Globally, type 2 diabetes mellitus (T2DM) is shifting in two directions: first, from being predominantly a disease of the affluent toward disadvantaged populations and nations; and second, from being a disease of old age toward exponential growth among young people [1] [2] [3] . Some of the factors associated with these shifting patterns include uneven distribution of health determinants and the unintended consequences of phenomena such as aging populations, growing industrialization, urbanization, and market globalization [4] [5] [6] [7] [8] . While the high-income countries have benefited from these phenomena, the low-income countries have paid the price with their health including increased obesity and its associated co-morbidities [7] [8] . In low and middle-income countries, T2DM has shifted from being the 15^th biggest cause of death in 1990 to the 7^th greatest in 2016 [8] [9] [10] . It has been reported that 80% of people diagnosed with T2DM are from low and middle-income countries [10] .

The growing prevalence of obesity, and particularly childhood obesity, has led to a shift in T2DM to younger populations [3] [6] [11] [12] [13] . The estimate from the International Obesity Task Force indicates that about 155 million school-going children worldwide are obese [14] . It has also been observed that T2DM now accounts for about 45% of new onset diabetes in adolescents, as against 3% about 10 years ago [15] .

New evidence is emerging indicating an association between hypovitaminosis D and insulin resistance amongst T2DM patients [12] [16] - [32] . Studies by researchers including Lips et al. [33] found that in patients with established T2DM and in the general population, low levels of calcidiol (also known as calcifediol, 25-hydroxycholecalciferol, or 25-hydroxyvitamin D [25(OH)D]) were associated with higher fasting glucose, insulin resistance and the metabolic syndrome [12] . 25(OH)D exists in two major forms: ergocalciferol (vitamin D2) which is largely ingested, and cholecalciferol (vitamin D3) which is synthesized in the human body [16] [34] . Both forms are inactive, but are converted into an active form by two enzymatic hydroxylation reactions: first, in the liver, forming 25-hydrox- yvitamin D mediated by 25-hydroxylase; and second in the kidneys mediated by 1 α-hydroxylase, forming the final activated product calcitriol (1,25 dihydroxyvitamin D) [16] [34] . 25-hydroxyvitamin D is the circulating form of vitamin D in plasma, and an inverse relationship has been established between this and the prevalence of T2DM and impaired glucose tolerance [12] [35] . Several prospective studies showed that a low serum 25(OH)D baseline was associated with incidence of T2DM [36] - [42] . A meta-analysis of prospective studies by Song et al. [43] also indicated an association between low circulating 25(OH)D levels and risk of T2DM.

Several studies have explained the mechanisms through which vitamin D levels are associated with T2DM [16] [33] [34] [44] . It has been proposed that vitamin D influences T2DM in two ways: first, the active form 1,25 dihydroxyvitamin D directly binds to β cell vitamin D receptors, thus facilitating insulin’s response to glucose [45] [46] ; and second, through the regulation of calcium homeostasis through β cell membranes [34] . In this second case, vitamin D deficiency (which alters the extracellular and intracellular β cell calcium pool) will adversely affect insulin secretion [39] [44] .

Vitamin D is present in some diets in micro-quantities, albeit rarely [45] . The major source of vitamin D (over 80%) is endogenous, but this requires ultraviolet B radiation from sunlight to activate its precursor 7-dehydrocholesterol in the skin [45] . The evidence indicates that indoor lifestyles and protection from the sun’s rays have led to vitamin D deficiency even in countries with abundant sunlight [45] .

In response to this vitamin D/T2DM theory, several studies have evaluated the effect of vitamin D supplementation on a variety of glycaemic outcomes, including the effect of 25(OH)D on glycosylated haemoglobin (HbA1c) levels, plasma glucose levels and homeostatic model assessment insulin resistance (HOMA-IR). These reported conflicting results. For example, studies have found 25(OH)D to be inversely related to HbA1c levels in diabetes mellitus [19] [28] [31] . Some studies reported a decrease in HbA1c and an increase in insulin sensitivity on vitamin D supplementation [47] . Others, such as Ahmadi et al. [22] , found no significant relationship between vitamin D and levels of HbA1c_, while Elkassaby et al. [20] in a study on T2DM patients with vitamin D deficiency observed a slight improvement in glycaemic outcomes measured by plasma glucose.

