Vitamin D

vitamin

Best with a mealTake with food

What is it

Vitamin D is a fat-soluble vitamin that the body uses to absorb calcium and maintain bone health. It is unusual among vitamins because the body can make it when skin is exposed to sunlight.

How it works

Vitamin D from food, supplements, or sun exposure is biologically inactive when it enters the body. The liver converts it to 25-hydroxyvitamin D — the form measured on blood tests — and the kidneys then activate it to 1,25-dihydroxyvitamin D (calcitriol). Calcitriol acts like a hormone, binding to vitamin D receptors found in nearly every tissue. Its best-known role is promoting calcium absorption in the gut and keeping calcium and phosphate levels high enough for bone mineralization. Without enough vitamin D, bones become thin, brittle, or misshapen. It is also involved in immune function, neuromuscular function, and cell growth regulation, with vitamin D receptors present in many tissues outside the skeleton.

Evidence for 6 uses

AI-assisted evidence assessment — talk to your doctor before relying on any single supplement.

Bone health and osteoporosis prevention

Grade A

Strong evidence

Vitamin D combined with calcium reduces fracture risk in older adults, particularly those with low baseline vitamin D status. This is the most established benefit, supported by decades of trials and used as the basis for the RDA.

Prevention of rickets and osteomalacia

Grade A

Strong evidence

Vitamin D supplementation prevents and treats rickets in children and osteomalacia in adults. Both are diseases of impaired bone mineralization caused directly by vitamin D deficiency.

Falls in older adults

Grade B

Good evidence

Several trials show modest reductions in fall risk among older adults supplementing with 700 to 1,000 IU per day, likely through effects on muscle strength. Results are inconsistent at very low or very high doses.

Respiratory infections

Grade C

Moderate evidence

Meta-analyses suggest a small protective effect of daily or weekly supplementation against acute respiratory tract infections, with the largest benefit in people who started out deficient. The effect size is modest.

Cardiovascular disease prevention

Grade D

Mixed evidence

Despite earlier observational links, large trials including VITAL have not shown that vitamin D supplements reduce heart attacks, strokes, or cardiovascular death in generally healthy adults.

Cancer prevention

Grade D

Mixed evidence

Trials including VITAL have not shown reductions in total cancer incidence, though some analyses suggest a possible reduction in cancer mortality. Evidence is mixed and effect sizes are small.

2 commercial forms

Vitamin D3 (cholecalciferol)

preferred form, raises blood levels more effectively

D3 is the form produced by skin in response to sunlight and is most commonly used in supplements. Studies show it raises and maintains serum 25(OH)D levels more effectively than D2 at equivalent doses.

Vitamin D2 (ergocalciferol)

plant-derived, slightly less effective

D2 comes from fungi and yeast and is used in some prescription products and vegan supplements. It works but tends to raise blood levels less than D3 and may decline faster between doses.

Dosage

The RDA is 600 IU (15 mcg) for adults up to age 70 and 800 IU (20 mcg) for adults over 70. Most supplements provide 1,000 to 5,000 IU per serving. The tolerable upper intake level for adults is 4,000 IU (100 mcg) per day from all sources combined. People with documented deficiency are sometimes prescribed much higher short-term doses, but that should be done under medical supervision with blood testing.

When and how to take it

Vitamin D is fat-soluble, so it absorbs best when taken with a meal that contains some dietary fat. A standard meal usually provides enough fat for good absorption. Consistency matters more than time of day — pick a meal you eat reliably and stick with it. Unlike water-soluble vitamins, vitamin D stores in body fat and the liver, so a daily dose is not strictly required. Some people take a larger dose once or twice a week with equivalent results, though daily dosing makes a routine easier to maintain.

Food sources

Food	Amount	%DV
Cod liver oil, 1 Tbsp	1,360 IU (34 mcg)	170%
Salmon (sockeye), 3 oz cooked	570 IU (14.2 mcg)	71%
Swordfish, 3 oz cooked	566 IU (14.1 mcg)	71%
Tuna (canned in water), 3 oz	40 IU (1 mcg)	5%
Sardines (canned in oil), 2 sardines	46 IU (1.2 mcg)	6%
Milk (fortified), 1 cup	120 IU (3 mcg)	15%
Orange juice (fortified), 1 cup	100 IU (2.5 mcg)	13%
Egg yolk, 1 large	44 IU (1.1 mcg)	6%
Mushrooms (UV-exposed), 1/2 cup	366 IU (9.2 mcg)	46%

Safety

Vitamin D from sun exposure or food does not cause toxicity. Excessive supplementation, however, can raise blood calcium to dangerous levels — a condition called hypercalcemia. Symptoms include nausea, vomiting, weakness, frequent urination, and in severe cases kidney stones, kidney damage, or heart rhythm problems. Toxicity is almost always the result of long-term intake above 10,000 IU per day, though sensitivity varies by individual. Most healthy adults tolerate supplemental doses up to 4,000 IU per day without issues. If you take more than that long-term, periodic blood testing of serum 25(OH)D and calcium is a reasonable safeguard.

Who should be cautious

People with kidney disease, hyperparathyroidism, sarcoidosis, lymphoma, or other granulomatous diseases should not take vitamin D supplements without medical guidance, since these conditions can cause abnormal vitamin D activation and hypercalcemia. Pregnant and breastfeeding women have the same RDA as other adults (600 IU), and exclusively breastfed infants are typically given 400 IU daily because breast milk alone does not provide enough.

Interactions

Vitamin D can interact with several medications. Steroids like prednisone can reduce calcium absorption and impair vitamin D metabolism. The weight-loss drug orlistat and bile acid sequestrants reduce vitamin D absorption — take them several hours apart. Thiazide diuretics combined with high-dose vitamin D may raise calcium levels excessively. Statins and certain anticonvulsants can also affect vitamin D status.

Frequently asked questions

How much vitamin D should I take per day?⌄

Most adults do well on 1,000 to 2,000 IU per day from supplements, which complements typical food and sun exposure. People with documented deficiency may need more under a doctor's supervision.

Should I take vitamin D with food?⌄

Yes. Vitamin D absorbs significantly better when taken with a meal that contains some fat. The fat does not need to be large — a regular meal works fine.

What is the difference between vitamin D2 and D3?⌄

D3 (cholecalciferol) comes from animal sources or sunlight on skin, while D2 (ergocalciferol) comes from plants and fungi. D3 generally raises blood levels more effectively than D2.

Can I get enough vitamin D from sunlight?⌄

Maybe, depending on where you live, your skin tone, and how much time you spend outside. People at higher latitudes, with darker skin, or who cover up when outside often need supplements, especially in winter.

How do I know if I am deficient?⌄

A blood test measuring serum 25-hydroxyvitamin D is the standard. Levels below 30 nmol/L (12 ng/mL) indicate deficiency; 50 nmol/L (20 ng/mL) or higher is considered sufficient for most people.

References

NIH ODS Vitamin D Health Professional Fact Sheet — NIH Office of Dietary Supplements link

Track Vitamin D with Pilora

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Disclaimer: These statements have not been evaluated by the FDA. This page is educational, not a substitute for personalized medical advice. Evidence grades are AI-assisted assessments — talk to your doctor before starting any new supplement, especially if you're pregnant, breastfeeding, on medications, or managing a chronic condition.