Vitamin D
Vitamin D is a fat-soluble vitamin with two main forms: vitamin D2 or ergocalciferol, which comes from plants and fungi, and vitamin D3 or cholecalciferol, the form human skin makes on exposure to ultraviolet B radiation. It is therefore unlike other vitamins in that the body can produce it under the right conditions.
Vitamin D is a fat-soluble vitamin, and it is unusual in that the body can make it: skin produces it on exposure to sunlight. Vitamin D from food or from the skin is not yet in its working form — the liver and then the kidney convert it, and the blood test used to assess vitamin D status measures the form produced by the liver, 25-hydroxyvitamin D. Nutrition references describe vitamin D as involved in the absorption of calcium from food. That is a description of the nutrient, not a statement about what a product does.
About this ingredient
Vitamin D is the collective name for two related forms. Vitamin D3 forms in the skin on exposure to UVB sunlight; vitamin D2 is made from yeast and mushrooms treated with UV light. One useful point is that skin production does not keep rising with time in the sun — it levels off as it forms, which is why sun exposure does not cause vitamin D toxicity.
Dietary sources
Few foods naturally contain much vitamin D. The main ones are oily marine fish such as salmon, sardine and mackerel, along with fish liver oils, egg yolk and UV-treated mushrooms, which is why many countries add vitamin D to milk and cereals. The US Recommended Dietary Allowance is 15 micrograms, or 600 IU, for most adults; 1 microgram of cholecalciferol equals 40 IU. Status is assessed by measuring 25-hydroxyvitamin D in blood, and a level below 50 nmol/L is considered inadequate. Beyond diet, the determining factors are latitude, season, time spent outdoors, skin tone, sunscreen use and how much of the body is covered by clothing. Most commercial vitamin D3 is derived from lanolin, the wax from sheep's wool, with a lichen-derived alternative for products avoiding animal material; vitamin D2 comes from yeast.
Forms & absorption
Vitamin D raw materials differ in two respects: form and unit. NIH ODS states that at nutritional doses vitamins D2 and D3 are equivalent, but that at high doses vitamin D2 is less potent — a distinction that matters when comparing high-dose products. Labels use either micrograms or IU, with 1 microgram equal to 40 IU, so 15 micrograms is 600 IU and 100 micrograms is 4,000 IU. Vitamin D is fat-soluble, so products are usually softgels in oil or oil-based drops taken with a meal containing fat. For people avoiding animal-derived material, yeast-derived vitamin D2 and lichen-derived vitamin D3 are the options that can be declared on a label.
Research status
The human research base for vitamin D is enormous and includes randomised controlled trials with tens of thousands of participants. Two things make it hard to interpret. First, participants' starting levels vary widely, and a trial in a population that is already replete will struggle to show anything that a trial in a deficient population would. Second, the doses and dosing schedules differ enormously, from daily intakes to monthly boluses. As an illustration, the BEST-D trial in the United Kingdom randomised 305 community-dwelling people aged 65 and over, and the mean blood vitamin D level rose from 50 nmol/L at baseline to 137 nmol/L and 102 nmol/L after twelve months on 4,000 IU and 2,000 IU a day respectively. No claim has been approved by the Thai FDA for the products on this site.
Safety & considerations
The US Food and Nutrition Board set a Tolerable Upper Intake Level for vitamin D of 100 micrograms, or 4,000 IU, a day for children and adults aged 9 and over. The figure is lower for younger children: 75 micrograms (3,000 IU) for ages 4 to 8 and 63 micrograms (2,500 IU) for ages 1 to 3. Vitamin D toxicity almost always arises from taking too many supplements rather than from sunlight, and a blood 25-hydroxyvitamin D level sustained above 500 nmol/L is considered potentially toxic. For Thailand, the Thai FDA guidance of 8 July 2024 gives a figure of not more than 15 micrograms of total vitamin D per day at item 367, as a condition attached to Atlantic cod liver oil — exactly the US recommended daily intake, and far below the US safety ceiling. If you are pregnant or breastfeeding, take prescription medicines, or already take a calcium supplement, speak to a doctor or pharmacist before starting, and always follow the amount printed on the label. Dietary supplements are not intended to prevent, treat or cure any disease.
Quality markers
Three things are worth checking on a vitamin D label. First, the form: it should say vitamin D2 (ergocalciferol) or vitamin D3 (cholecalciferol). Second, the unit — labels use micrograms or IU, and the two should be converted before comparing, at 1 microgram to 40 IU. Third, your total across products, since vitamin D appears in multivitamins, calcium supplements and many combination formulas, and the ceiling is a total daily figure. Every dietary supplement sold in Thailand also carries an FDA (อย.) registration number on the label.
- NIH Office of Dietary Supplements – Vitamin D Fact Sheet for Health Professionals ↗
- MedlinePlus (U.S. National Library of Medicine) – Vitamin D ↗
- Thai FDA – Advice on the use of key ingredients in dietary supplements (8 July 2024), item 367 Atlantic cod liver oil ↗
This page is general educational information about a nutrient — it describes the nutrient itself and is not a statement about any particular product. Dietary supplements are food, not medicine, and have no effect in preventing or treating disease. The references listed above are public-institution sources, not product claims.
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