Finally, although brain-derived neurotrophic factor (BDNF) is an important neurotrophin involved in neurogenesis and synaptic plasticity in the brain, and has been implicated in the pathophysiology of several psychiatric disorders [42], BDNF is also abundant in the periphery, and has been shown to be involved in the pathogenesis of IBS [43, 44]. Here, BDNF is linked to psychiatric disorder.