Insulin signaling often plays a role in the regulation of cancer including tumor initiation, progression, and response to treatment. In addition, the insulin-regulated PI3K-Akt-mTOR pathway plays an important role in the regulation of islet cell proliferation and this pathway is hyperactivated in human nonfunctional pancreatic neuroendocrine tumors (PanNETs). We therefore investigated the effect of a very low carbohydrate diet (ketogenic diet) on a mouse model that develops nonfunctional PanNETs to ask how reduced PI3K-Akt-mTOR signaling might affect the development and progression of non-functional PanNET. We found that this dietary intervention resulted in lower PI3K-Akt-mTOR signaling in islet cells and a significant reduction in PanNET formation and progression. We also found that this treatment had a significant effect on the suppression of pituitary NET development. Furthermore, we found that non-functional PanNET patients with lower blood glucose levels tend to have a better prognosis than patients with higher blood glucose levels. This preclinical study shows that a dietary intervention that results in lower serum insulin levels leads to lower insulin signal within the neuroendocrine cells and has a striking suppressive effect on the development and progression of both pancreatic and pituitary NETs.