Fat-soluble vitamins are a group of essential nutrients, consisting of A, D, E and K vitamins, which your body needs in small doses to maintain good health. This group of vitamins uses body fat for storage unlike the water-soluble vitamins such as the B and C vitamin groups. Fat-soluble vitamins differ from water-soluble vitamins in their storage and excretion processes. Fat-soluble vitamins can be found in supplements but are readily available in many foods as part of a healthful diet.
Fat-soluble vitamins use adipose, or fatty, tissue and liver tissue for storage in the body. Water-soluble vitamins need to be regularly ingested to keep levels topped up, but your body can store fat-soluble vitamins for later use.
Fat-soluble vitamins last longer in the body than do water-soluble vitamins. The long-term storage system is beneficial: Because your body does not need these vitamins every day, it can tuck them away until they are required for such functions as blood clotting, as in the case of vitamin K. Vitamin D is used in calcium production for bones and for absorbing calcium from the small intestine. From three 15-minute exposures to the sun a week, the body can build up enough of a stockpile of vitamin D for these essential processes when needed.
Absorption and Excretion
The fat-soluble vitamins are absorbed through the small intestine with dietary fat, which the vitamins use as a storage medium. People with fat-absorption conditions, such as Crohn's disease, can have trouble absorbing enough of these vitamins for proper health. The vitamins are also excreted more slowly than water-soluble vitamins.
Because of their longer-term storage in the body than water-soluble vitamins, fat-soluble vitamins are more likely to cause toxicity; the body cannot excrete them quickly enough. In general, regular eating habits do not cause toxicity, but taking supplements at high levels can create an imbalance of vitamins and cause illness. For example, vitamin A overdoses can cause headaches and nausea, and in the long-term, it can lead to blurred vision, slowed growth and dizziness.
According to Colorado State University, the level of fat-soluble vitamins in food does not reduce with cooking, unlike some water-soluble vitamins, because the chemicals are more resistant to heat and other cooking effects.