Background: The gut microbiota is attracting increasing interest as a factor possibly impacting colorectal cancer risk, therapy toxicity, and, as a consequence, patient’s quality of life. It has been observed that microbial imbalance in the gut and in cancer tissue is facilitated by a Western type of diet, rich in meat, sugars, and refined grains, while a Mediterranean diet, rich in low saturated fat and fibers, promotes gut eubiosis, and results in reduced risk of developing colorectal cancer. Specifically, a high fiber content diet has been associated with a reduced incidence of therapy related adverse events in patients with malignant melanoma.