Saturday, February 26, 2011

Vitamin D and our world 維生素D 的故事

Vitamin D has become a very hot topic in recent years. There are so much information available on the internet. Vitamin D can be naturally obtained through exposure to the sun, because of this reason, it is believed that people live north of 42 degrees latitude don't get enough natural Vitamin D. Do you think people live in the tropical region get enough Vitamin D from the sun?
No! Studies have shown they can become deficient in Vitamin D as well. I guess the heat is too intimidating to allow people to get outside. 

The precursor of Vitamin D is in our skin, and when we are exposed to the sun, the UV light can help us synthesize Vitamin D, which in turn be converted into 25-OH Vitamin D (25-monohydroxy vitamin D) in the liver. Then, the 25-OH Vitamin D will become 1,25-OH Vitamin D (1,25-dihydroxy vitamin D) in the kidney, which is the active form to increase gut absorption of calcium and remove phosphate, and the overall end result is to regulate the calcium level in our body.

It is believed that people don't get enough Vitamin D if only the face is exposed to the sun, the most effect areas are our arms, legs and body. Only 15 minutes a day can get us enough Vitamin D we need (600 IU). The sources of Vitamin D is not only limited to the sun,  mushrooms, salmon, eggs or meat all have high levels of Vitamin D. In addition, taking Vitamin D supplement is a good way to make sure we get enough. Remember that Vitamin D can help calcium absorption, so only calcium but no Vitamin D is still in vain. Also, take Vitamin D with some butter, because Vitamin D is fat-soluble.

We all know that Vitamin D deficiency will lead to rickets in children and osteomalacia in adults. But how about taking too much? Is it toxic?

Yes! Vitamin D overdose will lead to hypercalcemia (high calcium in the blood), and if severe, will cause renal failure. 

We have an interesting case recently. A 2-month-old baby was found to have high calcium. While looking for the source of high calcium, the clinical team found the mom was Vitamin D deficient and was taking supplements, but her level was normal. They would like to test the breast milk to see what the Vitamin D level was. However, there is no FDA-approved method available in the clinical laboratories for that. They then tested the baby. Indeed, the baby's blood Vitamin D level was elevated. So could that be the cause of her hypercalcemia?

No. The common ways to measure 25-OH Vitamin D is immunoassay and mass spectrometry. Due to increase in testing volume of Vitamin D in recent years, a lot of clinical laboratories implement mass spectrometry to measure Vitamin D due to its high throughput. The test principle is to detect specific ions for a typical molecule. However, 25-OH Vitamin D has a twin, the C-3 epimer 25-OH Vitamin D, they have the same molecular structure but different arrangements of the hydroxyl group. A significant portion (up to 60%) of the 25-OH Vitamin D in babies can be the C-3 epimer. Immunoassay might be able to distinguish the two, but mass spectrometry can't. Thus, mass spectrometry would give falsely-elevated Vitamin D results.

The C-3 epimer is not a problem in adults, because it doesn't exists in adults. Clinically, people who get Vitamin D level tested are adults, so this was never a problem.

The baby's sample was tested by a special mass spectrometry method which detects only the standard form of Vitamin D, and the result was normal. So I guess the clinical team has to find other reasons to explain her high calcium. She is doing fine though.

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