Vitamin D estimation in blood (25 hydroxy vit D levels) has become a very popular test all over the country. With the available test, a majority of the population is found to have lower levels.
Does this reflect a true deficiency of vitamin D3?
We have plenty of sunshine in our country and it seems paradoxical that people are deficient in vitamin D. So the main question is about the interpretation of the commercially available testa in our country. Vit D (cholecalciferol) is formed in the skin by the photolysis of 7 dehydrocholestrerol by ultra violet radiation from sunlight.
Cholecalciferol is hydroxylated to 25 hydroxy cholecalciferol in the liver. The present test estimates total 25 hydroxycholecalciferol consisting of 3 parts: a) the form bound to the vit D binding protein (85%); b) form bound to albumin (15%) and c) free or bio available form (0.03%).
What is important for bone health and vitamin D action is the free form. The protein bound form is determined genetically for storage purpose.
In Afro Americans, genetically, the protein-bound form is low since they were exposed to sunlight for generations. However, the free forms are quite normal. Hence, even though the total vitamin D3 level is low in their blood, they are not deficient in vitamin D since the free levels are normal.
In Harvard Medical School, it was found that Afro Americans had stronger bones in spite of lower total levels than Whites.
It is quite likely that we also have similar genetic backgrounds due to plenty of sunshine. Hence, the present test of total vitamin D3 does not reflect a deficiency. Unfortunately, the test is being done widely and is costly. It requires careful interpretation to avoid unnecessary vitamin D supplementation and its consequences.
Similarly, low levels of vitamin D 3 is found during a number of chronic diseases like heart attack, diabetes, hyper tension etc. Here again, it has been shown that it is not the cause of the chronic disease and supplementation does not help. It is only a marker of chronic disease.
(The article was contributed by Dr. Rajan Ravichandran, director, MIOT Institute of Nephrology.)