We know the obvious correlation between obesity and common diseases such as cardiovascular disease and diabetes, but not as commonly with degenerative diseases such as cancer with the exception of perhaps colon or breast cancer. A recent meta-analysis of 141 articles in the British medical journal The Lancet shows that increased body mass index (BMI) doubles the risk of adenocarcinoma of the esophagus in both men and women, and about doubles the risk of endometrial cancer and cancer of the gallbladder in women. Renal cancer risk in women is increased by one third, The study also notes increases in risk of colon cancer, thyroid cancer, malignant melanoma, post-menopausal breast cancer, pancreatic cancer, multiple myeloma, leukemia, and non-Hodgkin’s lymphoma in both sexes. Several possible mechanisms are discussed in the article, including IgF 1 (insulin like growth factor) which we commonly find elevated in cancer patients in our practice- and correlates strongly with elevated insulin levels. Insulin is one of the most inflammatory substances in the body, and the study also posits that increased inflammation and oxidative stress in the obese can lead either to initiation or proliferation of malignancies. 
 
Lancet. 2008;1371:536-537, 569-578.