By Allison Becker, ND, LAc

Naturopathic Doctor, Acupuncturist in private practice, Evansville, WI

Dr. Becker reports no financial relationships relevant to this field of study.

SYNOPSIS: Findings from a meta-analysis show vitamin D supplementation was associated with improved glycemic control in vitamin D-deficient or non-obese patients with type 2 diabetes.

SOURCE: Wu C, et al. Vitamin D supplementation and glycemic control in type 2 diabetes patients: A systemic review and meta-analysis. Metabolism 2017;73:67-76.

The subject of vitamin D supplementation and its effects on various aspects of health is a popular topic. Previous studies have demonstrated the benefits of vitamin D extend past bone and parathyroid health to include metabolic diseases such as type 2 diabetes and obesity. Mechanisms of action on metabolism are elucidated further in the commentary. Researchers have found that lower vitamin D levels are relatively common in those with type 2 diabetes.1,2 However, the causal relationship between vitamin D and type 2 diabetes is unclear. Wu et al attempted to clarify this relationship through a careful meta-analysis and review of previous studies.

Currently, more than 400 million people worldwide suffer from diabetes.3 Improving glycemic control is the primary goal in diabetes management. There are many effective dietary and pharmacological agents used in diabetes management. However, diabetes continues to rise at epidemic proportions. Gathering more tools to address this problem is wise. Other investigators have explored the use of vitamin D in glycemic control.4 Some studies have shown benefit, while others have shown no benefit. Previous studies have included dietary intake of vitamin D, patients who did not have diabetes, and patients with prediabetes. Wu et al conducted this review and meta-analysis to evaluate an association between vitamin D supplementation, specifically, and the reduction of fasting blood glucose (FBG) as well as glycosylated hemoglobin A1c (HbA1c) only in patients with type 2 diabetes. FBG reflects daily glycemic fluctuations. HbA1c indicates the average plasma glucose level over the previous eight to 12 weeks.

Wu et al started with 637 articles and narrowed their list to include just 26 in the final analysis. They searched PubMed, Web of Science, and the Cochrane Library for articles published through March 2017 and included international publications in any language. The authors’ numerous exclusions strengthened the final analysis. To be included in this review, studies had to be randomized, controlled trials of human subjects with type 2 diabetes that evaluated HbA1c and FBG. The studies used either vitamin D, vitamin D analogues, and/or vitamin D and calcium supplementation. The studies included also had to demonstrate some change in serum vitamin D and a difference between the intervention group and the control group. All 26 studies were printed in full-text articles.

Vitamin D deficiency was defined as serum 25(OH)D concentrations of < 20 ng/mL (50 nmol/L). Vitamin D insufficiency was defined as serum (OH)D concentrations of 20-30 ng/mL (50-75 nmol/L), and vitamin D sufficiency was defined as serum 25(OH)D concentrations of > 30 ng/mL (75 nmol/L). Other investigators have suggested that serum concentrations > 30 ng/mL maximize the noncalcemic benefits of vitamin D.5

Wu et al conducted a further analysis in subgroups including baseline 25(OH)D, body mass index (BMI), dosage of vitamin D supplementation, length of intervention, and change of 25(OH)D concentration. In this evaluation of moderating factors, Wu et al found vitamin D supplementation significantly benefitted FBG and HbA1c in those who were non-obese with a BMI < 30 kg/m2. BMI is an attempt to quantify the amount of tissue mass, including muscle, fat, and bone, in an individual to categorize them as underweight, normal weight, overweight, or obese.

Another subgroup analysis showed that HbA1c and FBG levels both dropped significantly when changes in 25(OH)D serum levels were < 20 ng/mL or between 10-20 ng/mL. The connection between physiological parameters and serum vitamin D levels is important, demonstrating benefits even with small laboratory changes with vitamin D repletion. The dosing of vitamin D also was highly variable, from as low as 1,000 IU per day to a single intramuscular injection of 300,000 IU. Additionally, the course of intervention ran from four to 48 weeks. Because of this high variability in dosing quantity and trial length, this analysis failed to yield specific clinical recommendations. However, in patients who have a serum 25(OH)D level < 20 ng/mL and are non-obese, vitamin D supplementation can lower their glycemic parameters. Unfortunately, the authors did not identify a specific dosing schedule that can reach a beneficial serum level.

