Vitamin D supplementation could be a useful adjunctive treatment for patients with type 2 diabetes, especially if they have suboptimal levels of the vitamin, according to a meta-analysis undertaken by US and Iranian researchers.
It has been estimated that in 2017, a staggering 462 million individuals had type 2 diabetes, corresponding to just over 6% of the global population. Despite there being several different therapies currently available for type 2 diabetes, new treatments will always be needed, due to the high prevalence of the disorder. Although normally administered to regulate calcium and phosphorus levels, in recent years, a purported role for vitamin D supplementation has been suggested for several diseases. For example, data from the prospective Nurses’ Health study found that higher vitamin D and calcium intake was associated with a 33% lower risk of type 2 diabetes. Nevertheless, studies that involved actual vitamin D supplementation produced mixed findings. For example, one study 6-month trial found that supplementing with the vitamin in patients with type 2 diabetes failed to affect either insulin sensitivity or secretion. In contrast, an 8-week intervention study demonstrated significant reductions in fasting plasma glucose, insulin and HOMA-IR.
As a result, in the current study, researchers performed a meta-analysis to examine the effect of using the vitamin on indices of glycaemic control including fasting plasma glucose (FPG), HbA1c and HOMA-IR.
Vitamin D supplementation and glycaemic measures
A total of 46 eligible trials were identified including 4313 patients with type 2 diabetes and a mean age of 56.5 years and of whom 2164 received the vitamin intervention. The majority of the studies (42) used an oral supplement, whereas in four trials, it was given via intramuscular injection.
The pooled analysis for HbA1c showed a significant reduction compared to placebo for vitamin D (weighted mean difference, WMD = -0.20, p < 0.001). Similarly, there was a significant reduction in FPG (WMD = -0.28 mmol/l, p < 0.001) and HOMA-IR (WMD = -0.42, p = 0.019) in those given vitamin D.
The authors concluded that although vitamin D supplementation had a positive impact on glycaemic indices, they cautioned that the substantial heterogeneity between the included studies raised the possibility of publication bias.
Farahmand MA et al. What is the impact of vitamin D supplementation on glycemic control in people with type-2 diabetes: a systematic review and meta-analysis of randomized controlled trials. BMC Endocr Disord 2023