Vitamin D2 and D3: how do they differ?

Vitamin D2 and vitamin D3 are the two main forms of vitamin D, and they differ in where they come from and in one small part of their structure. Vitamin D2 (ergocalciferol) forms when UVB light reaches ergosterol, a substance in mushrooms and yeast. Vitamin D3 (cholecalciferol) forms from 7-dehydrocholesterol in the skin of people and animals, and it is the form found in fish, egg yolk and liver. In a study of fish commonly eaten in Thailand, only D3 was found, while Thai mushrooms contained mainly D2. The Thai Reference Daily Intake (Thai RDI) for vitamin D is 15 µg a day.

This article explains the two forms by their structure, source and amounts in food. It does not compare which one to choose. For how the skin makes vitamin D and how much vitamin D people in Thailand have, see How does the body get vitamin D in a sunny country?

What are vitamin D2 and vitamin D3?

Both are fat-soluble substances with the same basic shape. They are named by number: D2 and D3 are the two main forms, and the NIH Office of Dietary Supplements (ODS) says they differ chemically only in their side-chain structures.

Where exactly the structure differs

A 2013 review from the Technical University of Denmark explains the difference: vitamin D2 has a double bond between carbons 22 and 23 and an extra methyl group at carbon 24 in its side chain. Both are secosteroids, meaning steroids with one ring broken open. The review notes that vitamin D is classified into forms numbered 2 to 6, and that D2 and D3 are the two main ones.

Two forms at a glance

The one extra carbon in D2 is the methyl group in the side chain, which is why its formula has 28 carbons rather than 27.

Where does each form come from?

Both forms are made by the same kind of reaction: UVB light opens one ring of a starting substance, which then turns into vitamin D.

Vitamin D3: skin and animals

The Danish review says vitamin D3 is produced when UVB at 290 to 315 nanometres reaches 7-dehydrocholesterol in the skin, which first becomes previtamin D3 and then, depending on temperature, vitamin D3. The ODS says beef liver, egg yolks and cheese contain small amounts of vitamin D, mainly as D3 and its metabolite 25-hydroxyvitamin D3. The review also notes that fish have the highest natural content of vitamin D, thought to build up through the food chain starting from microalgae, which contain both vitamin D3 and provitamin D3.

Vitamin D2: fungi and yeast

The same review says vitamin D2 is produced in fungi and yeast when UVB reaches ergosterol, the provitamin D2, and that small amounts of D2 can be found in plants that carry fungi. The ODS says mushrooms provide variable amounts of D2, and that some mushrooms on the market are treated with UV light to raise their D2.

Where supplement vitamin D comes from

The ODS says vitamin D2 in supplements is manufactured by UV irradiation of ergosterol in yeast, and vitamin D3 is typically produced by irradiation of 7-dehydrocholesterol from lanolin, which is obtained from sheep's wool. It adds that an animal-free version of vitamin D3 sourced from lichen is also available. A 2022 review of vitamin D sources lists reindeer lichen at 87 µg per 100 g.

Which Thai foods contain vitamin D2 or D3?

The Institute of Nutrition at Mahidol University has measured both forms separately in foods bought in Thailand.

Fish: only D3 was found

In a 2022 study of nine kinds of fish commonly eaten in Thailand, the researchers measured vitamin D2 and D3 and found that D3 was the only form they could detect. Raw freshwater fish had 2.42 to 48.5 µg per 100 g of edible portion, and raw marine fish 2.94 to 4.69 µg. Common silver barb (ปลาตะเพียน) had 48.5 µg, red tilapia (ปลาทับทิม) 31.0 µg and Nile tilapia (ปลานิล) 19.8 µg.

Mushrooms: mainly D2

A 2023 study of 12 kinds of mushroom, nine cultivated ones bought from three wholesale markets and three wild ones collected in a conservation area, found that vitamin D2 and ergosterol were the main forms. Ergosterol ranged widely, from 7,713 to 17,273 µg per 100 g. Vitamin D2 in the raw mushrooms was:

The values varied between samples; lung oyster mushroom, for example, had a standard deviation of 7.31 µg. After boiling, stir-frying and grilling, the mushrooms kept on average about 64%, 59% and 65% of their D2.

What UV light does to mushrooms

A 2023 study by Judprasong and colleagues, also from Mahidol University, exposed eight kinds of mushroom to UVB lamps on both sides for up to 3 hours in a laboratory cabinet. Vitamin D2 rose with exposure time in all of them. Enokitake had the highest level, rising from 6.68 to 1,880.22 µg per 100 g of dry weight after 3 hours. This figure is per dry weight, so it cannot be compared with the raw (fresh weight) mushroom table above. After cooking, the UV-treated mushrooms kept 53% to 89% of their D2. These were laboratory conditions, not mushrooms sold in markets.

Thai fish and mushrooms side by side

How does the body handle D2 and D3?

Both are converted to 25(OH)D

The ODS says vitamin D from the sun, food and supplements is not yet active: the liver converts it to 25-hydroxyvitamin D, or 25(OH)D, and the kidney converts that into the active form.

A blood test can measure each form

Some laboratory methods measure the two forms separately. In a long-term study of 1,239 employees of the Electricity Generating Authority of Thailand, with blood taken in 2009 and 2019 and measured by liquid chromatography and tandem mass spectrometry, the researchers measured 25(OH)D3, from vitamin D3, and 25(OH)D2, from vitamin D2, separately. Total 25(OH)D rose by 14.8% over the ten years, with increases in both 25(OH)D3 and 25(OH)D2. The participants were employees of one organisation, so the results may not reflect all Thai adults.

Micrograms and IU

The ODS and the NHS both give 1 microgram (µg) of vitamin D as equal to 40 international units (IU). So 15 µg is 600 IU, and 100 µg is 4,000 IU.

What does vitamin D do in the body?

Vitamin D has these roles:

• contributes to normal absorption of calcium and phosphorus and to normal blood calcium levels
• contributes to the maintenance of normal bones and teeth
• contributes to the maintenance of normal muscle function
• contributes to the normal function of the immune system

These roles describe normal physiology. Taking in more than the body needs does not add to them.

How much vitamin D do you need, and how much is too much?

The Thai RDI for vitamin D is 15 µg (600 IU) a day, for the general population aged 3 and over.

Upper limits

The NHS advises not taking more than 100 µg (4,000 IU) a day for adults, including pregnant and breastfeeding women and older people, and for children aged 11 to 17, and not more than 50 µg (2,000 IU) for children aged 1 to 10.

Who should talk to a doctor or pharmacist?

If you take regular medication, or if you are pregnant or breastfeeding, talk to a doctor or pharmacist before taking dietary supplements.

Dietary supplements are not intended to prevent, treat or cure any disease.