The few studies that measured the effects of vitamin D on HOMA-IR had varying outcomes. Some showed that vitamin D has a slight improvement effect on HOMA-IR [18] [32] ; others showed that vitamin D supplementation in patients with T2DM did not significantly reduce insulin resistance [17] .

Owing to inconsistencies in these results and because these RCTs suffer the same shortcomings (low statistical power, lack of precision and small sample size), this study will systematically review the effects of vitamin D on glycaemic outcomes in T2DM patients. A systematic review is used because the pooled effect of considering all RCTs increases the power of the research.

This research hypothesized that vitamin D will effectively improve glucose metabolism in type 2 Diabetes mellitus patients. Figure 4 is the forest plot of 15 trials that compared the effect of vitamin D on HbA1c. An increase in glycosylated hemoglobin (HbA1c) means that glucose metabolism was not improved with vitamin D supplementation. Conversely, a decrease in HbA1c depicts an improvement in glucose metabolism with vitamin D supplementation. The average HbA1c was slightly lower in the 25(OH)D than control, but the difference was

not statistically significant (P = 0.56; MD = −0.06; 95% CI = −0.26 to 0.14); the 95% confidence interval crosses the line of no effect.

Figure 5 is a forest plot of seven studies that compared the effect of vitamin D and placebo on blood glucose. An increase in blood glucose indicates that vitamin D supplementation did not improve glucose metabolism, while a decrease indicates an improvement. The figure shows a pooled estimated mean of −0.03 indicating an average decrease in plasma glucose in the 25(OH)D group compared to control. However, the difference was not statistically significant (P = 0.95; 95% CI: −0.69 to 0.63); the 95% confidence interval crosses the line of no effect.

Figure 6 is a forest plot of studies that compared the effect of vitamin D and placebo on homeostatic model assessment insulin resistance (HOMA-IR). The data show a pooled estimate of MD = −1.51, indicating an improvement in average HOMA-IR in the 25(OH)D group compared to control. However, the difference was not statistically significant (P = 0.16; 95% CI = −3.61 to 0.60), and the 95% confidence interval crosses the line of no effect.

Figure 7 and Figure 8 show a single study that compared the effect of vitamin D and non-pharmaceutical intervention (counselling) on glycosylated haemoglobin and blood glucose respectively. Nothing was administered to the control group in this study; however, participants were counselled on how to naturally increase vitamin D levels. The authors thought to separate this study from the studies that compared vitamin D with placebo, since the study has a different comparator which may affect the heterogeneity of the study results.

Our systematic review identified 929 studies and 148 RCTs that evaluated the effect of vitamin D supplementation on three glycaemic outcomes ? plasma glucose, HbA1c and HOMA-IR. A total of 15 studies compared vitamin D with placebo and measured glycosylated haemoglobin (HbA1c) as the endpoint outcome with a total of 791 participants, 402 in the vitamin D group and 389 in the placebo group. In relation to vitamin D and HbA1c, the results showed a slight reduction in HbA1c in the vitamin D group compared to the placebo group (mean difference = −0.06; 95% CI = −0.26 to 0.14; I² = 84%). However, the reduction was not statistically significant (P = 0.56). These findings are typical of most intervention studies evaluating the effect of vitamin D on T2DM, which give signals of nominal significance for diverse outcomes that are inconclusive and subject to caveats [48] [49] . It is also consistent with previous systematic reviews, including one by Seida et al. which included 15 RCTs and examined the effects of vitamin D on HbA1c in individuals with normal glucose tolerance and individuals with T2DM [49] .

The conclusive evidence of association between increases in 25(OH)D and decreases in HbA1c levels come from observational studies [50] [51] [52] [53] . For example, a longitudinal study by Munasinghe et al. [50] found that the mean serum 25(OH)D concentrations of participants on vitamin D supplementation increased from 90.8 nmol/L at baseline to 121.3 nmol/L at follow-up, while the mean HbA1c values decreased from 5.6% at baseline to 5.5% at follow-up. The prevalence of participants with increased diabetes risk, i.e. those with HbA1c values ≥ 5.8%, decreased from 29.5% to 17.4% [50] . This signifies the need for further investment into research in this area.

