Adiposity, type 2 diabetes and the metabolic syndrome in breast cancer

Summary Upper body obesity and the related metabolic disorder type 2 diabetes have been identified as risk factors for breast cancer, and associated with late‐stage disease and a poor prognosis. Components of the metabolic syndrome, including visceral adiposity, insulin resistance, hyperglycemia and hyperinsulinemia, with or without clinically manifest diabetes mellitus, low serum high‐density lipoprotein cholesterol and hypertension have all been related to increased breast cancer risk. The biochemical mechanisms include extraglandular oestrogen production, reduced sex hormone‐binding globulin with consequent elevation of the bioactive plasma free oestradiol and increased insulin biosynthesis, all of which exert mitogenic effects on both untransformed and neoplastic breast epithelial cells. Obesity, type 2 diabetes and the metabolic syndrome also have in common an increased production of leptin and a decreased production of adiponectin by adipose tissue, with consequent elevations and reductions, respectively, in the circulating levels of these two adipokines. These changes in plasma leptin and adiponectin, acting through endocrine and paracrine mechanisms, have been associated in several studies with an increase in breast cancer risk and, perhaps, to more aggressive tumours; studies in vitro showed that leptin stimulates, and adiponectin inhibits, tumour cell proliferation and the microvessel angiogenesis which is essential for breast cancer development and progression.