Made in skin with UVB light and found in few foods. Its main job is helping the gut absorb calcium.
How it works, step by step
UVB in skin converts 7-dehydrocholesterol into vitamin D3.
The liver converts it to 25-hydroxyvitamin D (the form measured in blood tests).
The kidneys convert that to active calcitriol.
Calcitriol tells the gut to absorb more calcium.
What does Vitamin D do?
The active form, calcitriol, acts like a hormone, increasing calcium and phosphorus absorption from the gut and supporting bone mineralization. Also involved in muscle and immune function.
Where does Vitamin D act in the body?
Small intestine, bone, kidneys, muscle, immune cells.
Which foods contain Vitamin D?
Skin synthesis from UVB; fatty fish (salmon, trout, sardines), egg yolks, UV-exposed mushrooms, and fortified foods like some milks, plant drinks and cereals.
How much Vitamin D do adults need?
RDA: 15 mcg (600 IU) for ages 1–70; 20 mcg (800 IU) over 70. Upper limit: 100 mcg (4,000 IU) for adults.
What happens with too little Vitamin D?
Long-term deficiency can cause rickets in children and osteomalacia (soft bones) in adults. Status can only be judged with a blood test (25-hydroxyvitamin D).
Can you get too much Vitamin D?
Toxicity almost always comes from high-dose supplements, not sun or food. It raises blood calcium, which can cause nausea, kidney problems and heart rhythm issues.
What does Vitamin D interact with?
Works with calcium and phosphorus. Some medicines (e.g., certain steroids and anticonvulsants) affect vitamin D metabolism.
What is known and unknown about Vitamin D?
Role in bone health is established. Large trials (e.g., VITAL) did not show supplements prevent cancer or heart disease in generally healthy adults.
Evidence & sources
The RDA for vitamin D is 600 IU (15 mcg) for ages 1–70 and 800 IU (20 mcg) above 70; the adult UL is 4,000 IU. Evidence: regulatory. Known: Set by the Institute of Medicine (2011) assuming minimal sun exposure. Unknown: Ideal blood levels are debated among expert bodies. Source: NIH Office of Dietary Supplements, Vitamin D — Health Professional Fact Sheet (Current revision).
Vitamin D supplements did not reduce cancer or cardiovascular events in a large trial of healthy adults. Evidence: strong. Known: The VITAL randomized trial (25,871 participants, 2,000 IU/day, median 5.3 years). Unknown: Effects in people with low starting levels are less clear. Source: Manson et al., New England Journal of Medicine, Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease (2019).