• Fiber has good results in relieving chronic idiopathic constipation, which makes up 4-20% of all constipation cases.
  • The best way to approach the treatment is to first eliminate a pre-existing hard stool with laxatives, then start on a low dose of fiber from diet and supplements, with increased water intake (and ideally exercise), gradually increasing the amount until it reaches 20-30g of fiber per day.
  • Psyllium husk is one of the best fiber supplements available.


The most effective types of fiber for constipation relief

There is substantial evidence that fiber helps in a type of constipation which has unknown causes (called Chronic Idiopathic Constipation) and is not a result of any medical conditions or drugs.

For the type of constipation with known causes (usually underlying medical conditions), more evidence is needed to prove the effectiveness of fiber.

Various categories of fiber play slightly different roles in constipation relief although the most important action of fiber is the bulking of stool.

Constipation may have various causes. Not all types of constipation can be helped with fiber. (read more on constipation symptoms and causes)

Here is some background information:

  • Water content in stool ranges from 60% to 90%.
  • Only 1% of fluid from diet and internal secretions form the bulk of the stool.
  • 9-10 liters of fluid per day goes through the gastrointestinal (GI) tract so even small variations of water amount passing through to the colon for stool absorption may cause either constipation or diarrhea.
  • Fiber and by-products of fiber fermentation capture fluid inside the colon and increase the bulk of the stool. (1)
  • See what are the healthiest high fiber foods for constipation relief here

Evidence of fiber in constipation relief

Chronic idiopathic constipation (CIC)

There is good evidence that fiber is effective in the relief of constipation in the cases of chronic idiopathic constipation (CIC).

CIC is of unknown causes and is not caused by an underlying medical condition or medications. Between 4% and 20% (depending on surveyed area) of the constipation cases are due to CIC. (2)

Constipation caused by specific conditions

There is lack of good quality evidence that fiber can aid in identifiable types of constipation, such as those caused by medical conditions, anatomical issues or those that are drug-related (e.g. constipation caused by obstruction, neurological problems, metabolic disorders or use of certain medicines/drugs).

In some cases of constipation (e.g. diverticulitis, bowel obstruction), fiber might even worsen the symptoms. (3)

What are the common side effects of using dietary fiber for constipation?

A fiber rich diet or fiber supplements are recommended for constipation. However, it is best to eliminate a pre-existing hard stool with laxatives before going on the fiber diet or fiber supplementation.

It should be noted that using fiber for constipation may trigger some unwanted side-effects such as: bloating, flatulence, constipation (e.g. in the case of insufficient water intake) and diarrhea.

For those that are not used to dietary or supplementary fiber, it is best to start with small dosages and increase gradually until reaching a dose of 20-30g of total dietary fiber per day.

When using fiber for constipation relief, it is necessary to increase fluid intake.

How fiber helps in constipation?

Fiber can help relieving constipation by:

  • Increasing the stool weight and volume (soluble, insoluble and fermentable fiber)
  • Combining with water (soluble fiber)
  • Increasing the bacterial mass (that feeds on fiber) and the resulting by-products from fermentation (fermentable fiber)
  • Irritation of the gut mucosa, causing intestinal secretions. This leads to lubrication and increase in the colonic movement – peristalsis (coarse insoluble fiber).

The larger and softer the stool, the easier and quicker it passes through the intestine. Fiber improves bowel movement frequency and consistency. (4)

The following chart makes it easier to generally understand how fiber contributes to constipation relief.

Fiber helps in constipation

Various fiber categories effect constipation differently

Fiber categories aren’t limited to only two types: soluble and insoluble.

There are various fiber categories, a combination of which may contribute to relief of constipation in different ways.

The function of fiber depends on their characteristics such as length of the chain of carbohydrates, solubility and fermentation properties.

Short chain carbohydrates such as oligosaccharides are highly fermentable due to their small size.

