*2.1.1 Exclusive enteral nutrition (EEN)*

This nutritional therapy has been recommended as first-line, steroid-sparing treatment for pediatric CD since the 1990s and provides the entire calorie and nutrient requirements using liquid formulations delivered either orally or through nasogastric tube or gastrostomy, for a period of 6–8 weeks. It is associated with remission rates of 60–80%, and the efficacy is not correlated with the formula types or the route of administration [1–6]. Most studies investigating the mechanism of action of EEN focused on the changes in gut flora and microbial metabolites as a potential mechanism, showing conflicting data. The first study on microbiota changes related to EEN used 16S ribosomal DNA polymerase chain reaction and temperature gradient gel electrophoresis and discovered important modification of the band profile associated with different bacterial species [7]. Later studies showed that although children treated with EEN had higher rates of mucosal healing, they still have a lower proportion of butyrate-producing bacteria in comparison with steroid-treated patients. A recent research revealed differences in the fecal metabolome of responders vs. non-responders to EEN [8, 9]. There are few data on the impact of EEN on the microbiota of adult CD patients; one study that investigated microbiota changes in adult CD patients treated with EEN for 2 weeks prior to intestinal resection for strictures found a significant decrease in alpha diversity and in the Enterobacteriaceae family, but which did not modify the postoperative recurrence [10].
