There’s a consensus that vitamin D deficiency is prevalent in the world.
Yet, reviews of vitamin D studies throughout the past decade show that analyses have yielded highly variable results, “lacking in both accuracy and precision,” researchers in Australia argue.
Take the 2014 umbrella review on vitamin D published in the British Medical Journal for example—it drew a lot of criticism from both the scientific community and industry for including what many considered flawed studies.
“Imprecise measurement of [vitamin D] in epidemiological research can result in incorrect null findings of associations with disease,” wrote a team of researchers from various Australian institutions, which included the Australian National University in Canberra, The University of Western Australia and Curtin University in Perth, and the QIMR Berghofer Medical Research Institute in Brisbane.
In their paper, posted open access in the Journal of AOAC International, the researchers explored the implications of inaccuracy in vitamin D analysis has posed a major roadblock in understanding the vitamin and how it relates to our health, and called for standardization of measuring the 25-hydroxyvitamin D [25(OH)D] in serum or plasma.
Implications to clinical decision making, effect on public health
When it comes to the long-studied health benefits of vitamin D, such as bone health, there's little argument against it. Where inaccuracy becomes a road block is when it comes to exploring outcomes of disease risk associated with 25(OH)D concentration, setting recommendations, and designing public health policies around the nutrient.
In terms of clinical decision making, the researchers pointed out that some assays used to analyze vitamin D are biased low, such as immunoassays, while others are biased high, such as liquid chromatography-mass spectrometry.
“With an assay that is biased high (it returns an inaccurately high result), patients who are truly vitamin D deficient may not be treated. Alternatively, from results of an assay that is biased low, patients risk being prescribed vitamin D supplements unnecessarily,” they wrote.
Because of possible association between vitamin D deficiency and a wide range of health risks, a high prevalence of vitamin D deficiency is a considerable concern. Hence there is “importance of the representativeness of the study population when extrapolating from epidemiological studies or surveys to whole populations,” they argued.
Because of this, a standard reference method of researching vitamin D adjusted to specific populations can help legislators design public health policies more accurately, such as the need for fortification or adjusting recommended daily intake—which, in the US, is still a contested value.
Industry experts agree with paper, researchers’ arguments
Commenting independently on the study, nutrition researcher and consultant Dr Taylor Wallace, who owns his own practice, concurred that there lacks standardization of measuring serum 25(OH)D.
He also agreed that vitamin D studies are most valuable when they are larger and have a more representative sample of participants.
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“For example, African-Americans seem to metabolize vitamin D very differently from Caucasians, but most of our reference data used assess vitamin D status based on studies of predominantly Caucasian individuals,” he told NutraIngredients-USA. “Having data on multiple populations helps enable us to give more personalized nutrition advice.”
Also in agreement is Andrea Wong, PhD, VP of scientific and regulatory affairs at the Council for Responsible Nutrition (CRN), a dietary supplement trade association.
“We agree with the authors that issues with precision and accuracy in 25(OH)D assays can impact everything from clinical decision-making for healthcare practitioners who provide personal guidance regarding diet and supplementation to their patients, to public health initiatives like food fortification or dietary guidelines,” she said.
“In the U.S., the Dietary Guidelines Advisory Committee (for the 2015-2020 Dietary Guidelines for Americans) used biomarker data, in addition to dietary intake data, to assess vitamin D status of population groups. We encourage the use of accurate measures of vitamin D levels to inform public health recommendations.”
What does accurate mean?
The Australian researchers pointed to the international Vitamin D Standardization Program (VDSP), established by the National Institutes of Health Office of Dietary Supplements in 2010, to address problems in measuring 25(OH)D concentrations in blood.
They defined accuracy as the degree to which the result returned by an assay is the same as assigned by a reference measurement protocol, whereas precision refers to the lack of random error in a test.
“The VDSP was originally developed to provide a reference measurement protocol against which assays could be standardized for use in national health surveys. Such standardization would enable comparisons across countries and, eventually, over time,” the researchers wrote.
“This would essentially allow the creation of a huge international cohort with repeated measures over time, providing an incredibly rich data set of routinely collected data.”
Source: Journal of AOAC International
Vol. 100 No. 5, 2017, DOI: 10.5740/jaoacint.17-0082
“Clinical, Research, and Public Health Implications of Poor Measurement of Vitamin D Status”
Authors: R. M. Lucas, S. Gorman, L. Black, R. E. Neale