Long chain carbohydrates such as the ones below aren’t as easily fermentable:

  • Soluble, highly fermentable, non-starch fiber;
  • Soluble (intermediate) and fermentable fiber;
  • Insoluble, slowly fermentable fiber;
  • Insoluble, non-fermentable fiber.

Using soluble and insoluble fiber for constipation relief

Using only the solubility as a category, these are the characteristics:

INSOLUBLE FIBER: Insoluble fiber is not fermented as well as soluble fiber and therefore doesn’t create significant biomass. It also doesn’t have a significant water holding quality.

Instead, it irritates and mechanically stimulates the gut surface (mucosa). Mucosa then increases secretion (lubricating and adding mass to the bulk) resulting in increased bowel movements (peristalsis).

Larger and coarser particles of insoluble fiber have a better laxative effect than smaller particles. Insoluble fiber is good for constipation only (not diarrhea). (3)

SOLUBLE FIBER: Increases stool bulk by increasing biomass and by-products of fermentation such as short chain fatty acids and gas. Soluble fiber is good for both constipation and diarrhea. (2, 5, 6, 7)

A considerable number of large systematic reviews have shown that soluble fiber helps in relieving chronic and idiopathic constipation and results in a significant improvement (rate of improvement is almost double) of general constipation symptoms, straining, pain during defecation and frequency. (2)

Psyllium husk seems to be the most effective over the counter supplement. It needs to be combined with plenty of fluids and, for the best results, accompanied by exercise (when possible).

Psyllium is the best evidence-supported fiber supplement in Functional Gastrointestinal Disorders. (3)

Effects of combined categories of fiber

The following is the table of fiber categories combinations and their effects on passing the stool. Note that some sources of fiber may contain more than one fiber types. (8)

Effects of various types of fiber on constipation

Type of fiberSourcesEffect on constipation
Insoluble, non-fermentable fiber. Long-chain carbohydrates (cellulose, sterculia, methylcellulose)Whole, high fiber grains and cereals, nuts, seeds, skins of fruit and vegetablesGood laxative effect;
Speeds up the movement of the stool through the intestine;
Very little gas produced.
Insoluble, slowly fermentable fiber (long chain carbohydrates)Wheat bran, wholegrain cereal, rye, brown rice, quinoa, some fruit, and vegetables, lignin from flaxseedsGood laxative effect;
Speeds up the movement of the stool through the intestine;
Increases bacteria mass;
High quantity of fructans in wheat bran may cause excessive bloating.
Soluble Intermediate fermentable fiber. Long –chain carbohydratesPsyllium, oatsGood laxative effect;
Speeds up the movement of the stool through the intestine;
Increases bacteria mass;
Moderate gas production.
Soluble, highly fermentable fiber long chain carbohydrates (pectin, resistant starch, guar gum, inulin)Legumes, pulses, rye bread, barley, not over-ripe bananas, buckwheat, millet, oats, potatoes, brown rice, pasta.Mild laxative effect;
Does not speed up the movement of the stool through the intestine;
Increases bacteria mass;
Produces moderate amount of gas.
Soluble, highly fermentable short-chain carbohydrates (Oligosaccharides: FOS, GOS)Legumes, pulses, wheat, rye, onions, garlic, asparagus, leeks, chicory, and artichoke.Weak laxative effect;
Does not speed up the travel of the stool through the intestine. May produce high amount of gas and cause flatulence and abdomen discomfort.
Soluble, viscous fiber minimally fermentableSome laxatives such as methylcellulose, calcium polycarbophil, and psyllium.Holds water very well and forms gel-like substance resulting in softening stool in the case of constipation, or firming of loose liquid in diarrhea;
Water is a crucial element in the stool softening effects: only 4.7% increase in stool water content results in substantial softening of the stool, and a 4.6 time increase in viscosity. (1)


You will find a summary of the most common nutrition myths and evidence-based nutrition facts here.


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