Conference Coverage

Vitamin D deficiency: Can we improve diagnosis?


AT ENDO 2023

Use of a ratio of vitamin D metabolites to assess vitamin D status rather than total 25-hydroxyvitamin D [25(OH)D] level may provide a better index of individual susceptibility to bone damage due to deficiency, new research suggests.

The study supports previous data suggesting that a ratio cut-off of greater than 100 is associated with the development of secondary hyperparathyroidism and the need for correction with supplementation, while a level greater than 50 suggests mild to moderate deficiency, Zhinous Shahidzadeh Yazdi, MD, noted in a poster presented at the annual meeting of the Endocrine Society

Current Endocrine Society guidelines published in 2011 advise measurement of plasma circulating 25(OH)D levels to evaluate vitamin D status in patients at risk for deficiency, defined as < 20 ng/mL (50 nmol/L). Revised guidelines are due out in early 2024.

“We don’t think measuring 25 hydroxy D is optimal because of the impact of vitamin D binding protein,” Dr. Yazdi said in an interview.

“Over 99% of all metabolites are bound to vitamin D binding protein, but only the free fraction is biologically active. By measuring total plasma 25(OH)D – as we do right now in clinic – we cannot account for the impact of vitamin D binding proteins, which vary by threefold across the population,” she added.

Thus, the total 25(OH)D deficiency cut-off of < 20 mg/mL currently recommended by the Endocrine Society may signal clinically significant vitamin D deficiency in one person but not another, noted Dr. Yazdi, a postdoctoral fellow at the University of Maryland, Baltimore.

Directly measuring binding protein or the free fraction would be ideal, but “there aren’t good commercial assays for those, and it’s more difficult to do. So, as an alternative, the vitamin D metabolite ratios implicitly adjust for individual differences in vitamin D binding protein,” she explained.

The ratio that Dr. Yazdi and colleagues propose to measure is that of the vitamin D metabolites 1,25(OH)2D/24,25 (OH)2D (shortened to 1,25D/24,25D), which they say reflect the body’s homeostatic response to vitamin D levels, and which rises in the setting of deficiency. It is a measurement > 100 in this ratio that they believe means the patient should receive vitamin D supplementation.

Controversial topic, ratio proposal is “very early in the game”

The issue of vitamin D deficiency has long generated controversy, particularly since publication of findings from the VITAL study in 2022, which showed vitamin D supplements did not significantly reduce the risk of fracture among adults in midlife and older compared with placebo.

According to the senior author of the new study, Simeon I. Taylor, MD, professor of medicine at the University of Maryland, what still remains controversial after VITAL is the question: “How can you identify people who have sufficiently bad vitamin D deficiency that it’s adversely impacting their bones?”

He added that there is a suggestion that small subpopulations in VITAL really did benefit from vitamin D supplementation, but the study “wasn’t designed to look at that.”

Indeed, the authors of an editorial accompanying the publication of the VITAL study said the findings mean there is no justification for measuring 25(OH)D in the general population or for treating to a target level.

Asked to comment on Dr. Yazdi and colleagues’ ratio proposal for diagnosing vitamin D deficiency, the coauthor of the VITAL study editorial, Clifford J. Rosen, MD, said in an interview: “I do think it’s important to point out that changes in the vitamin D binding protein can have a significant impact on the level of 25 [OH] D ... People should recognize that.”

Dr. Clifford J. Rosen, director of the Center for Clinical and Translational Research, Maine Medical Center Research Institute, Scarborough

Dr. Clifford J. Rosen

And, Dr. Rosen noted, “I like the idea that the ... [ratio] is a measure of what’s happening in the body in response to vitamin D stores. So, when you supplement, it comes back up ... In certain individuals at high risk for fractures, for example, you might want to consider a more extensive workup like they’re suggesting.”

However, Dr. Rosen, of the Rosen Musculoskeletal Laboratory at Maine Medical Center Research Institute, Scarborough, added: “If the 25[OH]D level is below 20 [ng/mL] you’re going to treat regardless. When we think about sensitivity, a 25[OH]D level less than 20 [ng/mL] is a good screen ... Those individuals need to be treated, especially if they have low bone mass or fractures.”

To validate the ratio for clinical use, Dr. Rosen said, larger numbers of individuals would need to be evaluated. Moreover, “you’d need to run a standard of vitamin D binding protein by mass spectrometry versus their assumed method using ratios. Ratios are always a little tricky to interpret. So, I think this is very early in the game.”

And measuring the ratio of 1,25D/24,25D “is quite expensive,” he added.

He also pointed out that “calcium intake is really critical. You can have a [25(OH)D] level of 18 ng/mL and not have any of those secondary changes because [you’re] taking adequate calcium ... So, that always is a consideration that has to be worked into the evaluation.”


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