[Obesity, insulin resistance and type 2 diabetes: risk factors for breast cancer].

Obesity and type 2 diabetes are two risk factors of breast cancer, especially after menopause. Underlying mechanisms are multiple and include hyperinsulinism due to insulin resistance (insulin, as insulin-like-growth factor -IGF-, is a growth factor), hyperleptinaemia associated with hypoadiponectinaemia, and high levels of estrogens resulting from aromatization of androgens in adipose tissue. In presence of type 2 diabetes associated with obesity, hyperglycaemia might provide energy substrate promoting tumour growth. These data have therapeutic implications with expected favourable effects of weight loss, resulting in a reduction of fat mass and insulin resistance, and the promising results recently reported with metformin contrasting with the negative effects of exogenous administration of high doses of insulin.