Most experts and textbooks divide vitamins into those that dissolve readily in water and those that dissolve in fat. This is helpful because the body handles water and fat quite differently.
Water-soluble vitamins are readily absorbed from the intestine and used by the body; any extra tends to be removed quickly in the urine. Most must be replenished regularly from dietary sources.
Fat-soluble vitamins offer more of a challenge. Certain people have medical conditions that cause problems with absorbing fat-soluble vitamins. Despite adequate dietary intake, they need extra. On the flip side, fat-soluble vitamins are stored in body tissues (such as the liver) and released very slowly over time. As a result, taking too much can be dangerous.
The 13 major vitamins include the water-soluble vitamins (thiamin, riboflavin, niacin, B6, folic acid, B12, pantothenic acid, biotin and C) and the fat-soluble vitamins (A, D, E and K).