Increasing fiber intake is a common recommendation for people with constipation. But the effects of fiber likely depend on what type of fiber is added.
Quite a few studies have looked at whether fiber is beneficial for constipation by assessing its ability to increase the number of stools a person passes in a week. And while a greater defecation frequency can be an indicator of benefit, it doesn’t always mean the unpleasant aspects of constipation — like abdominal pain — are actually improved.
When looking solely at patient-assessed constipation symptoms (things like stomach discomfort, bloating, painful bowel movements, and straining during defecation), several randomized controlled trials have reported no clear benefits from fiber supplementation. This was the finding of a 3-week trial using galacto-oligosaccharides (GOS; 5.5 or 11 grams per day),[1] a 2-week study using polydextrose (4, 8, or 12 grams per day),[2] and a 4-week study using wheat bran (12.5 grams of fiber per day).[3]
Conversely, some randomized controlled trials have reported improvements to patient-reported constipation symptoms from fiber supplements. Several trials on psyllium supplementation (typically in dosages of 10–20 grams of fiber per day) have reported reductions in straining[4][5] and pain during defecation.[4][6] One 4-week trial likewise found supplementation with 24 grams of pectin (a fiber found in various fruits and vegetables) led to reductions in constipation symptoms, including less bloating.[7]
This then raises the question: what is it about fibers like psyllium and pectin that makes them good at improving symptoms? The answer may be viscosity.
Fiber is believed to benefit constipation in large part by binding water in the intestines, thereby softening the stool and making it move more smoothly through the intestines. Both pectin and psyllium are very effective at binding water, which means they should increase the hydration of feces. This water-binding property, known as viscosity, varies depending on the fiber in question. Other viscous fibers and fiber-sources with evidence of benefit for constipation include flaxseeds,[8] partially hydrolyzed guar gum,[9][10] β-glucan (e.g., from oats),[11][12] and inulin.[13][14]
Finally, it’s worth noting that many types of fiber (both viscous and non-viscous) are readily fermentable by intestinal bacteria. This can result in significant gas production, potentially leading to gastrointestinal effects like flatulence and bloating, especially in susceptible individuals or when high amounts are consumed. Among the viscous fibers, inulin seems to be one of the most fermentable,[15] whereas psyllium tends to be one of the least fermentable.[16]