About 90 per cent of the vitamin D, also known as calciferol, needed by a person is produced in the body. When skin is exposed to the ultraviolet (B) rays, provitamin D present in the skin gets converted into previtamin D. It is isomerised by body heat to the precursor of vitamin D3, which is then transported to the liver through blood. Here, vitamin D3 gets converted to 25-hydroxy vitamin D and is sent to the kidneys where its active form calcitriol is formed. Calcitriol is important for maintaining calcium balance in the body. It also functions as a hormone and regulates the concentration of calcium and phosphate in blood.
The recommended exposure time to obtain this UV dose depends on the skin type, time and location as well as ambient conditions and clothing. Studies carried out in developed countries show that a full body exposure to UVB radiation that results in pinkness of skin (one erythemal dose) is equal to an oral intake of 250–625μg (10,000–25,000 IU) of 25-hydroxy vitamin D. Exposing one-quarter of skin, for instance just hands, arms and face, to one erythemal dose of UVB rays can form dietary equivalent vitamin D dose of about 1,000 IU. However, no such calculation has been done for India, where the complexion of people varies from light to dark.
Initially, most of the known benefits of vitamin D were restricted to those to the bones. But in recent years, the vitamin has been given the status of a miracle molecule due to the large number of extra-skeletal benefits it seems to have. Research around the world shows that it can prevent multiple sclerosis, diabetes, cancer, pre-eclampsia during pregnancy, low birth weight, and also improve immune response to TB, cognitive decline, Parkinson’s disease, asthma and obesity.
But a recent report by USA’s Institute of Medicine (IOM) punctures the bubble—the 2011 report says the evidence that links vitamin D to all the myriad diseases is weak. The potential roles of vitamin D are currently best described as hypotheses of emerging interest, and the conflicting nature of available evidence cannot be used to establish health benefits with any level of confidence, IOM says.
At present standards for vitamin D are set down by using the level of 25D in the blood. But a study based on postmortems shows a large proportion of people who had less than 10 ng/ml of 25D had normal bone histology.
Moreover, vitamin D is not the only nutrient required for adequate calcium levels, which ensure bone health. Calcium levels are also controlled by sodium and potassium levels in the diet. For instance, high intake of sodium increases excretion of calcium through urine, but it can be checked by adding more potassium in the diet. Consumption of alcohol, coffee and tea too reduce calcium absorption. Experts suggest consumption of carbonated soft drinks with high levels of phosphate can also lead to reduced bone mass and heighten risk of fracture.
|How accurate are tests?|
“Nearly everyone who comes to us for a test is deficient in vitamin D,” says Onjal Taywade, consultant biochemist at Dr Lal Pathlabs in Delhi. With symptoms of the deficiency ranging from aches in the bones to fatigue, almost everyone is eligible for the test. Biswajit Sen, a senior consultant pathologist at Dr Dangs Lab Pvt Ltd, says until three years ago, one or two patients came for this test. Now at least 40 patients visit his lab every day for the test. The test can cost anything from Rs 1,000 to Rs 3,000. But it is fast emerging that the diagnostic tests currently available in the country are not reliable.
Though RIA is used in India, it needs technical expertise as well as compliance of radioactive safety norms as mandated by local governing authorities. So the preferred technology of most diagnostic labs is chemiluminescent immunoassay (CLIA), which offers automated estimation of Vitamin D. However, accuracy is the lowest in CLIA. It tends to overestimate the frequency of vitamin D deficiency, according to a study published in the January 2013 issue of Clinica Chimica Acta. The researchers tested the DiaSorin LIAISON test kit based on the CLIA technology for the quantitative determination of 25D. In the study, researchers from University of Calgary, Alberta, compared this instrument with LC-MS/MS. They found that the chance of error was 36 per cent in DiaSorin Liaison, and 9 per cent in LC-MS/MS. DiaSorin LIASON is widely used in India.
A study by US researchers had also found inaccurate readings by similar test kits. They used Abbott Architect and Siemans Centaur2 to analyse the level of vitamin D in 163 blood samples. They compared the results with findings from LC-MS/MS. LC-MS/MS results showed that 33 of the 163 specimens showed vitamin D deficiency. The Abbott test showed that 45 specimens had vitamin D deficiency and the Siemens test showed that 71 subjects had vitamin D deficiency.
Earle W Holmes, professor at the Department of Pathology at Stritch School of Medicine of Loyola University Chicago, the US, who had carried out the study, said inaccurate test results could lead to misdiagnoses of patients and confound efforts of physicians, nutritionists and researchers to identify the optimal levels of vitamin D for good health.
“Since Vitamin D deficiency still does not fall under critical health problem category, there are no recommendations or strict guidelines for use of specific methodologies,” says Das. At SRL, both RIA and CLIA are being used. During the last financial year, 90,000 tests were carried out at SRL. They increased to 1,50,000 tests during this financial year. “We have recently acquired the LC-MS/MS technology. Vitamin D assay based on this cutting-edge technology would be available to the customer in a month’s time. We are the first lab in India to provide this technology,” Das says. In view of the affordability issues in India, the price for this test would be comparable to that of RIA or CLIA, Das reveals.
Biswajit Sen supports the rapid tests, which he says are reliable. But he emphasises that there is a need to keep tab over how samples are managed. Many diagnostic centres just collect samples, which are then sent to somewhere else for testing. There is a likelihood that the samples are spoiled in the process of transportation.
In many countries, the rising number of tests being carried out has increased the cost of healthcare to the government. In Auckland, New Zealand, annual requests for vitamin D measurement quadrupled between 2000 (8,500) and 2010 (32,800). In 2011, the total annual laboratory cost due to vitamin D testing was about NZ$1 million.
“Such findings have widespread consequences in terms of quality of care, unnecessary cost, and potential overdiagnosis. Further studies are needed to determine whether this increased testing translates into improved vitamin D status in the population and subsequent health outcomes.” Kellie Bilinski and Steven Boyages of The University of Sydney wrote in the British Medical Journal in July 2012.