Abstract
Aim. To examine the relationship between plasma 25-hydroxyvitamin D (25(OH)D) levels and blood hemoglobin A1c (HbA1c) levels in diabetic patients at various stages of chronic kidney disease (CKD). Methods. We screened for data collected between 2003 and 2012. The correlation between 25(OH)D and HbA1c levels was studied in patients categorized according to the severity of CKD and their vitamin D status. A multivariate linear regression model was used to determine whether 25(OH)D and HbA1c levels were independently associated after adjustment for a number of covariates (including erythrocyte metformin levels). Results. We identified 542 reports from 245 patients. The mean HbA1c value was 6.7 ± 1.0% in vitamin D sufficiency, 7.3 ± 1.5% in insufficiency, and 8.4 ± 2.0% in deficiency (P < 0.0001). There was a negative correlation between 25(OH)D and HbA1c levels for the population as a whole (r = −0.387, P < 0.0001) and in the CKD severity subgroups (r = −0.384, P < 0.0001 and r = −0.333, P < 0.0001 for CKD stages 1–3 and 4-5, resp.). In the multivariate analysis, the 25(OH)D level was the only factor associated with HbA1c (P < 0.0001). Conclusion. 25(OH)D levels were negatively correlated with HbA1c levels independently of study covariates.
Highlights
In addition to the pivotal role of vitamin D in calcium/phosphorus homeostasis and bone physiology [1, 2], several lines of evidence suggest that vitamin D status may have a significant role in glucose homeostasis in general [3] and on pathophysiology and progression of metabolic syndrome and Type 2 diabetes in particular [4]
It has been reported that most patients with Type 2 diabetes have low 25(OH)D levels and that hemoglobin A1c (HbA1c) levels are negatively correlated with vitamin D status [8]
We further refined the analysis of the association between vitamin D and HbA1c by taking account of erythrocyte metformin levels, since (i) the latter one may rise as renal function declines and (ii) this drug is known to influence HbA1c levels [28]
Summary
In addition to the pivotal role of vitamin D in calcium/phosphorus homeostasis and bone physiology [1, 2], several lines of evidence suggest that vitamin D status may have a significant role in glucose homeostasis in general [3] and on pathophysiology and progression of metabolic syndrome and Type 2 diabetes in particular [4]. It has been reported that most patients with Type 2 diabetes have low 25(OH)D levels and that hemoglobin A1c (HbA1c) levels are negatively correlated with vitamin D status [8]. In view of the above findings, one can hypothesize that vitamin D supplementation decreases insulin resistance and reduces HbA1c levels in patients with diabetes. As it is well known that ageing and chronic kidney disease (CKD) are associated with changes in vitamin D metabolism and insulin resistance [10, 11], these studies would have been enriched by taking into account the impact of a number of factors on glucose metabolism, such as age, renal status, and antidiabetic medications but to name a few
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