A growing body of research shows that insulin plays a key role in fueling tumors. This could point to a link between obesity, diabetes, and cancer. Normal cells are not particularly sensitive to insulin; breast tumor cells in culture, in contrast, can't live without it. Insulin is most commonly known for its role in diabetes, but insulin and a related hormone known as insulin-like growth factor (IGF) could be crucial factors in cancer. This might explain why obese and diabetic individuals have a far higher risk of getting cancer, and a greater risk of dying from it if they do.
Excess body fat seems to account for between one-quarter and one-half of the occurrence of many cancer types.
According to an article in Science Magazine:
“... [T]he likely first step in the progression to cancer is the increase in insulin signaling ... Genetic defects follow ... [O]ne result is a significant increase in the generation of reactive oxygen species -- free radicals -- that can induce mutations in the genome.”