Elmer McCollum demonstrated the first existence of vitamin D in 1922. Due to its effects in treating rickets disease caused by rats, vitamin D is also called an antirachitic factor. As the human body synthesizes this vitamin when exposed to sunlight, it is called the sunshine vitamin as well.
Characteristics of Vit D
Vitamin D exists in various forms, of which Vitamin D2 and Vitamin D3 are the important ones.
They are called ergocalciferol and cholecalciferol respectively.
Cod liver oil is the best natural source of vitamin D.
Egg yolk, dairy products, and some fruits and vegetables also contain this vitamin but in smaller amounts.
Vit D2 has a plant origin while Vit D3 originated from animals.
Vitamin D2 is produced from ergosterol in plants by its irradiation by ultraviolet light.
Vit D3 is produced from 7-dehydrocholesterol in animals through a similar process.
The molecule has a steroidal structure with four rings.
The main structural difference between the D3 and D2 versions is the presence of a double bond in the D2 molecule, existing between the carbon atoms 22-23. It also has a methyl group in C24.
Vit D is an odourless, white crystalline, fat-soluble substance.
It is heat resistant and oxidation resistant and is also not affected by alkalis or acids.
Metabolism of Vitamin D
Under the UVB light, 7-dehydrocholesterol in the epidermis of humans undergo a chemical reaction to initially form pre-vitamin D3. This is a non-enzymatic reaction. This will further make internal changes in the molecule to give rise to Vit D3.
Vitamin D3 thus formed is transported to other parts of the body with the help of VDBP binding.
Only one-half of the produced vitamin is transported to the liver where further processing happens until it is transformed into its circulating form.
The other half is stored in the adi[ose tissues. This will be stored for up to 2 months.
In the liver, Vit D is converted into its useful form of 25OHD3, with the help of a cytochrome.
The conversion of the circulating form of Vit D into its active form takes place in the kidneys.
In the kidney tubules, the 25OHD3 molecules undergo 1-alpha hydroxylation to form 1,25 (OH) 2D3 with an additional OH group in the first carbon of A ring.
The 23-hydroxylase enzymes in the inner membrane of mitochondria in kidney cells perform a stepwise reaction to convert the active form of Vit D3 into an inactive calcitroic acid.
Biological Importance of Vit D
The primary role of vitamin D is to help the body absorb, retain, and utilize calcium and phosphorus. Both of these are necessary to build bone structure.
Studies have also shown that it can help reduce inflammation, prevent cancer cell growth, and control infections.
Deficiency of this vitamin leads to health issues in the bones, inflammations, lower immunity, and rickets disease.