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Why are Some Vitamins Water Soluble and Some Fat Soluble?

Michael Pollick
Updated Mar 03, 2024
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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.

The Health Board is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Michael Pollick
By Michael Pollick , Writer
As a frequent contributor to The Health Board, Michael Pollick uses his passion for research and writing to cover a wide range of topics. His curiosity drives him to study subjects in-depth, resulting in informative and engaging articles. Prior to becoming a professional writer, Michael honed his skills as an English tutor, poet, voice-over artist, and DJ.

Discussion Comments

By anon276089 — On Jun 21, 2012

I've been taking vitamin E in large doses and it resulted in a blood pressure spike. I was told to stop taking Vitamin E because in large doses over long periods of time, it may do more harm then good. Lower doses are fine, but the problem is many people do not know what is considered a low, acceptable dose and a high dose.

For anyone who has high blood pressure, do not take more than 200 IU of Vitamin E. Larger doses can result in an actual increase in your blood pressure. Consult a GP to determine if vitamins are safe for you to take and do not interact with any medications you may be taking. Vitamins can interact with some drugs, making them either less or more potent, so it's important to make sure you don't put yourself at risk.

Overall, vitamins are safe. You just have to be sure that you actually need to take vitamins. Most of us don't need them at all. We're just taking them because there is a spin that taking vitamins will do this and that for you and you health.

The best way of increasing general well being and health is to make lifestyle changes, stop smoking, alcohol exercise, eat fresh vegetables and fruit and avoid large amounts of saturated fats, red meat, etc.

By Charlie89 — On Nov 11, 2010

I'm a little bit confused. If it's difficult to overdose on water soluble vitamins since they tend to just get flushed out of the body, then how do people actually do it?

I mean, do they take enough of something that it just overwhelms the water content of the body and sticks on the insides like some kind of water soluble gel?

I could understand that if somebody was chronically dehydrated, yet still taking a lot of vitamins, but how does it happen in normal people? Or do all the articles just play up the risk of a water soluble vitamin overdose?

Can you explain this to me?

By EarlyForest — On Nov 11, 2010

So I know that vitamin B is water soluble, but I was looking at a supplement the other day that offered 2000 times the daily recommended dosage for vitamin B.

How can that be safe, even if it is water soluble? I mean, eventually, if you keep putting that much of something into your body, won't some of it stay behind?

The supplement did have a warning that long-term usage could lead to numbness, which I know is a symptom of a vitamin B overdose, so I guess I'm just confused as to why they would put that much in there to begin with.

Is there some reasonable explanation for bombarding your body with vitamin B, or is it just a crazy supplement.

Do you guys have any experience with this?

By Planch — On Nov 11, 2010

This article was just so helpful for my daughter. We are doing a unit on the difference between water solubility and fat solubility, and I was having a little bit of a hard time explaining the difference to her.

The point of the unit, you see, is to understand why we have things like water soluble plastic polymers and water soluble fertilizers, things like that, as well as the importance of water soluble vitamins.

This article was just so helpful in explaining the difference between fat soluble and water soluble vitamins -- I can't thank you enough!

Michael Pollick

Michael Pollick


As a frequent contributor to The Health Board, Michael Pollick uses his passion for research and writing to cover a wide...
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