Modulation of insulin resistance in conditions associated with high insulin resistance NADERPOORNEGAR 2018 Insulin resistance is a key factor in the development of diabetes. My thesis focuses on modulation of insulin resistance in obesity and polycystic ovary syndrome, two conditions characterised by insulin resistance. I explore the relationships between insulin resistance and kidney and liver function as well as faecal microbiota in overweight or obese individuals. I also show that vitamin D supplementation has no effect on insulin resistance in this population. Furthermore, I review the literature on the effect of obesity on polycystic ovary syndrome, the most common endocrine disease in reproductive-aged women. Finally, I compare the effect of combined lifestyle modification and metformin with lifestyle modification alone, on insulin resistance and other outcomes in polycystic ovary syndrome.