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Vitamin D Supplementation for Diabetes Prevention

Aug 4, 2018

Vitamin D supplementation for diabetes: shows potential to lower HbA1c, FPG, insulin resistance, and 2-hour post-prandial glucose levels.

People with prediabetes are at a much higher risk of not only developing type 2 diabetes, but also complications such as heart disease, stroke, and peripheral nerve damage. These complications can also arise without the progression of type 2 diabetes. A correlation between vitamin D levels and prediabetes has been found after reviewing many clinical trials. Compared to patients with normal glycemic control, people with prediabetes have lower levels of 25-hydroxyvitamin D [25(OH)D]. With vitamin D supplementation, insulin resistance and impaired insulin secretion may be prevented.


A meta-analysis was done on 28 randomized placebo-controlled clinical trials where patients received either vitamin D supplementation alone or in combination with calcium. The studies collected were from 2007 to 2017 and were from a variety of countries. The effect of this supplementation on glycemic control and insulin resistance was the primary outcome of resistance. To evaluate this, HbA1c, fasting blood glucose, 2-hour post-prandial glucose, and homeostatic model assessment (HOMA-IR) for insulin resistance were assessed in adult populations with prediabetes, obesity, or overweight. The impact of obesity and the coadministration of calcium on 25(OH)D levels was also evaluated. The data sources used for this investigation included PubMed/Medline, Cochrane library, Cumulative Index to Nursing and Allied Health Literature, and Google Scholar. Adults had to be 18 years old or older and the supplementation had to be administered for at least two months to allow for changes in the glucose blood markers.

The vitamin D was supplemented daily, twice a week, or once weekly. Serum levels of 25(OH)D were assessed at baseline and at the end of the intervention along with HbA1c, insulin resistance, fasting glucose levels, and post-prandial levels. Vitamin D serum levels were collected in ng/mL and then multiplied by a factor of 2.496 to record the levels in nmol/L. Plasma glucose levels were multiplied by a factor of 0.0555 to convert measurements from mg/dL to mmol/L. A sensitivity analysis was done by removing one study each time and repeating the analysis (leave-one -out method) to determine whether that study made an impact on the results.

Daily doses of vitamin D ranged from 10.5 mcg/day to 175 mcg/day. Weekly doses ranged from 500 to 2222 mcg/week. The average 25(OH)D serum levels at baseline ranged from 25 to 76 nmol/L. Sixteen studies assessed the effect of vitamin D on HbA1c. Of these 16 studies, 7 reported reductions in HbA1c compared to placebo (p<0.002). Twenty-five studies reported fasting blood glucose levels after supplementation of vitamin D. Eight of these studies reported lower fasting levels, 4 reported higher levels, and 13 reported no change in fasting blood glucose levels. The combined data showed a reduction in fasting blood glucose by -0.46 mmol/L (p<0.001). Twenty studies assessed the effects of vitamin D on insulin resistance using HOMA-IR.

Eight of these studies reported a decrease in insulin resistance with vitamin D supplementation, one study found an increase, and 11 studies found no effect on insulin resistance. The combined data showed a reduction of HOMA-IR by -0.39 (p<0.007). Ten studies evaluated the effect of vitamin D supplementation on 2-hour post-prandial blood glucose. Two of these studies reported positive results in post-prandial glucose levels, one study reported negative results, and seven reported no effects. Combined data showed that vitamin D supplementation does not affect 2-hour post-prandial glucose levels. Vitamin D in combination with calcium showed no significant difference in HbA1c or HOMA-IR when compared to vitamin D supplementation alone. In contrast, vitamin D in combination with calcium significantly improved glycemic control by lowering 2-hour post-prandial glucose levels (p<0.02) and fasting blood glucose levels (p<0.002). There was no difference in the change of any of these parameters when evaluating persons with obesity versus persons without obesity.

In conclusion, vitamin D supplementation for diabetes was found to lower HbA1c, fasting blood glucose, and HOMA-IR. It was also found that these results were dose dependent. The higher the dose of vitamin D, the greater effects it had on all four parameters, including 2-hour post-prandial levels. Subgroup analysis demonstrated that the vitamin D and calcium combination improved fasting blood glucose levels and post-prandial levels. Also, longer durations of supplementation (6 months) demonstrated better effects on insulin resistance compared to shorter durations of supplementation (3 months). The level of vitamin D to produce these effects is about 88 mcg/day (3500 IU/day). Due to these findings, supplementation of vitamin D may be extremely beneficial for those at risk of developing type 2 diabetes.

Practice Pearls:

  • People with prediabetes are at risk for developing the complications of diabetes or diabetes itself.
  • Low levels of vitamin D have commonly been found in those patients with prediabetes and diabetes.
  • Vitamin D supplementation for diabetes has the potential to lower HbA1c, FPG, insulin resistance, and 2-hour post-prandial glucose levels.
  • Vitamin D supplementation with or without calcium has demonstrated to be beneficial in the prevention of type 2 diabetes.

Reference – Vitamin D supplementation for diabetes:

Mirhosseini, Naghmeh, et al. “Vitamin D Supplementation, Glycemic Control, and Insulin Resistance in Prediabetics: A Meta-Analysis.” Journal of the Endocrine Society, vol. 2, no. 7, 2018, pp. 687–709., doi:10.1210/js.2017-00472.

Amanda Cortes, 2019 LECOM School of Pharmacy PharmD Candidate