The gastrointestinal tract is confronted with a cornucopia of diverse chemicals, pathogens and physicochemical states that it must analyze and react to appropriately to optimize nutrition and to defend against harm. It presents the largest and most vulnerable surface to the outside world. Integrated responses to these challenges require the gut to sense its environment. This it does through a range of detection systems for specific chemical entities, pathogenic organisms and their products, and physico-chemical properties of its contents. Receptors for nutrients include taste receptors, free fatty acid receptors, peptide and phytochemical receptors, that are primarily located on enteroendocrine cells. Hormones released by enteroendocrine cells commonly act via the nervous system to optimise digestion. Pathogen detection is both through antigen presentation to T cells and through pattern recognition receptors (PRRs). Activation of PRRs triggers local tissue defence, for example, by causing release of antimicrobials from Paneth cells. Toxic chemicals, including pharmaceuticals, are sensed and then avoided, expelled or metabolised. Sensory information is communicated to four major effector systems: the enteroendocrine hormonal signalling system; the innervation of the gut, both intrinsic and extrinsic; the gut immune system; and the local tissue defence system.
Phytochemicals are a special component of foods. They occur in low amounts, and contribute little directly to nutrition. However, they provide signals to the intestine that can have beneficial downstream effects, for example by enhancing nutrient uptake and stimulation of the gut immune system. They are thus valuable food additives that can promote gut health and improve animal productivity.