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Vitamin D: D3 vs D2, IU to mcg, Timing and Limits

Also known as vitamin D3, vitamin D2, cholecalciferol, ergocalciferol, calciferol, the sunshine vitamin

Short answer

Vitamin D is a fat-soluble vitamin the body uses to absorb calcium and keep bones mineralized. Labels list it in mcg, often with IU: 1 mcg equals 40 IU. Fat in the gut helps absorption. The NIH upper limit for adults is 100 mcg (4,000 IU) a day, and too much can raise blood calcium [1].

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What is Vitamin D?

Vitamin D is a fat-soluble vitamin found in a few foods, added to others, and made in the skin when UVB rays from sunlight reach it [1]. The body converts it in the liver and then the kidney into its active form [1]. It helps the gut absorb calcium and keeps calcium and phosphate at the levels bones need to mineralize [1]. It also plays roles in cell growth, nerve and muscle function, and immune function [1].

Forms of Vitamin D you'll see on labels

  • Vitamin D3 (cholecalciferol)

    Usually made from lanolin in sheep's wool; an animal-free version from lichen also exists. Most evidence shows D3 raises blood 25(OH)D levels more, and for longer, than D2 [1].

  • Vitamin D2 (ergocalciferol)

    Made by exposing yeast ergosterol to UV light. Both D2 and D3 are well absorbed and both raise blood 25(OH)D levels [1].

  • 25(OH)D3 (calcifediol)

    Used in some studies and is 3 to 5 times as potent as D3 per microgram, but NIH says no such supplements appear to be sold to US consumers at this time [1].

How much Vitamin D do adults need?

NIH lists these Recommended Dietary Allowances (RDA) for adults. Values are in mcg with IU in brackets (1 mcg = 40 IU). Reference values, not a personal recommendation.

GroupAmount per day
Adults 19–5015 mcg (600 IU)
Adults 51–7015 mcg (600 IU)
Adults over 7020 mcg (800 IU)
Pregnancy (19–50)15 mcg (600 IU)
Lactation (19–50)15 mcg (600 IU)

Upper limit: 100 mcg (4,000 IU) a day for adults 19+, including during pregnancy and lactation [1]

How Vitamin D appears on a label

The FDA requires labels to list vitamin D in mcg per serving, and labels may also show IU in brackets [1]. To convert, 1 mcg equals 40 IU, so 25 mcg is 1,000 IU and 100 mcg is 4,000 IU [1][2]. The Daily Value (DV) used for % DV is 20 mcg (800 IU) for adults and children 4 and older [1]. The label should say whether it is D2 (ergocalciferol) or D3 (cholecalciferol).

When to take Vitamin D

No official source names a best time of day to take vitamin D. NIH notes that fat in the gut helps vitamin D absorption, but some is absorbed even without fat [1]. In the UK, the NHS advises everyone to consider a daily 10 mcg supplement in autumn and winter, when sunlight is too weak to make enough [2].

What to keep apart from Vitamin D

  • Calcium

    Vitamin D helps the gut absorb calcium [1]. In one large trial, women taking calcium plus vitamin D had a higher risk of kidney stones; other shorter trials found more high blood or urine calcium, but not more stones [1].

  • Orlistat (weight-loss medicine)

    Orlistat with a low-fat diet can reduce vitamin D absorption from food and supplements [1]. Ask your pharmacist.

  • Statins

    High vitamin D intakes might reduce the effect of atorvastatin, lovastatin and simvastatin, because they share a metabolizing enzyme [1]. Ask your pharmacist.

  • Steroid medicines (corticosteroids)

    Medicines such as prednisone can reduce calcium absorption and impair vitamin D metabolism [1]. Ask your pharmacist.

  • Thiazide diuretics

    Taken with vitamin D supplements, these might lead to high blood calcium, especially in older adults or people with kidney problems [1]. Ask your pharmacist.

Foods that contain Vitamin D

  • Cod liver oil
  • Trout, salmon, tuna and mackerel
  • Sardines
  • UV-exposed mushrooms
  • Fortified milk and plant milks
  • Fortified breakfast cereals
  • Egg yolks
  • Beef liver
  • Cheese (small amounts)

Who should ask a doctor or pharmacist first

Talk to a doctor or pharmacist first if you take orlistat, a statin, a steroid medicine or a thiazide diuretic [1]. The same applies if you have kidney disease, high blood calcium, or a condition that limits fat absorption, such as celiac disease, Crohn's disease or cystic fibrosis [1]. NHS advice is to follow your doctor if they recommend a different amount, as some conditions mean you may not safely take as much [2].

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Vitamin D: questions people ask

1 mcg of vitamin D equals 40 IU [1]. So 10 mcg is 400 IU, 25 mcg is 1,000 IU, and 100 mcg is 4,000 IU [2].

Both forms are well absorbed and both raise blood 25(OH)D levels [1]. NIH says most evidence shows D3 raises those levels more and keeps them up longer than D2 [1].

4,000 IU (100 mcg) is the NIH upper limit for adults, the most per day unlikely to cause harm [1]. NIH notes that even intakes below the upper limit might have adverse effects over time [1]. Ask your doctor what amount suits you.

In children, deficiency can cause rickets, with soft bones and bone deformities [1]. In adults it can cause osteomalacia, with weak bones and bone pain [1]. Only a blood test and a doctor can tell if you are deficient.

The main test is serum 25(OH)D. NIH's expert panel says 50 nmol/L (20 ng/mL) or more is enough for most people, and the risk of deficiency rises below 30 nmol/L (12 ng/mL) [1]. Levels above 125 nmol/L (50 ng/mL) have been linked to possible adverse effects [1].

No. Experts do not believe too much sun exposure causes vitamin D toxicity [1][2]. Toxicity is almost always from supplements [1].

No. UVB rays do not pass through glass, so sunlight indoors through a window does not produce vitamin D [1].

NIH says fat in the gut helps vitamin D absorption, but some is absorbed without fat [1]. No official source names a best time of day.

Sources

  1. Vitamin D: Fact Sheet for Health Professionals — NIH Office of Dietary Supplements
  2. Vitamin D - Vitamins and minerals — NHS

Educational information, not medical advice. The amounts above are official reference values, not a recommendation for you. Check with your doctor or pharmacist before starting or changing supplements, especially if you take medication, are pregnant or have a health condition. Supin sells a subscription, not supplements. Last reviewed .

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