Vitamin D3 & the Liver

Your liver is the largest internal organ you have, and it has hundreds of functions. The Merck Manual explains that vitamin D3 formed on your skin is a prohormone and the liver synthesizes it into calcidiol, or 25-hydroxyvitamin D, which is the prehormone for calcitriol, one of the most potent and important steroid hormones in your system. A steroid hormone is any hormone that is formed using cholesterol.

How It Works

Without a properly functioning liver, you would not be able to utilize vitamin D3. Besides forming the prehormone calcidiol, your liver is the warehouse for vitamin D3. When the body needs more of the hormone calcitriol, the liver releases calcidiol into the bloodstream and the blood carries the prehormone form to the kidneys where it is changed to the hormone form.

The Facts

There is an advantage to having a liver warehouse filled with D3. It prevents deficiency of the hormone calcitriol, which if severe, can cause serious symptoms. A blood test can determine the level of vitamin D3 in your blood. If you maintain this level in the high normal range and your liver is healthy, you won't become deficient in the hormone.

Theories/Speculation

If there is enough of the hormone in your system and a good reserve, the kidneys return the extra prehormone to the bloodstream and it is distributed to the tissues. Many tissues can make their own calcitriol if there is enough calcidiol available. This synthesis of hormone by the tissues may have the advantage of helping to prevent and fight solid tissue cancers. A 2007 study by P. Tuohimaa and colleagues and reported in the "European Journal of Cancer" concluded that "Vitamin D production in the skin seems to decrease the risk of several solid cancers (especially stomach, colorectal, liver and gallbladder, pancreas, lung, female breast, prostate, bladder and kidney cancers)."

Considerations

There is no real risk to having your blood tested for the prehormone calcidiol. This test is called the 25, hydroxyvitamin D test and can be included with other blood tests that your doctor orders. Most physicians are happy to check this level for you. There is risk in having an insufficient amount of vitamin D3 and there is a potential cancer fighting benefit in having a high normal reading.

Warning

There is some risk of toxicity with vitamin D3 when it is taken as a supplement in very large amounts, but the current RDA of 400 IU might not be enough for everyone. This is especially true if you have dark skin or minimal sun exposure, use sun blockers or live in a Northern latitude. Testing can help you decide if you do need supplements and just how much you need.