In the clinical assessment of vitamin D concentrations, free 25-hydroxyvitamin D shows little added benefit to the current standard of total 25(OH)D, with deficiencies in each associated with at least a twofold risk of all-cause mortality, new research shows.
“In this prospective, population-based study of middle-aged and older European men, total 25(OH)D levels below 20 µg/L were independently associated with a twofold increased all-cause mortality,” the researchers reported.
“Lower concentrations of free 25(OH)D were also predictive of mortality, but did not provide any additional information,” they noted. “The data do not support routine measurement of free 25(OH)D or 1,25(OH)2D [1,25-dihydroxyvitamin D] over total 25(OH)D levels.”
Despite vitamin D deficiency being well established as playing a role in a wide range of adverse health effects, including cardiovascular disease and mortality, there has been a lack of consensus on the optimal concentration of total 25(OH)D, with studies showing inconsistent levels to define insufficiency and deficiency.
One aspect of the debate has focused on precisely how to measure the concentrations, with some evidence supporting the “free hormone hypothesis,” which suggests that free 25(OH)D could represent a better indicator than the standard total 25(OH)D of functional availability of vitamin D, and have stronger clinical utility.
To investigate both issues, Marian Dejaeger, MD, PhD, and colleagues evaluated prospective data on 1,915 men recruited from eight centers around Europe in the European Male Aging Study in a report published in the Journal of Clinical Endocrinology & Metabolism
The men, who were aged between 40 and 79 years, had a mean follow-up of 12.3 years; during that time, about a quarter (23.5%) of them died.
In addition to other factors, including being older, having a higher body mass index, and having at least two comorbidities, men who died had significantly lower levels of total 25(OH)D, total 1,25(OH)2D, free 25(OH)D, and free 1,25(OH)2D, as well as higher parathyroid hormone and creatinine values.
After adjustment for key confounders, including body mass index, smoking, alcohol consumption, kidney function, number of comorbidities at baseline and other factors, men with a total 25(OH)D below 20 µg/L had a significantly increased risk of mortality, compared with those who had normal levels of vitamin D, defined as above 30 µg/L (hazard ratio, 2.03; P < .001).
In terms of free 25(OH)D, the lowest three free 25(OH)D quintiles (under 4.43 ng/L) similarly had a significantly higher mortality risk, compared with the highest quintile (HR, 2.09; P < .01) after adjustment for the confounders.
Further observations of all quintiles of other measures of 1,25(OH)2D and vitamin D binding protein (DBP) showed no associations with mortality after adjusting for confounders.
Methods of Measurement
An important caveat of the study is the type of method used to measure free 25(OH)D. The authors calculated free 25(OH)D using a formula, as opposed to the alternative of direct measurement with an enzyme-linked immunosorbent assay kit, and there can be important differences between the two approaches, said Daniel Bikle, MD, PhD, a professor of medicine and dermatology at the San Francisco Veterans Affairs Medical Center and University of California, San Francisco, in a comment on the research.
“The biggest problem is that calculating free 25(OH)D does not give an accurate estimate of the real free level, so making conclusions regarding its role in clinical situations is subject to error,” said Bikle, who recently authored a review of the free hormone hypothesis.
A calculation approach “depends heavily on the total 25(OH)D level, so in a population with reasonably normal DBP and albumin levels, the correlation with total 25(OH)D is very high, so I am not surprised by the results showing no additional value,” he said in an interview.
The authors addressed their use of the calculation over the direct measurement in the study, noting that there is a “high correlation between both methods.”
But they added that, “as no equilibrium analysis method is available for free 25(OH)D, nor for free 1,25(OH)2D, no method can be considered superior.”
Dejaeger, of the department of public health and primary care, Katholieke Universiteit Leuven (Belgium), added that she agreed that high or low DBP could potentially shift some correlations, but noted that other research has shown calculated and direct measures to match relatively well.
“So we partly agree [with Bikle] not being surprised that we did not find an added value because we also found little variation in DBP, but we are not convinced that a different measurement method could make the difference here.”
Another caveat of the study is that, despite half of the measurements being taken in the summer, more than 90% of subjects in the study’s cohort had vitamin D insufficiency, defined in the study as total 25(OH)D levels below 30 µg/L, and as many as 70% had deficiency, with levels below 20 µg/L.
Therefore, “as the number of participants with high levels of total 25(OH)D in our study is small, a true threshold concentration for optimal vitamin D status cannot be defined on basis of our data,” the authors noted.
Under current recommendations, the Endocrine Society indicates that concentrations below 30 µg/L are insufficient, while other groups, including the Institute of Medicine, suggest concentrations of 20 µg/L or above are adequate.
Free Hormone Hypothesis
Under the free hormone hypothesis, which is observed with thyroid hormones and sex steroids, the very small fraction of free hormones that are not bound to protein carriers can enter cells and help facilitate biologic activity.
The hypothesis of a role of free 25(OH)D in mortality was supported by a recent study, in which free 25(OH)D levels – but not total 25(OH)D levels, were found to be independently associated with an increased risk of all-cause and cardiovascular mortality among patients with coronary artery disease.
However, two other studies are more consistent with the new findings, including one study showing no added value of free 25(OH)D as a marker for bone mineral density in older women, and another study showing no value as a marker of metabolic variables in healthy children.
“Currently, there are no hard data to support routine measurements of free 25(OH)D or 1,25(OH)2D over total 25(OH)D, the current standard of assessing vitamin D status, as stated in guidelines from different scientific bodies,” Dejaeger said in an interview.
The study received support from Versus Arthritis and the National Institute for Health Research Manchester Biomedical Research Centre. Dejaeger and Bikle had no disclosures to report.
This article originally appeared on MDedge.com, part of the Medscape Professional Network.
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