Obesity is a growing health concern worldwide. According to research, it is associated with an increased risk for cancer of at least 13 anatomic sites, including endometrial, esophageal, renal and pancreatic adenocarcinomas; hepatocellular carcinoma; gastric cardia cancer; meningioma; multiple myeloma; colorectal, postmenopausal breast, ovarian, gallbladder and thyroid cancers. Although the mechanism of this relationship is unclear, the existence of diet-based therapies (ketotherapy) as a form of cancer treatment suggests that diet somehow affects the activities of cancer cells or its environment. Preclinical studies are underway to gain a better understanding of how excess body weight affects the tumor microenvironment (TME) or how it impacts antitumor immunity.