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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.
a) the form bound to the vitamin D binding protein
(85%)
b) form bound to albumin (15%)
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 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 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.
Reference:
1) 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.
2)The Lancet Diabetes & Endocrinology volume II issue
1,page 76-89 Jan 2014 |
