Bioavailable, free and total 25-(OH)D provide similar results in assessing bone health
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There is minimal difference among bioavailable, free and total serum 25-hydroxyvitamin D for predicting bone health in older adults, according to data presented at the American Society for Bone and Mineral Research virtual meeting.
“There has been increased interest in the measurement of bioactive metabolites, bioavailable and free serum 25-(OH)D, as superior predictors of vitamin D beneficial effects on bone metabolism,” Malak El Sabeh, MD, a postdoctoral research fellow at American University of Beirut, told Healio. “In our randomized vitamin D trial, conducted in ambulatory elderly, we demonstrated substantial increments in total, bioavailable and free 25-(OH)D levels after low and high vitamin D dose supplementation and significant differences between the two doses. We, however, showed no advantage in measuring the bioactive metabolites in better predicting indices of bone and mineral metabolism in this elderly population.”
El Sabeh and colleagues analyzed data collected in a double-blind, randomized controlled trial with 221 adults aged 65 years or older with a BMI of 25 kg/m2 or greater and serum 25-(OH)D levels between 10 ng/mL and 30 ng/mL. Half of the participants were placed into a low-dose group taking 600 IU of calcium and oral vitamin D3 supplements per day, and the other half were placed into a high-dose group and had a daily intake of 3,750 IU of vitamin D3 per day. Serum levels were measured at baseline, 6 and 12 months, and bone mineral density was measured at baseline and 12 months.
The findings showed an association between total 25-(OH)D levels and bioavailable (r = .942) and free 25-(OH)D levels (r =.943, P < .001 for both). Total 25-(OH)D, bioavailable and free 25-(OH)D levels were associated with similar increases in vitamin D levels in both the low- and high-dose groups at 6 and 12 months, with the high-dose group showing a vitamin D increase 1.3 to 1.4 times higher than the low-dose group for all three measures (P < .001). All three measures were also negatively correlated with parathyroid hormone (P < .001).
None of the three measurements was correlated with serum calcium, indices of bone remodeling or spine, hip or total body BMD. Total 25-(OH)D — but not bioavailable or free 25-(OH)D — was correlated with BMD change in the femoral neck at 12 months.
“Total, bioavailable and free 25-(OH)D were very tightly correlated with each other and had very similar relationships with the indices of bone and mineral metabolism,” El Sabeh said. “The measurement of bioavailable or free 25-(OH)D level is more costly and tedious, compared to established standardized total 25-(OH) assays. Such measurements are therefore unwarranted in an ambulatory elderly population, free of kidney or liver disease. Our findings do not apply to populations with different hormonal homeostatic milieu, such as children or pregnant women.”
In the presentation, El Sabeh said more studies are needed to examine the metabolites in populations where vitamin D-binding protein and albumin are affected and to examine the lack of standards and reference range for 25-(OH)D metabolites.
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El Sabeh M, et al. The relationship of total, bioavailable and free 25(OH)D with indices of bone health and metabolism in older adults: a randomized controlled trial. Presented at: American Society for Bone and Mineral Research Annual Meeting; Sept. 11-15, 2020 (virtual meeting).
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