Among adults at high risk of type 2 diabetes — the leading cause of kidney disease — those who received vitamin D supplements as opposed to placebo for close to 3 years did not have significant differences in kidney function, in a secondary analysis of the Vitamin D and Type 2 Diabetes (D2d) study.
However, most of these adults with prediabetes plus obesity or overweight also had sufficient serum levels of 25-hydroxyvitamin D (25[OH]D) and a low risk for adverse kidney outcomes at study entry.
“The benefits of vitamin D might be greater in people with low blood vitamin D levels and/or reduced kidney function,” lead author Sun H. Kim, MD, Stanford University School of Medicine, California, speculated in a statement from the American Society of Nephrology.
The study was published online August 6 in the Clinical Journal of the American Society of Nephrology.
“The D2d study is unique because we recruited individuals with high-risk prediabetes, having two out of three abnormal glucose values, and we recruited more than 2000 participants, representing the largest vitamin D diabetes prevention trial to date,” Kim pointed out.
Although the study did not show a benefit of vitamin D supplements on kidney function outcomes, 43% of participants were already taking up to 1000 IU of vitamin D daily when they entered the study, she noted.
A subgroup analysis of individuals who were not taking vitamin D at study entry found that vitamin D supplements were associated with lowered proteinuria, “which means that it could have a beneficial effect on kidney health,” said Kim, cautioning that “additional studies are needed to look into this further.”
Effect of Vitamin D on Three Kidney Function Outcomes
Although low levels of serum 25(OH)D are associated with kidney disease, few trials have evaluated how vitamin D supplements might affect kidney function, Kim and colleagues write.
The D2d trial, they note, found that vitamin D supplements did not lower the risk of incident diabetes in people with prediabetes recruited from medical centers across the United States, as previously reported in 2019.
However, since then, meta-analyses that included the D2d trial have reported a significant 11%-12% reduction in diabetes risk in people with prediabetes who took vitamin D supplements.
The current secondary analysis of D2d aimed to investigate whether vitamin D supplements affect kidney function in people with prediabetes.
A total of 2166 participants in D2d with complete kidney function data were included in the analysis.
The three study outcomes were change in estimated glomerular filtration rate (eGFR) from baseline, change in urine albumin-to-creatinine ratio (UACR) from baseline, and worsening Kidney Disease: Improving Global Outcomes (KDIGO) risk score (which takes eGFR and UACR into account).
At baseline, patients were a mean age of 60, had a mean body mass index (BMI) of 32 kg/m2, and 44% were women.
Most (79%) had hypertension, 52% were receiving antihypertensives, and 33% were receiving an angiotensin-converting enzyme (ACE) inhibitor or an angiotensin receptor blocker (ARB).
Participants had a mean serum 25(OH) level of 28 ng/mL.
They had a mean eGFR of 87 mL/min/1.73m2 and a mean UACR of 11 mg/g. Only 10% had a moderate, high, or very high KDIGO risk score.
Participants were randomized to receive a daily gel pill containing 4000 IU vitamin D3 (cholecalciferol) or placebo.
Medication adherence was high (83%) in both groups during a median follow-up of 2.9 years.
There was no significant between-group difference in the following kidney function outcomes:
- 28 patients in the vitamin D group and 30 patients in the placebo group had a worsening KDIGO risk score.
- The mean difference in eGFR from baseline was –1.0 mL/min/1.73m2 in the vitamin D group and –0.1 mL/min/1.73 m2 in the placebo group.
- The mean difference in UACR from baseline was 2.7 mg/g in the vitamin D group and 2.0 mg/g in the placebo group.
The authors have reported no relevant financial relationships.
CJASN. Published online August 6, 2021. Abstract