Vitamin D Metabolites

Vitamin D is a complex steroid hormone whose production begins with cholesterol in the skin and ends with activation in the kidney or other tissues. There are a number of vitamin D metabolites in between these steps, and some help assess overall vitamin D status.

The 25(OH)D form is currently the most commonly used to assess vitamin D deficiency. However, it is considered the inactive “storage” form and must be converted to its most active hormonal form, 1,25(OH)2D (calcitriol), to exert its effects. Research suggests that 24,25(OH)2D may have some biological activity as well when converted to 1,24,25(OH)3D, a form that appears to play a role in insulin release (Ahmed 2020).

The 24,25(OH)2D metabolite is derived from the enzymatic catabolism of excess 25(OH)D, a key step in regulating local intracellular vitamin D levels. An elevated ratio of 25(OH)D to 24,25(OH)2D suggests reduced CYP24A1 enzyme activity and therefore reduced 25(OH)D catabolism. An increasing ratio is also associated with hypercalcemia caused by a CYP24A1 mutation (Dirks 2018).

Although 25(OH)D is the most common metabolite, low levels may not always correlate with signs of vitamin D deficiency, leading some researchers to recommend the measurement of other metabolites as well. The Vitamin D Metabolite Ratio (VMR) may provide a better assessment of vitamin D status than just assessing a single marker.

Vitamin D Metabolite Ratio (VMR)

The VMR can incorporate various metabolites of vitamin D, including 25(OH)D, 1,25(OH)2D, and 24,25(OH)2D. In one study of 460 type 2 diabetics, 3 VMRs were assessed. The ratio between 1,25(OH)2D and 25(OH)D was the best predictor of diabetic and cardiovascular complications, including hypertension, dyslipidemia, coronary artery disease, stroke, diabetic retinopathy, and diabetic neuropathy. The 24,25(OH)2D to 25(OH)D VMR was associated with diabetic retinopathy and peripheral artery disease (Ahmed 2020).

It is important to note that a higher level of 1,25(OH)2D, above 45.83 pg/mL (110 pmol/L), has been associated with autoimmunity, likely due to disruption of normal feedback mechanisms, which can significantly alter VMR calculations based on this biomarker (Blaney 2009).

Many factors should be taken into consideration when assessing vitamin D status, including (Fraser 2020):

• Measurement of 25(OH)D alone may be inaccurate due to cross-reactivity with other vitamin D metabolites.
• When 25(OH)D decreases below 20 ng/mL (50 nmol/L), the ratio of 1,25(OH)2D to 24,25(OH)2D will increase exponentially likely due to decreased catabolism of 25(OH)D to 24.25(OH)2D
• 24,25(OH2D assessment assists in the diagnosis of idiopathic infantile hypercalcemia
• Elevations in the ratio of 25(OH)D to 24,25(OH)2D above 80 can help identify 24-hydroxylase deficiency and abnormalities in the CYP24A1 gene. Healthy populations maintain a ratio of 7-23 though the ratio increases as vitamin D deficiency advances. Moderate elevation in the ratio may indicate renal impairment and disorders of the bone.
• The interaction of vitamin D metabolites with the vitamin D receptor (VDR) ultimately determines vitamin D's effects on bone physiology and healing, calcium homeostasis, muscle function, and the immune response to infection and inflammation

Additional factors being explored to improve the assessment of vitamin D status include free and bioavailable 25(OH)D, 3-epi-25(OH)D, and vitamin D binding protein. However, methods for measuring these markers have not yet been standardized and are not used clinically at this time. It is also important to consider risk factors for vitamin D deficiency, including dark skin pigmentation, malabsorption, obesity, advanced age, and lack of sunlight exposure.

Vitamin D insufficiency is associated with cancer, cardiovascular disease, neuropsychiatric disorders, autoimmune diseases, and endocrinopathies. Consistent monitoring of vitamin D status is warranted in these conditions. Measuring 1,25(OH)2D is specifically indicated in chronic kidney disease, vitamin-D resistant or pseudovitamin D-deficiency rickets, phosphate-losing disorders, oncogenic osteomalacia, and chronic granuloma-forming conditions, including sarcoidosis and lymphoma (Alonso 2022).

Conclusion

Although various ratios between vitamin D metabolites may have some value in assessing vitamin D status, VMRs are not yet considered substitutes for 25(OH)D in the diagnosis of vitamin D deficiency at this time (Aloia 2017, Cavalier 2018, Francic 2019).

References

Ahmed, Lina H M et al. “Vitamin D3 metabolite ratio as an indicator of vitamin D status and its association with diabetes complications.” BMC endocrine disorders vol. 20,1 161. 27 Oct. 2020, doi:10.1186/s12902-020-00641-1

Aloia, John et al. “The vitamin D metabolite ratio (VMR) as a predictor of functional biomarkers of bone health.” Clinical endocrinology vol. 86,5 (2017): 674-679. doi:10.1111/cen.13319 

Alonso, N et al. “Vitamin D Metabolites: Analytical Challenges and Clinical Relevance.” Calcified tissue international, 1–20. 3 Mar. 2022, doi:10.1007/s00223-022-00961-5

Blaney, Greg P et al. “Vitamin D metabolites as clinical markers in autoimmune and chronic disease.” Annals of the New York Academy of Sciences vol. 1173 (2009): 384-90. doi:10.1111/j.1749-6632.2009.04875.x

Cavalier, Etienne, and Jean-Claude Souberbielle. “Vitamin D and its metabolites: from now and beyond.” EJIFCC vol. 29,2 105-110. 11 Jul. 2018

Dirks, Niek F et al. “The When, What & How of Measuring Vitamin D Metabolism in Clinical Medicine.” Nutrients vol. 10,4 482. 13 Apr. 2018, doi:10.3390/nu10040482

Fraser, William D et al. “Vitamin D Measurement, the Debates Continue, New Analytes Have Emerged, Developments Have Variable Outcomes.” Calcified tissue international vol. 106,1 (2020): 3-13. doi:10.1007/s00223-019-00620-2

Francic, Vito et al. “The Effect of Vitamin D Supplementation on its Metabolism and the Vitamin D Metabolite Ratio.” Nutrients vol. 11,10 2539. 21 Oct. 2019, doi:10.3390/nu11102539

Lips, Paul. “Relative value of 25(OH)D and 1,25(OH)2D measurements.” Journal of bone and mineral research : the official journal of the American Society for Bone and Mineral Research vol. 22,11 (2007): 1668-71. doi:10.1359/jbmr.070716