Vitamin D3: What It Is, How Much to Take, and D3 vs D2 vs D3 + K2

Vitamin D3 (cholecalciferol) is the form of vitamin D your skin makes from sunlight and the form in most supplements. It helps the gut absorb calcium for bones. NIH sets the adult RDA at 600 IU (15 mcg) a day, or 800 IU (20 mcg) after age 70, with an upper limit of 4,000 IU (100 mcg). D3 raises blood vitamin D more than D2.

Vitamin D3 vs D2 vs D3 + K2: Comparison Table

Sources: NIH ODS vitamin D and vitamin K fact sheets; Tripkovic 2012; Manson 2019 (VITAL).

What is vitamin D3?

Vitamin D3 is one of the two main forms of vitamin D in food and supplements; the other is D2. They differ only in their side chain. Whatever the source, vitamin D is inactive until the liver converts it to 25-hydroxyvitamin D [25(OH)D], the form blood tests measure, and the kidney converts that to active calcitriol. Vitamin D lets the gut absorb calcium and keeps calcium and phosphate high enough for bone mineralization, preventing rickets in children and osteomalacia in adults (NIH ODS).

Vitamin D units: IU vs mcg conversion

Labels now list vitamin D in micrograms, often with IU in brackets. The conversion is fixed: 1 mcg = 40 IU. So 10 mcg = 400 IU, 25 mcg = 1,000 IU, 50 mcg = 2,000 IU, 125 mcg = 5,000 IU and 1,250 mcg = 50,000 IU. Blood tests use a different pair of units: 1 ng/mL = 2.5 nmol/L. A result of “50” means very different things depending on which unit your lab used: 50 nmol/L (20 ng/mL) is adequate; 50 ng/mL (125 nmol/L) is the level above which NIH says adverse effects become possible.

How much vitamin D3 should you take per day?

NIH ODS lists the RDA as 600 IU (15 mcg) for ages 1–70, including pregnancy and breastfeeding, and 800 IU (20 mcg) over 70; infants need 400 IU (10 mcg). These assume minimal sun. The Tolerable Upper Intake Level is 4,000 IU (100 mcg) a day for anyone 9 and older. Many supplements are 1,000–2,000 IU, which sits between the RDA and the UL. Doses above 4,000 IU, or weekly 50,000 IU capsules, are treatment doses that should be prescribed and monitored. The Endocrine Society’s 2024 guideline does not recommend routine testing or extra supplementation for healthy adults aged 19–74.

What vitamin D blood level is enough?

The Food and Nutrition Board, as summarized by NIH ODS, says 25(OH)D below 30 nmol/L (12 ng/mL) carries a risk of deficiency, 30–50 nmol/L (12–20 ng/mL) may be inadequate for some people, 50 nmol/L (20 ng/mL) or more is adequate for most, and above 125 nmol/L (50 ng/mL) has been linked to potential adverse effects. Some labs and clinicians target 30 ng/mL or more; that higher target is not the official FNB threshold.

Vitamin D3 vs D2: which is better?

D3. A 2012 meta-analysis of randomized trials in the American Journal of Clinical Nutrition found D3 raised serum 25(OH)D significantly more than D2. The difference was strongest with large bolus doses and was lost with daily dosing in that analysis (Tripkovic 2012). NIH ODS concludes most evidence shows D3 raises levels more and keeps them higher longer. D2 remains an option for vegans, although lichen-derived vegan D3 now exists. Details: vitamin D2 vs D3.

Do you need K2 with vitamin D3?

Probably not, for most people. Vitamin K activates bone and blood-vessel proteins, and that biology is why D3 + K2 products exist. But the NIH vitamin K fact sheet says “it is not clear whether vitamin K supplementation affects the risk of osteoporosis,” and that most U.S. diets already contain an adequate amount of vitamin K. Anyone on warfarin should not start K2 without their prescriber. See vitamin D3 vs D3 + K2.

What does vitamin D3 do, and what doesn't it do?

