In the general population, low blood vitamin D levels have been associated with higher risks of the development, or in some cases the exacerbation, of various diseases, including type 2 diabetes and kidney disease. The diagnosis of prediabetes, a physiologic warning sign of increased risk for type 2 diabetes and associated kidney disease, was a required inclusion criteria for this study, published in the Journal of the American Society of Nephrology. Lead author, Sun H. Kim, MD, MS, who is affiliated with the Stanford University School of Medicine, and colleagues conducted a secondary analysis of a randomized trial to evaluate the potential effects of supplemental vitamin D on kidney health and function in individuals with prediabetes.
 
This Vitamin D and Type 2 Diabetes (D2d) study examined 2,423 adults with a mean age of 60 years, body mass index 32 kg/m2, serum 25-hydroxyvitamin (D 25[OH]D) 28 ng/mL, estimated glomular filtration rate (eGFR) 87 mL/min per 1.73 m2, urine albumin-to-creatinine ratio (UACR) 11 mg/g, 79% with hypertension, and 10% with moderate, high, or very high worsening in kidney disease measured using the Improving Global Outcomes risk score. All patients were overweight or obese, had prediabetes, and were randomized to receive either vitamin D3 4,000 IU per day or placebo for a median duration of 2.9 years.

According to Dr. Kim, “The D2d study is unique because we recruited individuals with high-risk pre-diabetes, having 2-out-of-3 abnormal glucose values, and we recruited more than 2,000 participants, representing the largest vitamin D diabetes prevention trial to date. Our results did not show a benefit of vitamin D supplements on kidney function. About 43% of the study population was taking outside-of-study vitamin D, up to 1,000 IU daily, at study entry, though. Among those who were not taking any vitamin D on their own, there was a suggestion for vitamin D lowering the amount of urine protein over time, which means that it could have a beneficial effect on kidney health. Additional studies are needed to look into this further,” he added.

The authors reported the mean difference in eGFR in the vitamin D group from baseline as −1.0 mL/min per 1.73 m2 (95% CI, −1.3 to −0.7) and −0.1 mL/min per 1.73 m2 (95% CI, −0.4 to 0.2) in the placebo group. Mean difference in UACR was 2.7 mg/g and 2.0 mg/g for placebo group, respectively (95% CI, 1.2-4.3 and 95% CI, 0.5-3.6).

Despite the popularity of vitamin D supplementation and common clinical observance of deficiency, Dr. Kim concluded that more research is needed in patients with even lower vitamin D concentrations and poorer kidney function to detect a true difference. “The majority of the study population had sufficient blood vitamin D levels and normal kidney function," she said. "Benefits of vitamin D might be greater in people with low blood vitamin D levels and/or reduced kidney function,” said Dr. Kim.

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