Researchers at Iran's Mashhad University of Medical Sciences, Shahrekord University of Medical Sciences, Ferdowsi University, Islamic Azad University and Babol University of Medical Sciences, as well as the UK's Brighton & Sussex Medical School, conducted a systemic review and meta-analysis to evaluate the impact of vitamin D supplements on glycaemic control indices.
They assessed insulin resistance (HOMA-IR), glycated haemoglobin (HbA1C), fasting blood glucose (FBG) and quantitative insulin-sensitivity check index (QUICKI), as well as lipid profile in diabetic patients.
They searched eight databases for RCTs or cross-sectional and cohort studies that had been published up to December 2017 and used comprehensive meta-analysis software (CMA) for all statistical analyses.
D against diabetes
They eventually settled on 82 studies, 37 of which were used for the meta-analysis.
They then reported that vitamin D supplementation was linked to significant improvement in FBG and HbA1C in patients with type 2 diabetes mellitus (T2DM), although similar statistically significant changes were not seen in women with gestational diabetes (which affects approximately 4.7% of pregnant women in Iran).
However, treatment using vitamin D supplements led to an improvement in HOMA-IR in women with gestational diabetes, as well as in individuals with T2DM.
At the same time, the researchers found that the pooled result of the cross-sectional meta-analysis showed that serum vitamin D concentration was significantly lower in diabetics than in healthy controls.
These results were supported by earlier research, where vitamin D supplementation had exhibited "potentially beneficial effects" via an improvement in glucose tolerance test (GTT) and a reduction in the prevalence of gestational diabetes — diabetic pregnant women who had consumed vitamin D saw improved glycaemic status measurements and HbA1C levels, with no complications reported.
When it came to type 1 diabetes mellitus (T1DM), they reported that vitamin D supplementation played a "potentially important role" by suppressing T-cell activation and improving HbA1C and FBG.
Vitamin D supplementation was also recommended in conjunction with other types of supplementation, such as calcium. The combination was found to lower T2DM risk, and have a positive effect on BMI, hip circumference and systolic blood pressure in T2DM patients.
On the other hand, some of the studies showed that vitamin D supplementation did not affect insulin resistance or FBG, and reported no significant association between gestational diabetes and serum vitamin D levels.
First and future indications
The researchers stated that the current review was the first meta-analysis to evaluate the impact of vitamin D on glycaemic indices in "diverse subgroups of diabetes, and also summarises cross-sectional, cohort and interventional studies in diabetic patients".
They also noted, however, a "major limitation": the lack of pre- and post-baseline measures for glycaemic indices, as observed in several papers, which meant they had to be excluded from the meta-analysis.
In conclusion, they wrote: "This systematic and meta-analysis review demonstrated that vitamin D had major effects on (the) FBG of type 2 diabetics and pregnant women with diabetes. Therefore, vitamin D could be used as an adjuvant therapy along with the other treatments for those patients.
"Also, vitamin D supplementation leads to an improvement in HOMA-IR, FBG and HbA1C. Furthermore, the overall pooled result of the cross-sectional meta-analysis showed the level of vitamin D is significantly lower in diabetic patients than healthy controls.
However, further studies are required to better understand the relationship between vitamin D supplementation and glucose homeostasis indices in type 2 diabetes patients and pregnant diabetic women."
Source: Complementary Therapies in Clinical Practice
"The effects of vitamin D supplementation on indices of glycemic control in Iranian diabetics: A systematic review and meta-analysis"
Authors: Reza Sahebi, et al.