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Dietary fiber

Dietary fiber is the indigestible portion of food derived from plants. Although a universally accepted definition for dietary fiber does not exist, it is generally agreed that the term means carbohydrates that are not digested in the upper part of the gastrointestinal tract. These carbohydrates are basically different from the readily digestible glycemic carbohydrates such as sugars and starches.

US and UK health authorities recommends that adults consume 20–35 g of dietary fiber per day, but the average daily intake among the population in the western world is only 12–18 g.

Diets naturally high in fiber can be considered to bring about several advantages:

  • helps prevent constipation
  • reduces the risk of colon cancer
  • improvements in gastrointestinal health
  • improvements in treatment of diabetes
  • reduction of hyperlipidemia, hypertension and coronary heart disease factors
  • reduction in the risk of developing some cancers

When fiber reach the colon the result is a partial or a total fermentation leading to the production of fatty acids and gas, thereby affecting gastrointestinal function.

When fiber is recommended for functional bowel disorders such as constipation and irritable bowel syndrome, the use of a soluble supplement such as psyllium (HUSK) is best supported by the available evidence.

Fiber has long been used for the treatment of various gastrointestinal conditions including constipation, diarrhea, ulcerative colitis, obesity, hypercholesterolemia and diabetes.

The fermentation of fiber influences fecal bulking, water retention and stimulates growth of “friendly” gut bacteria. The unwanted side-effect of fiber ingestion and subsequent fermentation, however, is the production of gas which may cause abdominal pain, bloating, and flatulence.

Stool consistency is highly correlated with stool water content, and a relatively small change in stool water content (5%) can lead to a relatively large stool softening effect.

Fiber Supplements in constipation:

There does appear to be a significant improvement in constipation symptoms and abdominal discomfort compared with placebo for soluble fiber (psyllium, inulin). (psyllium or HUSK is readily avalible in pharmacies an many foodstores. Dosage recommendations are usually printed on the package.

Dietary fiber in constipation:

A recent clinical trial examined treatment with dried plums (prunes, 6 g/day fiber) compared with psyllium (6 g/day fiber) in 40 patients. Dried plums not only contain fiber but also sorbitol and fructans, non-absorbable carbohydrates that, when fermented by colonic bacteria, create an osmotic load that can dramatically alter stool frequency and consistency.

Treatment with dried plums resulted in a greater improvement in constipation symptoms as reflected by a significant increase in the number of complete spontaneous bowel movements and in stool consistency (softer stools) when compared to treatment with psyllium.

Also, more subjects reported subjective improvement in overall constipation symptoms, although the mean global onstipation symptom scores were similar between groups and psyllium also improved constipation symptoms when compared with baseline.

Conclusion Constipation:

As there may be significant benefit and little risk of serious adverse events, increasing dietary fiber or the addition of fiber supplements seems a reasonable initial strategy in the management of constipation. Patients may enjoy improvements in bowel movement frequency and consistency. Effects on other symptoms such as abdominal pain or bloating are more variable.

A practical advice on initiating therapy with fiber supplements includes starting at a nominal dose and slowly titrating up as tolerated over the course of weeks to a target dose of 20–30 g of total dietary and supplementary fiber per day.

It is also recommended to clear hard stool with a laxative before initiating fiber therapy, which may avoid cramping pain.

Fiber for Irritable Bowel Syndrome (IBS)

Historically, increasing dietary fiber intake has been a standard recommendation for patients with IBS. A high-fiber diet (30 g of fiber/day) for the treatment of IBS is reported to improve stool consistency, bowel frequency and urgency but may not change abdominal distension, diarrhea, or flatulence.

Fiber Supplements in IBS: Psyllium is reported to improve overall IBS symptoms. Overall psyllium will lead to general improvement in 25%, deterioration in 20% and no change in 55%.

Dietary fiber in IBS: There is very limited data regarding the effect of increasing fiber intake in the form of ordinary foods. It is suggested that the most popular sources of dietary fiber, such as bran, cereals, vegetables, and fruits, might actually aggravate symptoms in IBS as these foods may also contain large amounts of FODMAPs (fermentable oligo-, di-and monosaccharides and polyols) such as fructans, fructose, galactose and sugar alcohols.

The symptoms that may be aggravated are flatulence, bloating, and abdominal pain. Read more about FODMAPs and low-FODMAPs diet


Success in finding an effective treatment strategy for treating functional gastrointestinal disorders such ad constipation and IBS is challenging.

In these conditions, the use of a soluble fiber supplement such as ispaghula/psyllium is best supported by the available evidence.

In constipated patients, it can be helpful for pre-existing hard stool to be eliminated (e.g., with a laxative) before initiating fiber therapy. Fiber should be started at a nominal dose and slowly increased over the course of weeks to a target dose of 20–30 g of total dietary and supplementary fiber per day.

Even when used judiciously, fiber can exacerbate problems with abdominal distension, flatulence, constipation, and diarrhea, especially if fiber therapy increases the intake of FODMAPs.

Read more about IBS, constipation, intestinal gas and bloating.