Fiber supplements have complex effects on gut permeability and bacterial composition. Low fibre diets increase permeability. Dietary supplementation with insoluble fibre, such as pure cellulose, decreases permeability. Dietary supplementation with highly soluble fibre sources, such as fruit pectin or guar gum, has a biphasic effect. At low levels they reverse the hyperpermeability of low residue diets, probably by a mechanical bulking effect which stimulates synthesis of mucosal growth factors. At high levels of supplementation, they produce hyperpermeability, probably by inducing synthesis of bacterial enzymes which degrade intestinal mucins[148-151]. For maximum benefit with regard to intestinal permeability, dietary fibre supplementation should therefore contain a predominance of hypoallergenic insoluble fibre.