To understand insulin resistance, you need to know a little bit about how the body uses food.
Your stomach breaks down (or digest) the food you eat into a simple sugar called glucose. Glucose is your body's main source of energy. After digestion, glucose passes into your blood and is ready for your cells to use as energy. The amount of glucose in your blood is your blood glucose level or blood sugar level. Your cells need the hormone insulin to get the glucose out of the blood. Insulin "opens" your cells so that glucose can get in and become energy.
Normally, the body makes the right amount of insulin so that the level of glucose in the blood stays within a healthy range. Insulin resistance means that the cells don't use the insulin the body makes. As a result, higher than normal levels of insulin are needed to get glucose into the cells for energy.
Insulin resistance can cause the body to store more fat and can make that fat hard to lose. Continued problems with glucose and insulin can also lead to the metabolic syndrome and diabetes. Insulin resistance can also upset the balance of hormones in the body. In particular, it can cause the body to make more androgens - a main feature of PCOS.
How common is it in women with PCOS? Current studies suggest that 50-70% of women with PCOS have some level of insulin resistance.