Psyllium is the common word used to refer to fibers taken from the plant known as Plantago ovata (Plantago psyllium is used synonymously, and is where the fiber name is derived from); the fiber is characterized by being water soluble (hydrophilic) and gel forming (viscous), while possessing low fermentability. It is commonly known by the brand name Metamucil.
Psyllium is used clinically as a bulk laxative, an agent that has laxative effects but secondary to increasing fecal size; a gentler laxative relative to agents like caffeine or senna alexandrina. This bulk occurs due to water and gas absorption in the small intestines and colon to give chyme (made from digested food) more size and softness. This bulk is retained in the colon despite microflora as psyllium is poorly fermented (highly fermented fibers may be metabolized by bacteria in the colon, and water retaining properties with the fiber would be lost in this scenario).
Psyllium increases fecal size and moisture, and the most common characteristics of stool following supplementation of psyllium are 'soft, sleek, and easily passable.' Relative to other sources of dietary fiber, psyllium appears to be more effective at forming feces and appears to be one of the few fiber sources not associated with excessive flatulence.
Beyond the fecal properties, psyllium appears to be able to reduce total cholesterol and LDL-C in persons with high cholesterol (secondary to the gel forming properties leeching bile acids, and cholesterol being used up to replace hepatic bile acids) and there is sometimes a slight reduction of HDL-C as well. This is common to all water soluble dietary fibers and is not unique to psyllium.
There appears to be some glucose reducing properties associated with psyllium supplementation that may beneficial for diabetes. These are not overly potent, but appear reliable as long as psyllium is taken; cessation of psyllium usage is associated with a loss of the glucose reduction, and this may be common to all soluble dietary fibers rather than just psyllium.
Psyllium may reduce appetite slightly when taken in high doses, but does not appear to be potent or reliable; long term studies using psyllium in the doses for fecal management have failed to find weight reducing properties of psyllium suggesting it is not a good weight management intervention.
In very rare cases, supplementation with psyllium — especially in high amounts or when not consumed with sufficient water — can lead to gastrointestinal obstructions. This occurs when the psyllium fiber forms a mass (known as a bezoar) which becomes stuck in an area of the gut. These obstructions are more likely to occur when there is gastrointestinal hypomotility (slow movement of food through the gut) or abnormalities of gastrointestinal anatomy.