Although Wu et al were careful to streamline the data included in this analysis by excluding many other studies, there were some weaknesses in the methods to gather data. First, not all study authors reported hypoglycemic pharmacologic use. Authors of seven studies included in this analysis did not report diabetes pharmaceutical use; therefore, lower blood sugars could be attributed to pharmaceuticals rather than vitamin D supplementation. There was not a subgroup analysis conducted to eliminate this variable. Second, the numbers of participants in the included studies were relatively small. On average, studies included 30-40 participants. Third, most authors did not include the effects of sun exposure, dietary intake, or regular exercise, which all contribute to vitamin D synthesis.

Despite these limitations, we can conclude that patients with diabetes and deficient vitamin D levels would benefit from some amount of vitamin D supplementation. The exact amount and for how long has yet to be determined.


Previous studies regarding vitamin D and glycemic control have been conflicting and confusing. Some have demonstrated benefit, and others have found no benefit. No trials have shown a worsening of glycemic parameters with vitamin D supplementation. Most studies contained confounding variables and interfering factors that weaken their arguments in favor of or against vitamin D. Wu et al tried to resolve this confusion by presenting data from a highly selective group of studies, breaking the collected data into subgroups, and analyzing the data using different statistical methods. They focused specifically on vitamin D supplementation in people with type 2 diabetes and evaluated HbA1c and FBG as their specific parameters. Most of the body’s vitamin D is synthesized in the skin upon exposure to sunlight. Upon stimulation in the skin, vitamin D3 is produced from pro-hormone 7-dehydrocholesterol. Vitamin D3 (cholecalciferol) is the more bioactive form of vitamin D than vitamin D2 (ergocalciferol).6 Vitamin D2 does not bind as well to receptors in human tissue as vitamin D3. Most experts agree vitamin D3 should be used in practice.7 In this meta-analysis, Wu et al did not distinguish between the use of vitamins D3 and D2, and this discrepancy may contribute to a decreased metabolic effect overall.

To understand the mechanism of vitamin D, it is best to think of it more as a hormone than a vitamin. Instead of serving as a cofactor and facilitator for metabolic processes like a vitamin, vitamin D regulates metabolic processes.8 The effect of vitamin D on glucose metabolism is not entirely clear. However, it is understood vitamin D deficiency impairs glucose-mediated insulin secretion in pancreatic beta cells.9 Data suggest that normalization of vitamin D stimulates insulin secretion in rats that are deficient in vitamin D.10 It is postulated vitamin D’s glucose regulation occurs in two ways: by regulating plasma calcium levels, which regulate insulin synthesis and secretion, and through a direct action on pancreatic beta-cell function.11

In this meta-analysis, some studies included patients receiving insulin treatments, and serum insulin was not measured as a parameter. The specific effect of vitamin D supplementation on serum insulin is an area of research to be explored. Non-obese patients with diabetes received the most glycemic benefit from vitamin D supplementation in this meta-analysis. Although obesity is a risk factor for vitamin D deficiency,12 these findings suggest improving vitamin D status in obese patients does not significantly improve their glycemic parameters. In one study, Hypponen et al reported that as BMI increased, serum 25(OH)D decreased and HbA1c increased,13 suggesting the association between vitamin D and glucose metabolism might depend on body habitus. The most clinically relevant information in this analysis is to understand vitamin D as biologically active influence on metabolism. Further research on mechanism and specific clinical protocols has yet to be developed. However, in our non-obese patients with diabetes who are deficient in vitamin D, supplementing with vitamin D3 on a daily basis could yield glycemic benefits.


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