Fiber
Fiber, or dietary fiber, is the part of plant foods the small intestine cannot digest. It comes in soluble and insoluble types, and gut bacteria ferment much of it.
Also listed as Dietary Fiber
Does a fiber supplement make bowel movements easier?
A 2022 review brought together 16 randomized trials involving 1,251 adults with chronic constipation. Fiber improved treatment response and stool frequency overall; psyllium and pectin showed significant response benefits in the analyses by type. Results varied across trials, and flatulence was more common with fiber. 1
Larger improvements in stool frequency appeared in subgroups using more than 10 g daily and treatment lasting at least four weeks. Those are study comparisons, not a starting dose or a reason to increase the amount quickly. 1
Fiber, especially psyllium in the reviewed trials, can improve chronic constipation. The response depends on the preparation and person. 1
How much can soluble fiber lower LDL?
Across 181 randomized trials with 14,505 participants, soluble fiber supplements lowered LDL cholesterol by an average of 8.28 mg/dL compared with control treatments. This is a useful, measurable change in a blood marker; the review did not establish how many heart attacks a particular supplement prevents. 2
Soluble fiber produces a modest average LDL reduction. The pooled result describes several preparations rather than one universal product. 2
What did studies in type 2 diabetes find?
A review of 29 trials with 1,517 adults with type 2 diabetes found lower fasting glucose and HbA1c with soluble fiber. HbA1c, a measure of longer-term blood sugar, fell by an average of 0.63 percentage points compared with controls. Many results differed substantially between studies, and the authors called for stronger long-term trials. 3
These findings concern people with diabetes. They do not tell a person with normal blood sugar to expect the same reduction, or establish fiber as a replacement for diabetes treatment. 3
Soluble fiber improved glucose measures in trials of adults with type 2 diabetes, with substantial variation in the size of benefit. 3
Can fiber change the gut microbiome?
Yes, but “change” needs a more precise description. A review of 64 studies in 2,099 healthy adults found increases in Bifidobacterium and Lactobacillus and a small increase in stool butyrate, a substance produced during fermentation. It found no overall increase in alpha diversity, a measure of variety within a microbial community. 4
Fructans and galacto-oligosaccharides were especially associated with increases in bifidobacteria. These findings support a biological effect and further research; they do not show that a higher bacterial count necessarily relieves an individual person’s digestive symptoms. 4
Some bacterial groups change with fiber, while overall diversity did not improve in this review. Symptom relief is a separate question. 4
A useful result still has a boundary
The constipation trials measured bowel function, the diabetes trials measured glucose control, and the microbiome trials measured bacteria and fermentation products. Reading those results separately avoids turning one favorable measurement into a promise of better digestion, weight control and metabolic health all at once. 1 3 4
Effects by health goal
Open an effect to see the studies behind its rating. Bars show the size of the effect; the letter grades the evidence.
Cardiovascular1 effect
Gut Health4 effects
No study is linked to this rating.
No study is linked to this rating.
Weight Management3 effects
Other effects1 effect
All studies
20 publications are linked to Fiber, newest first.