Recent data indicate that the main pathways linking obesity and cancer include: 1) hyperinsulinemia and abnormal insulin-like growth factor-I (IGF-I) systems and signals; 2) sex hormone biosynthesis and pathways; 3) chronic low-grade inflammation and oxidative stress; 4) pathophysiological changes of adipocytes; 5) microenvironment and cell disturbance; 6) disorders of circadian rhythm and dietary nutrients; and 8) changes in the intestinal microbiome (63). Here, IGF1 is linked to obesity due to melanocortin 4 receptor deficiency.