Psyllium, derived from the plants known as ispaghula, Plantago psyllium, or Plantago ovata, is a primarily soluble, bulk-forming fiber supplement.^{[11][14][13][25]} Psyllium is commonly used in clinical settings for chronic constipation, because it tends to be gentler than stimulant laxatives.^[26] Currently, psyllium has a generally recognized as safe (GRAS) status in the US as a food additive.

Psyllium fiber is made of indigestible carbohydrate chains consisting of approximately 15–25% fermentable components, with the remainder being poorly fermentable or unfermentable in the gut.^[27][28] The main carbohydrates found in psyllium are arabinose and xylose, which combine to form arabinoxylans — a complex fiber which may contribute to a healthier gut microbiome.^[27][28][29]

The strongest evidence stands for constipation relief through increased water resorption, which increases fecal weight and improves stool consistency.^{[30][1][31][32][25][33]} As a function of its partial fermentability and gel-forming properties, psyllium is thought to promote beneficial changes to the gut microbiota in people with constipation, but so far this has only been seen when psyllium is combined with other dietary fibers.^[34]

Those same gel-forming properties may also be responsible for the reductions in serum glucose concentrations and HbA1c levels that are sometimes seen when psyllium is used by people with type 2 diabetes.^{[35][6][7][36][35][37]}

Psyllium has shown potential for other digestive health and metabolic health measures as well. Use of psyllium has been associated with sustained remission in people with ulcerative colitis and reduced symptoms of gastroesophageal reflux disease.^[38][39] When taken with or prior to a meal, psyllium may produce small reductions in body mass measurements for people with higher BMIs.^[5][40][41] However, body composition changes may be more of a result of appetite suppression.^{[42][43][8][9][10][37][8][9][10]}

For improvements in cardiovascular conditions, psyllium’s effects are most consistent when used for LDL cholesterol reduction. Still, some studies have shown promise for improvements in other blood lipid measures and blood pressure as well.^{[44][45][46][47][4][3][35][48][49]} However, it does seem that there is a lot of variation in the studies themselves, so further research is needed to determine effective ranges for dose, duration, timing, and frequency of psyllium use.^{[50][3][51][4][35][52][53][49][54][55][12][44][45][56][47]}

Cases of anaphylaxis (a severe and life threatening allergic reaction) have been reported with psyllium consumption, so people with a possible allergy to psyllium or other members of the Plantaginaceae family should be cautious.^[16][17] For people working with psyllium regularly, there is a risk of acquired sensitivity (or allergic exacerbation) due to exposure or inhalation of powdered particles.^{[18][19][20][21][22][23][24]} Allergic responses are possibly due to the non-husk portions of psyllium seed, so psyllium husk that has been completely separated from other seed components may not have the same risk of adverse effects.^[57]

Generally, all fiber is pro-flatus, meaning it causes gas buildup in the digestive system because of slowed motility and bacterial fermentation. Excess gas in the intestines may result in unwanted bloating, stomach cramping, feelings of fullness, flatulence, and “pull my finger” jokes.^{[58][59][60][61]} Psyllium fiber, however, appears to produce less gas relative to other fibers, possibly due to its limited fermentation.^{[62][63][64][25]}

Psyllium may worsen constipation if fluid intake is inadequate, and in severe cases, this can even lead to bowel obstruction. Due to the risk of bowel obstruction in the absence of fluids, psyllium use is not recommended in anyone who may be incapacitated and/or unable to maintain adequate fluid intake.^[15][14]

Psyllium encourages water resorption in the intestines, turning it into a gel that slows digestion, increases fecal weight, and improves stool consistency.^[65][14] These qualities are what psyllium is best known for and likely explain its actions as a laxative and appetite suppressant.^[11][66]

Compared to similar fibers, psyllium is less fermentable in the gut, which means it’s less likely to produce flatulence, stomach pain, and nausea.^[11] Lower fermentability also means psyllium produces fewer short-chain fatty acids in the colon. However, notably, butyrate (a short-chain fatty acid) is produced after psyllium consumption (10 grams of husk twice per day), which could help prevent the growth of potentially pathogenic bacteria while improving mineral absorption.^[38][67][68]

Other proposed mechanisms for psyllium’s reported actions include increased glucose uptake and fatty acid oxidation in skeletal muscle (for improved glucose regulation);^[69][70] the release of satiety-promoting intestinal peptides, slowed gastric emptying, and regulation of the postmeal glycemic response (for appetite suppression);^{[71][66][72][73]} and/or increased bile acid production (for cholesterol reductions).^[74][75][76] However, it should be noted that much of this evidence is still preclinical (from animal and in vitro research), so these observed mechanisms may not necessarily translate into clinical effects.