Fig. 2.

Interacting pathways between microbiota, prebiotics, dietary fibre, gut and brain affecting constipation. Several pathway impact on gut motility. There is bidirectional communication between gut and brain via vagal and spinal nerves, with serotonin as the main neurotransmitter⁽³⁴⁾. SCFA produced by the microbiota can directly stimulate nerves cells in the gut, or indirectly stimulate enteroendocrine cells to produce serotonin and consequently trigger peristaltic reflexes⁽³⁴⁾. SCFA can also directly stimulate receptors on the vagus nerve⁽⁴¹⁾. Secondary bile acids (BA) produced by the microbiota can affect motility, while BA themselves affect the composition of the microbiota⁽⁴²⁾. The microbial composition of the gut is influenced by the enteric nervous system through immunological defence secretions, permeability or motility⁽⁴¹⁾. Transit time influences the composition of the microbiota through exposure to water and nutrients. Prebiotics such as galacto-oligosaccharides can increase SCFA-producing Bifidobacterium spp. and increase motility in constipated individuals⁽⁵¹⁾. Non-fermentable dietary fibre such as coarse wheat bran and psyllium husks soften stools making them easier to pass by mechanisms that do not directly involve the microbiota⁽⁶⁸⁾ but affects the environment in which they reside.