Vitamin D3

vitaminVitamin D3 glucosiduronate

Take with food

What is it

Vitamin D3, also called cholecalciferol, is the form of vitamin D produced by skin in response to sunlight and the most common form used in supplements. It is converted in the liver and kidneys to the active hormone calcitriol.

How it works

Vitamin D3 is biologically inactive when first absorbed. The liver hydroxylates it to 25-hydroxyvitamin D — the storage form measured on blood tests — and the kidneys convert that to 1,25-dihydroxyvitamin D, the active hormone. The active form regulates calcium absorption, bone remodeling, and a wide range of cellular processes through vitamin D receptors found in most tissues. Compared to vitamin D2 (ergocalciferol), D3 binds more tightly to vitamin D binding protein in the blood, which gives it a longer half-life and raises serum 25(OH)D levels more effectively at the same dose. This is why most clinicians and supplement manufacturers prefer D3.

Evidence for 5 uses

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

Raising serum vitamin D levels

Grade A

Strong evidence

D3 raises and maintains serum 25(OH)D more effectively than D2 at equivalent doses, based on head-to-head trials. This is the most consistent finding distinguishing the two forms.

Bone density and fracture prevention

Grade A

Strong evidence

Vitamin D3 combined with calcium reduces fracture risk in older adults with low baseline vitamin D, supporting bone mineralization through enhanced calcium absorption.

Falls prevention in older adults

Grade B

Good evidence

Daily doses around 800 to 1,000 IU appear to reduce fall risk in older adults, likely through effects on muscle strength and balance.

Respiratory infection prevention

Grade C

Moderate evidence

Meta-analyses suggest a small protective effect against acute respiratory infections, especially in people who start out deficient. The benefit is modest in well-nourished populations.

Cardiovascular disease prevention

Grade D

Mixed evidence

Large trials including VITAL did not show that vitamin D3 supplementation prevents heart attacks, strokes, or cardiovascular death in generally healthy adults.

Dosage

The RDA for total vitamin D (D2 plus D3) is 600 IU (15 mcg) per day for adults up to age 70 and 800 IU (20 mcg) over age 70. Common supplemental doses are 1,000 to 5,000 IU of D3. The upper limit for adults is 4,000 IU (100 mcg) per day from all sources combined; higher doses should only be taken under medical supervision with periodic blood testing.

When and how to take it

Vitamin D3 is fat-soluble and absorbs best when taken with a meal that contains some fat. Most people do well taking it with breakfast or their largest meal of the day. Consistency matters more than the exact time, since stored vitamin D has a long half-life. Because D3 stores in body fat and liver, you do not have to dose daily — some people take weekly or even monthly doses with similar effect. Daily dosing is usually easier to remember.

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%
Egg yolk, 1 large	44 IU (1.1 mcg)	6%
Sardines (canned), 2 sardines	46 IU (1.2 mcg)	6%
Milk (fortified), 1 cup	120 IU (3 mcg)	15%
Beef liver, 3 oz cooked	42 IU (1 mcg)	5%

Safety

Vitamin D3 in typical supplemental doses is safe for most adults. Chronic high doses above 10,000 IU per day can lead to hypercalcemia, with symptoms including nausea, frequent urination, weakness, kidney stones, and in severe cases kidney damage or heart rhythm problems. There is no known toxicity from sunlight exposure or food sources. For most healthy adults, doses up to 4,000 IU per day are well tolerated long-term. If you take more, periodic monitoring of serum 25(OH)D and calcium is sensible.

Who should be cautious

People with kidney disease, hyperparathyroidism, sarcoidosis, or lymphoma should not supplement with vitamin D3 without medical guidance, as these conditions can cause abnormal vitamin D activation and hypercalcemia. Pregnant and breastfeeding women have the same RDA as other adults. Infants typically need 400 IU daily if exclusively breastfed.

Interactions

Steroids like prednisone interfere with vitamin D metabolism. The weight-loss drug orlistat and bile acid sequestrants reduce absorption — separate them from your D3 dose by several hours. Thiazide diuretics combined with high-dose vitamin D can raise calcium levels excessively. Certain anticonvulsants and statins may also affect vitamin D status.

Frequently asked questions

Is vitamin D3 better than vitamin D2?⌄

For raising and maintaining blood vitamin D levels, D3 is more effective dose-for-dose. Most experts recommend D3 unless you specifically need a vegan option.

How much vitamin D3 should I take?⌄

Most adults do well on 1,000 to 2,000 IU per day. People with documented deficiency may need more, ideally under medical supervision with blood testing.

Can vitamin D3 be vegan?⌄

Most D3 supplements are derived from lanolin (sheep's wool) and are not vegan. Vegan D3 made from lichen is available but more expensive.

Should I take D3 in the morning or evening?⌄

Either works. Take it with a meal containing fat for best absorption. Some people find evening doses interfere with sleep, but evidence for this is anecdotal.

Do I need vitamin D3 if I get sun exposure?⌄

Possibly not, but it depends on latitude, skin tone, sunscreen use, and lifestyle. People at northern latitudes, with darker skin, or who spend most time indoors often benefit from supplements.

References

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

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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.