The influence of vitamin D supplementation on IGF-1 levels in humans: A systematic review and meta-analysis

•Vitamin D dosage ≤1000 IU/day significantly increased IGF-1 levels.•Intervention duration ≤12 weeks significantly increased IGF-1 levels.•In cohort study, under 60 years subjects with a higher dietary vitamin D intake had significantly higher IGF-1 levels. Inconsistencies exist with regard to influence of vitamin D supplementation on IGF-1 levels. The inconsistencies could be attributed to several factors, such as dosage and duration of intervention, among others. To address these inconsistencies, this study was conducted to determine the impact of vitamin D supplementation on IGF-1 levels through a systematic review and meta-analysis of randomized controlled trials (RCTs). A comprehensive systematic search was carried out in PubMed/MEDLINE, Web of Science, SCOPUS and Embase for RCTs that investigated the impact of vitamin D intake on circulating IGF-1 levels from inception until June 2019. Weighted mean difference (WMD) with the 95 % CI were applied for estimating combined effect size. Subgroup analysis was performed to specify the source of heterogeneity among studies. Pooled results from eight studies demonstrated an overall non-significant increase in IGF-1 following vitamin D supplementation (WMD: 4 ng/ml, 95 % CI: −4 to 11). However, a significant degree of heterogeneity among studies was observed (I2 = 66 %). The subgroup analyses showed that vitamin D dosage of ≤1000 IU/day (WMD: 10 ng/ml) significantly increased IGF-1 compared to the vitamin D dosage of