Vitamin D estimation in blood (25 hydroxy vit D levels) has become a very popular test all over the country. With the present available test majority of the population are found to have lower levels.
Does this reflect a true deficiency of vitamin D3?
We have plenty of sunshine in our country and it looks paradoxical that we are deficient in vitamin D. So the main question is the interpretation of the present commercially available test in our country. Vitamin D (cholecalciferol) is formed in the skin by the photolysis of 7dehydrocholestrerol by Ultra Violet Radiation from sun light. Cholecalciferol is hydroxylated to 25 hydroxy cholecalciferol in the liver.
The Present test estimates total 25 hydroxycholecalciferol consisting of 3 parts.
- The form bound to the vitamin D binding protein (85%)
- Form bound to albumin (15%)
- 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 blood they are not deficient for vitamin D since the free levels are normal. This has been studied in Harvard Medical School where they found that the afro Americans had stronger bones in spite of lower total levels than the whites. It is also quite likely that we also have similar genetic back ground due to plenty of sun shine. 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 in 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.
- Vitamin D binding protein and vitamin D status of black Americans and white Americans. N.Engl J Med 2013 369,1991-2000. Nov 21 ,2013.
- The Lancet Diabetes & Endocrinology volume II issue 1,page 76-89. Jan 2014