There have been several epidemiological studies that have indicated a relationship between the incidence of cancer and obesity, as well as an increased risk of malignancies associated with obesity and excess body fat.
More research is being conducted to explain the mechanisms causing this association. Some of the possibilities include insulin, estrogen and adipocytokines. The interesting thing is that fat (adipose tissue) is not a storage organ as we once thought, but is considered an endocrine organ with lots of functions. Fat tissue consists of free fatty acids with well defined physiological and pathological effects on glucose balance and on proteins called adipocytokines that act to control many metabolic functions. These proteins have been implicated in the development of insulin resistance which leads to diabetes and heart disease.
Obesity not only puts people at greater risk for malignancies, but it makes screening for and treating cancers harder which increases cancer mortality.
Reference: Levitan, D. “Obesity and cancer: Effects on risk screening and mortality” HemOnc today, Volume 11-Number 1, Jan. 10, 2010.