For effects of vitamin D on plasma glucose, a total of seven studies [17] [20] [21] [24] [25] [29] were examined; these had a total of 273 participants, 143 in the vitamin D group and 130 in the control group. The study showed a MD = −0.03 mmol/l and a 95% CI = −0.69 to 0.63 in favour of the 25(OH)D group. Consistent with previous intervention studies which showed nominal improvements in plasma glucose, these findings were statistically insignificant (P = 0.93; 95% CI = −0.69 to 0.63). Similarly, Seida et al.’s [49] systematic review of 25 RCTs showed minimal effects (MD = −0.18 mg/dL; 95% CI = −1.26 to 0.90; I² = 21%) of vitamin D on blood glucose in individuals with normal glucose tolerance, pre-diabetes and patients with T2DM. Similar results were shown in another systematic review by George, Pearson and Witham, [54] . More conclusive evidence of the link between vitamin D and FPG came from an observational study by Pannu et al. [52] which showed that every 10 nmol/L increment in serum 25(OH)D significantly reduced the adjusted odds ratio (AOR) of a higher FPG (AOR = 0.91, (0.86, 0.97); P = 0.002] and a higher HbA1c (AOR 0.94, (0.90, 0.98); P = 0.009).

Homeostatic Model Assessment Insulin Resistance (HOMA-IR) is the most common measure of insulin sensitivity. We found three studies which met our inclusion criteria [17] [18] [32] and measured effects of vitamin D on HOMA-IR. Together these had 147 participants, 67 in the vitamin D group and 65 in the control group. The result showed that vitamin D has a slight improvement effect on HOMA-IR (MD = −1.51; 95% CI = −3.61 to 0.60; I² = 67%); however, a P value = 0.16 shows that the effect of vitamin D was not significant. This study showed that vitamin D supplementation in patients with T2DM did not significantly reduce insulin resistance. Comparably, Seida et al. [49] included six studies that examined the effects of vitamin D supplementation on HOMA-IR in T2DM patients [55] . They also found no significant difference in the vitamin D group compared to the placebo group (MD = −1.46; 95% CI = −4.27 to 1.34).

Overall, a lack of consensus on the effect of vitamin D supplementation continues to exist among the intervention studies. The consistent feature amongst all of them is lack of statistical significance. Even systematic reviews that combine several RCTs failed to increase statistical power and estimates of precision [49] [56] . This calls for further research into this issue using larger sample sizes.

In conclusion, although vitamin D has been extensively studied in relation to glycaemic outcomes and indications that increased plasma vitamin D concentrations might be linked to prevention of T2DM, firm universal conclusions about its benefits cannot be drawn. Further studies with better designed trials and larger sample sizes are needed to draw firmer conclusions.

It is important to note that the major strength of this review is its strict adherence to the methods and guidelines included in the Cochrane handbook of systematic reviews. It also included the most recent RCTs which provide up-to-date evidence. However, it was not without its limitations: the funding for this study only covered the lead researcher’s time and did not cover resources; consequently, several studies that looked promising from the reviews of their abstracts were excluded due to limited funds to purchase the full text.

The authors would like to thank Roger Whiteley for proofreading and editorial support. Our very special gratitude goes to the reviewers of this paper, who will have expended considerable effort on our behalf.

The authors declare no conflicts of interest.

There was no funding allocated to the study. However, the University of Chester provided time for the lead author to do this research.

Not applicable to this paper.

Not applicable to this paper.

・ MM wrote the entire manuscript.

・ AB wrote the protocol for the study and contributed to writing the manuscript.

・ AO and EA contributed in data analysis and

・ AY did all the editing of the manuscript.

Mabhala, M.A., Babanumi, A., Olagunju, A., Akata, E. and Yohannes, A. (2017) Evaluating the Use of Vitamin D Supplementation to Improve Glycaemic Outcome in Type 2 Diabetes Mellitus Patients: A Systematic Review of Evidence. Journal of Diabetes Mellitus, 7, 223-240. https://doi.org/10.4236/jdm.2017.74018