Question:

Dear experts,

I was told that vitamine B6 and all water-soluble vitamines are quickly excreted from the body if it is in excess amount in the body. That is the reason why when I take Vitamine B supplements, I also smell them when I urinate and this is as opposed to fat-soluble vitamines, which may be built up in large amount if taken for a prolongd period of time. But when I check the excretion time for vitamin B6 in the book, its half-life is 15 to 20 days. Can you explain to me why it take so long for the body to excrete it?

Answer:

You are correct that vitamin B6 is a water-soluble vitamin and that its half-life is 15 to 20 days.  You are also correct that excess water-soluble vitamins are quickly removed from the body by the kidneys.  Vitamin B6, also known as pyridoxine, is metabolized in the liver after it is ingested to its active form called pyridoxal-5′-phosphate or P5P.  P5P is highly bound to protein in the blood, and drug that is bound to protein cannot be excreted by the kidney.  So when your body gets the dose that it needs, it converts almost all that dose to P5P which is then tightly held onto by protein resulting in a half life of 15 to 20 days.  Eventually, P5P is again metabolized in the liver to 4-pyridoxic acid which is then eliminated by the kidneys.  It is when you take pyridoxine in excess that it is quickly eliminated.  If the liver receives more pyridoxine than it can handle, additional drug reaches the blood without being converted to P5P.  Pyridoxine is not as highly bound to protein and can then be more easily eliminated by the kidneys.  Also, if there is too much P5P in the blood, more P5P will be unbound and also able to be metabolized and then eliminated until your body again reaches equilibrium.  

A simplified way to think of this would be to imagine a cup.  When it is filled to the brim with water, it would hold that water, until it is slowly evaporated.  If too much water is added, it will overflow and the excess will be quickly lost.  Similarly, the body has a maximum amount of vitamin B6 that it can contain, and anything beyond that is quickly spilled out through the kidneys.  It is important to note though, that just like with any other medication, it is possible for toxic effects to develop if excess amounts are ingested.

  

Submitted by: 

Natalie Deel, PharmD Candidate 2011 

University of Toledo  

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