BACKGROUND: Chronic low-grade inflammation is common in obesity and related chronic conditions, including type 2 diabetes. Vitamin D is proposed to have anti-inflammatory properties; however the effect of vitamin D supplementation on inflammation in type 2 diabetes has not been established.
PURPOSE: To conduct a systematic review and meta-analysis of the effects of vitamin D supplementation on inflammatory markers in patients with type 2 diabetes, and to identify relevant knowledge gaps.
DATA SOURCES: Medline, CINAHL, EMBASE and All EBM were systematically searched from inception to 25-January-2017 for randomised controlled trials (RCTs) investigating the effects of vitamin D supplementation on inflammatory markers in type 2 diabetes.
STUDY SELECTION: Two independent reviewers screened full-text articles of RCTs (no date or language limits) of vitamin D supplementation (any form, route, duration, or co-supplementation) compared to placebo/usual care on inflammatory markers in patients with type 2 diabetes.
DATA EXTRACTION: Two independent reviewers extracted data and assessed quality using the grading of recommendations, assessment, development and evaluation (GRADE) approach.
DATA SYNTHESIS: Twenty-eight RCTs met inclusion criteria, and 20 had available data for pooling. In meta-analyses of 20 RCTs (n=1,270), vitamin D-supplemented groups had lower follow-up levels of C-reactive protein (standardized mean difference (SMD[95%CI]):-0.23 mg/L(-0.37,-0.09);p=0.002), tumour necrosis factor (SMD:-0.49 pg/ml(-0.84,-0.15);p=0.005), and erythrocyte sedimentation rate (SMD:-0.47 mm/hr(-0.89,-0.05);p=0.03), and higher levels of leptin (SMD:0.42 µg/L(0.04, 0.81);p=0.03) compared to control groups. No differences were observed for adiponectin, interleukin-6, or E-selectin. In meta-regression and subgroup analyses, results were not influenced by age, sex, BMI, diabetes duration, baseline vitamin D status, or supplementation dose or duration. There was no evidence of heterogeneity (I2=0-38%;Phet>0.1) or publication bias and most studies had low to moderate risk of bias.
CONCLUSIONS: This meta-analysis of RCTs provides level one evidence that vitamin D supplementation may improve chronic low-grade inflammation in patients with type 2 diabetes.