Low molecular weight chemical molecules known as vitamins are needed in modest amounts in the diet.
Most of the vitamins are not synthesized in the human body but are synthesized by the plants. Hence these essential nutrients are mainly obtained through the food.
Though most of them are present in the diet as such, some are present as precursors known as provitamins.
Vitamins are divided into two major categories. They are fat-soluble (A, D, E and K) and water-soluble vitamins (B-complex and vitamin C).
B complex vitamins include thiamine (B1), riboflavin (B2), niacin, pyridoxine (B6) biotin, folic acid, pantothenic acid and cobalamin (B12).
Inositol, choline and para-aminobenzoic acid are vitamin-like substances sometimes classified as part of the B complex, but no convincing evidence has been shown so far to be included as vitamins.
All the fat-soluble vitamins and some B vitamins exist in multiple forms.
The active forms of vitamin A are retinol, retinal and retinoic acid and vitamin D is available as ergocalciferol (D¬2¬) and cholecalciferol (D3).
The vitamin E family includes four tocopherols and four tocotrienols but α-tocopherol being the most abundant and active form.
There are interconvertible and interchangeable variations of vitamins.