Most vitamin compounds are water-soluble vitamins. This means that water soluble vitamins are those that dissolve in water and are therefore not stored in the body. They are either absorbed and used by the body immediately or eliminated through waste. Thus, the water-soluble vitamins require a daily supply to be effective (because they are either used up immediately or eliminated through waste). Fat-soluble vitamins, on the other hand, are not dissolved by water, but rather dissolved by fat (so if you take the supplement by mouth, it is recommended you take them with foods that are high in fat content) and then stored in fatty cells or organs such as the liver, either for immediate use or stored for later use.

Fat-soluble vitamins, for the most part, are available from fatty or oily foods and are stored in fat cells or in the liver. The four main vitamin compounds that are fat soluble are Vitamin E, Vitamin D, Vitamin K, and Vitamin A. In general, these compounds benefit the immune system, the circulatory system, and skin and skeletal system (they promote healthy skin, bones, eyes, teeth, hair, and nails, and protect these systems from the effects of aging).

That leaves the rest of the vitamins out there as water-soluble vitamin compounds. The main two vitamin compounds that are water soluble are Vitamin B and Vitamin C. Other vitamin compounds that are water soluble include Folic Acid, Biotin, and Pantothenic Acid. Vitamin C is also known as Ascorbic Acid. Vitamin B compounds (known as B complex vitamins) include Thiamin (Vitamin B1), Riboflavin (Vitamin B2), Niacin (Vitamin B3), Pyridoxine (Vitamin B6), and Cobalamin (Vitamin B12). In this article, I explain the differences between water soluble vitamins and fat soluble vitamins, and how they are used by the body. The benefits of water-soluble vitamins and their sources from foods are many (and in a related way, the effects of deficiency from each vitamin are many), thus, I go into greater detail of benefits and sources of each in separate articles. Here, I differentiate between how each type of vitamin is used by the body and the risks of over consumption based on whether excess amounts of each vitamin is stored in the body or excreted by the body.

Because fat-soluble vitamins are stored in the body, any excess amount of the vitamin taken that cannot be used immediately can be used later, and this does pose a risk (albeit a rare one) of overdosing or poisoning from taking too much of a fat-soluble vitamin at one time or over a short period. Water-soluble vitamins generally do not pose such a risk if obtained through food, as the body uses up whatever it can immediately and any excess amount of the vitamin would be eliminated through waste. Through supplements, however, over consumption of some water-soluble vitamins can be damaging to the liver, for instance. There are no known problems with over consumption of Thiamin (Vitamin B1), Riboflavin (Vitamin B2), Cobalamin (Vitamin B12), Biotin, and Pantothenic Acid. Problems that occur from over consumption of water-soluble vitamins are rare but have been shown to occur despite the fact that the body is designed to excrete water-soluble vitamins when in excess. Over consumption of Niacin (Vitamin B3) supplements can cause flush, skin rashes, or liver damage. Over consumption of Pyroxidine (Vitamin B6) can cause nerve damage over time. Over consumption of Folic Acid can mask Cobalamin (Vitamin B12) deficiencies. Over consumption of pantothenic acid can rarely cause diarrhea and water retention. Finally, over consumption of Vitamin C, in rare cases, can cause kidney stones, gout, diarrhea, and rebound scurvy.