Vitamin D3 reliably corrects deficiency and supports calcium absorption. Big trials have been disappointing for everything else in people who were not deficient. In VITAL, 25,871 adults taking 2,000 IU/day for a median 5.3 years had no lower rate of invasive cancer or cardiovascular events than placebo (Manson 2019). A 2022 VITAL analysis found no reduction in fractures either. NIH ODS also summarizes trials showing no benefit for depression, type 2 diabetes prevention in people with adequate levels, or weight loss.

Who should take vitamin D3?

NIH ODS names the groups most likely to fall short: breastfed infants (the American Academy of Pediatrics advises 400 IU/day from shortly after birth), older adults, people who get little sun, people with darker skin, people with fat-malabsorption conditions such as Crohn’s, celiac disease or cystic fibrosis, and people with obesity or a history of gastric bypass. Few foods are rich in vitamin D: a tablespoon of cod liver oil has 1,360 IU, 3 ounces of cooked sockeye salmon about 570 IU and a cup of fortified milk about 120 IU.

Vitamin D3 toxicity and interactions

Toxicity almost always comes from supplements, not sun or food. It causes high blood calcium, with nausea, vomiting, weakness, kidney stones and, in extreme cases, kidney failure and heart rhythm problems; serum 25(OH)D is typically above 150 ng/mL (375 nmol/L). NIH ODS lists interactions with orlistat (lowers absorption), statins, corticosteroids (impair vitamin D metabolism) and thiazide diuretics (risk of high calcium). Take D3 with a meal that contains fat; dietary fat in the gut improves absorption. See our supplement timing guide.

Vitamin D3 FAQ

What is the difference between vitamin D and vitamin D3?

Vitamin D is the family name. Vitamin D3 (cholecalciferol) is one form, made in skin and found in animal foods and most supplements. Vitamin D2 (ergocalciferol) is the other main form, made from yeast or mushrooms.

How many IU is 1 mcg of vitamin D?

1 microgram of vitamin D equals 40 IU. So 25 mcg is 1,000 IU, 50 mcg is 2,000 IU and 125 mcg is 5,000 IU.

How much vitamin D3 should an adult take daily?

The NIH RDA is 600 IU (15 mcg) a day for adults up to age 70 and 800 IU (20 mcg) over 70. The upper limit is 4,000 IU (100 mcg) a day unless a clinician prescribes more.

Is 5,000 IU of vitamin D3 too much?

5,000 IU (125 mcg) is above the 4,000 IU daily upper limit set by the Food and Nutrition Board. It may be prescribed short term to treat deficiency, but it should be monitored by a clinician rather than taken indefinitely on your own.

Is vitamin D3 better than D2?

Yes for raising blood levels. A 2012 meta-analysis found vitamin D3 raised serum 25-hydroxyvitamin D more than D2, and NIH says D3 keeps levels higher for longer.

Should I take vitamin D3 with K2?

Most people do not need to. NIH says it is not clear whether vitamin K supplementation affects osteoporosis risk, and that most U.S. diets contain enough vitamin K. People on warfarin should not add K2 without their prescriber.

What is a normal vitamin D level?

According to the Food and Nutrition Board, a 25-hydroxyvitamin D level of 20 ng/mL (50 nmol/L) or higher is adequate for most people, below 12 ng/mL (30 nmol/L) indicates deficiency risk, and above 50 ng/mL (125 nmol/L) may cause adverse effects.

When is the best time to take vitamin D3?

With a meal that contains some fat, because fat in the gut improves vitamin D absorption. The time of day matters less than taking it consistently.

Does vitamin D3 prevent cancer or heart disease?

Not in people who are not deficient. The VITAL trial gave 2,000 IU a day to 25,871 adults for a median of 5.3 years and found no lower rate of invasive cancer or cardiovascular events than placebo.

Sources

1. NIH Office of Dietary Supplements. Vitamin D: Fact Sheet for Health Professionals
2. NIH Office of Dietary Supplements. Vitamin K: Fact Sheet for Health Professionals
3. Tripkovic L, et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. Am J Clin Nutr. 2012;95(6):1357-64
4. Manson JE, et al. Vitamin D supplements and prevention of cancer and cardiovascular disease (VITAL). N Engl J Med. 2019;380(1):33-44