The classification of the severity of acute pancreatitis has changed with respect to the Atlanta update of 2012. The recommendations for nutritional therapy in cases of severe acute pancreatitis are not supported by high-level studies, as studies contain a mix of patients with different degrees of severity. The Atlanta 2002 classification is used to define severe acute pancreatitis and, in most of experimental studies, controls are patients with parenteral nutrition. A narrative review of the current published evidence is carried out analyzing the clinical and epidemiological characteristics of the patients in these results and characteristics to be included in future studies are proposed.