This study reports 1,24,25(OH)3D3 levels in humans across a spectrum of measured GFRs. To address the limitations of a cross-sectional study, we assessed the same parameters longitudinally in rat circulation with induction and progression of CRF. 1.25(OH)2D3 catabolism reflected by 1,25-VMR increased while 25(OH)D3 catabolism (25-VMR) decreased as mGFR decreased, a finding we also observed longitudinally in rats.
The progressive reduction of 25-VMR with decreasing mGFR in humans, and with induction and progression of CRF in rats, supports the work of others12 and indicates stagnation of 25(OH)D3 catabolism.. The biological significance of 24-hydroxy metabolites in circulation is unclear; however, there is evidence that 1,24,25(OH)3D3 is biologically active and 24.25(OH)2D3 facilitates the repair of fractures13,14,15,16. The overall reduction of 24-hydroxylation in CKD has been proposed to explain the low levels of 24,25(OH)2D3, however, this theory requires further evaluation since 1,25-VMR does not have the same tendency as 25-VMR. The kidneys are the main site of origin of 1,25(OH)2D3 in the circulation, but increased or unchanged expression of CYP24A1 in renal tissue has been reported17,18,19.20. Although these results are not consistent with the reduction in 25-VMR, they are consistent with 1,25-VMR.
Participants taking cholecalciferol had higher levels of 1,24,25(OH)3D3. One possibility is that cholecalciferol increased 1,25(OH)2D3 production and subsequent catabolism, despite low kidney function. If this hypothesis were true, it could justify the use of cholecalciferol in patients with kidney disease. Alternatively, it could also suggest the presence of pathways that convert 25(OH)D3 at 1,24,25(OH)3D3 independent of a 1,25(OH)2D3 intermediate as suggested by Martineau et al.16 In a study of participants with moderate to severe CKD, cholecalciferol supplementation increased 25(OH)D levels3 substantially, but the change of 24.25(OH)2D3 was more than proportional to the increase in 25(OH)D suggesting that supplementation increased 25(OH)D supply to CYP24A1 and/or increased CYP24A1 activity21. 1.25(OH) levels2D3 did not change. In a study involving multi-ethnic atherosclerosis study participants, lower 25(OH)D and lower 25-VMR were associated with greater treatment response to cholecalciferol supplementation, such as evaluated by a change in PTH22. Similar to the previous study, there was no change in the 1,25(OH)2D3 in participants supplemented with cholecalciferol.
Study strengths include LC-MS/MS assessment of vitamin D metabolites which allows for accurate assessment of these structurally very similar metabolites, a limitation of other methods4.23 as well as GFR measurement, as opposed to estimation based on endogenous markers. The cross-sectional design of the human study limits interpretation, but longitudinal rat data demonstrated the identical course of changes in vitamin D metabolites with progression of CKD. Participants were predominantly white limiting generalizability. Future studies should consider acute challenges of different metabolites to assess specificity and transformation kinetics. We recognize that variability in metabolite-to-parent ratios may not solely reflect true enzyme activity. For example, Hsu et al. demonstrated higher clearance of 25(OH)D in black individuals, despite lower 25-VMR24. Moreover, since 1α-hydroxy metabolites circulate at approximately 1000 times lower concentration than their non-1α-hydroxy counterparts, differences unrelated to 24-hydroxylation, e.g., intra-individual variation or dosage variability, can significantly alter the 1,25-VMR, without altering the other metabolites circulating at a higher level, and therefore the circadian rhythm and the metabolism of 1,24,25(OH)3D3 is an important area for future research before using it as a potential diagnostic tool. Development of a vitamin D profile that includes VMRs and assesses response over time to supplementation may better define adequacy in this population and guide future therapy as suggested by Melamed et al.25.
In summary, circulating 1,25(OH)-24-hydroxy2D3 the ratio does not decrease like 25-VMR when kidney function declines. The clinical implications of the discrepancy between 25(OH)D catabolism3 against 1.25(OH)2D3 requires further study. Understanding vitamin D catabolism and the potential that 24-hydroxylation products may have biological activity can help inform treatment strategies in the future.