risk

When and how to take Vitamin D3

Vitamin D3 is fat-soluble, so take it with the largest meal of the day — one that contains some fat — for the best absorption. Time of day does not matter much; some people who take it late report lighter sleep, so if that is you, take it earlier.

Can you take Vitamin D3 with other supplements?

It pairs naturally with vitamin K2 and magnesium, both of which help the body use vitamin D. There is no need to space it from most other supplements.

Vitamin D3 side effects

Very well tolerated at normal doses. Problems come only from long-term megadoses, which can raise blood calcium — stay within the amount on the label unless a blood test and clinician say otherwise.