A vitamin is an organic compound and a vital nutrient that an organism requires in limited amounts but cannot synthesize the compound in sufficient quantities, and it must be obtained through the diet. Term "vitamin" is conditional upon the circumstances and the particular organism. For example, ascorbic acid (one form of vitamin C) is a vitamin for humans, but not for most other animal organisms. Supplementation is important for the treatment of certain health problems, but there is little evidence of nutritional benefit when used by healthy people.
13 Vitamins are universally recognized at present: A, B1, B2, B3, B5, B6, B7, B9, B12, C, D, E, K.
Vitamins are classified by their biological and chemical activity, not their structure. Vitamers by definition are convertible to the active form of the vitamin in the body.
Anti-vitamins are chemical compounds that inhibit the absorption or actions of vitamins. For example, avidin is a protein in egg whites that inhibits the absorption of biotin.