INS and Insulin resistance: Therefore, to investigate the role of vitamin D supplementation in ameliorating markers of insulin resistance we completed a systematic review of RCTs assessing the effect of vitamin D, without the use of additional supplements or other pharmacological treatments known to affect IR, on fasting glucose, fasting insulin, glycated haemoglobin (HbA1c) or homeostatic model assessment for insulin resistance (HOMA-IR) in women with PCOS.