Vitamin D2 and D3 are two important forms of vitamin D. D2 comes from plants, D3 comes mainly from animal sources, or is made by the human body when our skin is exposed to sunlight. D3 absorbs better than D2 and is more effective. Fortified milk or juice is more likely to contain D2 because of its lower production cost.

Vitamin D3
Vitamin D3 is produced when cholesterol in our skin is exposed to sunlight, and certain foods of animal origin also contain small amounts of vitamin D3.

Our skin stores a specific type of cholesterol, called 7-dehydrocholesterol, which is converted to provitamin D3 when exposed to UVB (wavelength 270-300nm). Another process converts it to cholecalciferol, which is then activated into active vitamin D in the liver and kidneys. Active vitamin D is called 1,25 dihydroxy vitamin D3 (1,25 (OH) D) or calcitriol.

Although the process of converting 7-dehydrocholesterol into active vitamin D3 is complicated, it is quite effective. It is estimated that only 10 minutes of sufficient sunlight per day can produce 10 mg of vitamin D3.

Vitamin D2
Vitamin D2 supplements can also be synthesized by irradiating fungi and plants that naturally contain ergosterol. Drisdol is another name for vitamin D2 supplementation. The production of supplemental D2 is cheaper than that of supplemental D3. However, it cannot effectively increase the level of vitamin D in the blood and is not as stable as synthetic vitamin D3. Vitamin D2 still needs to be converted into D3 active substances in the body.

Vitamin D2, also known as ergot calciferol, occurs naturally in mushrooms exposed to sunlight. Mushrooms contain a yeast compound called ergosterol, which is converted to ergocalciferol when exposed to ultraviolet light.

Vitamin D3 is more effective than D2 and can bind vitamin D receptors more effectively. It is also better absorbed and more easily converted into active D.

All forms of vitamin D2 and D3 obtained from our diet or supplements need to be converted in the liver and kidneys first.