The human body treats most vitamins like a well-stocked medicine cabinet; each "bottle" remains safely stored away until needed as a catalyst or carrier for other essential chemicals. Only a very small amount of vitamins are actually necessary to maintain a healthy body, so any excess should be excreted out of the body over time. Some vitamins, such as the B complex and C, are considered water soluble, which means they dissolve in the bloodstream and will eventually be excreted through the urinary tract. Others, such as A, D, E and K, are considered fat soluble, which means they will be stored in fat cells or the liver and eventually be excreted through the lymph system.
There are a number of reasons why vitamins are either water soluble or fat soluble, mostly because of the roles they perform in the body and the nature of their chemical compositions. Water soluble vitamins like the B complex and C are necessary for short-term projects such as boosting the nervous system or providing antioxidants for cell repair. Once the body has obtained enough water soluble vitamins to perform the tasks at hand, there is no need to store the remaining supply. Excess amounts of vitamin C and the B complex are filtered out of the bloodstream by the kidneys and excreted in urine. This is why it is sometimes more difficult, though certainly not impossible, to overdose on water soluble vitamins than to do so on fat soluble vitamins.
Fat soluble vitamins, on the other hand, are processed in a different manner. Vitamins such as A, D, E and K usually survive the initial digestive process and enter the lower intestines, where they are absorbed and dissolved by fat cells called lipids. Some of these vitamin-storing lipids eventually make their way to the liver or other fat deposits for long-term storage. Fat soluble vitamins help other chemical and nutrients perform tasks such as calcium absorption or collagen replacement. The body only needs a small amount of fat soluble vitamins, but the excretion process takes much longer. Fat cells literally have to melt away in order to get rid of excessive fat soluble vitamins, and the released vitamins pass slowly through the lymph system.
It is possible to consume a toxic level of fat soluble vitamins through supplement overdoses or a long-term unbalanced diet. Consuming too many water soluble vitamins such as Vitamin C can also cause serious medical problems, but these vitamins can be eliminated quickly through the urinary tract. Fat soluble vitamins must be stored in the liver until they can be safely metabolized, so it is important to avoid taking extreme doses of vitamins A, D, E or K for their purported health benefits. The body only needs trace amounts of fat soluble vitamins at any one time, so overdoses can become toxic quickly.