Many women with PCOS are overweight, although about one-third to one-half of women with PCOS actually is of normal weight. Thus, just because a woman does not have obesity does not mean that she cannot have PCOS.
Many women with PCOS also have some degree of insulin resistance. Insulin is a hormone produced by the pancreas that is responsible for processing the sugars (i.e. carbohydrates) that we ingest in our diets. Insulin resistance is a condition in which the tissues of the body do not respond appropriately to normal levels of insulin. This forces the pancreas to produce increasing amounts of insulin to process the same amount of sugars. As the insulin resistance becomes worse, the pancreas is forced to produce ever-increasing amounts of insulin. If the insulin resistance becomes so severe that the pancreas is not able to produce enough insulin to meet the body’s needs then the patient develops diabetes mellitus.
Insulin resistance in PCOS is made worse by overweightness or obesity. A sign of insulin resistance is acanthosis nigricans, a darkening of the skin, although not all insulin resistant women have acanthosis nigricans.
The exact cause of insulin resistance in women with PCOS is not yet clear. A great deal of research is being directed at discovering more about insulin resistance, why it occurs, and how best to treat it.
However, what is well established is that weight reduction in overweight women with PCOS will greatly improve insulin resistance.
In addition to improving insulin resistance, weight reduction can also improve many of the other signs and symptoms of PCOS.
For example, overweight women with PCOS and insulin resistance who lose weight can resume normal ovulation, normal menstrual periods, and normal fertility. Thus, although scientists are still in the process of understanding the association between obesity, insulin resistance and PCOS, what is clear is that there appears to be some common link that ties all these factors together.
It is important to understand that not all obese women are affected by PCOS.
Conversely, not all women with PCOS are overweight. It is also important to note that there appears to be an association between having PCOS, being overweight, and an increased risk of developing diabetes, lipid abnormalities, or cardiovascular disease.
http://www.ae-society.org/obesity.html
Obesity and insulin resistance in PCOS 
