How is vitamin D made and activated?
When skin is exposed to UVB light, vitamin D3 is made from a cholesterol derivative. You can also get it from food (fatty fish, egg yolks, fortified milk and cereals) or supplements. The liver converts vitamin D into 25(OH)D, and the kidneys turn that into the active form, 1,25(OH)2D, which helps the gut absorb calcium and phosphorus.
How do you read a vitamin D blood test?
Vitamin D status is measured by blood 25(OH)D. The US Institute of Medicine (IOM, 2011) concluded that 20 ng/mL (50 nmol/L) or higher is enough for bone health in most people, and below 12 ng/mL (30 nmol/L) signals a risk of deficiency. Some groups, like the Endocrine Society, have favored 30 ng/mL or higher, so cutoffs aren't fully agreed upon. Severe deficiency causes rickets in children and osteomalacia in adults.
Large supplement trials
- VITAL (2019): 2,000 IU a day in 25,871 adults aged 50+ didn't reduce cancer or major cardiovascular events.
- D2d (2019): 4,000 IU a day in 2,423 people with prediabetes raised blood levels substantially, but the drop in diabetes wasn't statistically significant (HR 0.88).
- Fractures: in trials and meta-analyses of the general adult population, vitamin D alone didn't clearly reduce fractures.
In observational studies, people with low levels had higher disease risk, but randomized controlled trials mostly didn't confirm that supplementing lowers that risk. One interpretation is that low vitamin D may be a marker of poor health rather than its cause.
Cautions
Because vitamin D is fat-soluble, taking too much for a long time can cause high blood calcium and kidney stones. If you've been diagnosed with a deficiency or are being treated for osteoporosis, supplement as your doctor advises, and avoid taking high-dose products long term on your own.
This article is general health information and is not a substitute for diagnosis or treatment. If symptoms persist, see a doctor.
FAQ
- How much vitamin D do adults need?
- US recommendations are 600 IU a day for adults up to age 70 and 800 IU after that; the upper limit for most adults is 4,000 IU a day unless a doctor prescribes more.
Sources reviewed: 2026-10-25
References
- Institute of Medicine. (2011). Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: National Academies Press.
- Manson, J. E., et al. (2019). Vitamin D supplements and prevention of cancer and cardiovascular disease. New England Journal of Medicine, 380(1), 33–44.
- Pittas, A. G., et al. (2019). Vitamin D supplementation and prevention of type 2 diabetes. New England Journal of Medicine, 381(6), 520–530.