Glossary Detail

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name of the term: Fiber

Short introduction

  • Fiber refers to nutrients in the diet that are not digested by gastrointestinal enzymes.
  • Fiber is widely recognized as beneficial for overall human health, and high fiber intake is linked with reduced risk for a number of chronic conditions.
  • Progress has been slow on agreeing to a universal classification of dietary fiber.
  • The Institute of Medicine (IOM) defines fiber in two parts:
    • Dietary fiber: non-digestible carbohydrates and lignin, occurring intact in plants.
    • Functional fiber: isolated non-digestible carbohydrates with beneficial effects.
  • Each type of fiber has its own effect in the body, influenced by characteristics as solubility, viscosity and fermentability – these should be included in the classification.
    • Viscous fibers have gel-forming properties in the intestinal tract.
    • Fermentable fibers can be metabolized (processed, used) by colonic bacteria.
    • Soluble fibers are more completely fermented (more broken down in the body) and have a higher viscosity than insoluble fibers.
    • Not all soluble fibers are viscous and some insoluble fibers may be well fermented.
  • Group 1
    • Dietary fiber e.g.: lignin, cellulose, pectins, gums, β-glucans.
    • Soluble fiber e.g.: wheat dextrin, gums, pectins, fructooligosaccharides, inulin, β-glucans, psyllium.
    • Fermentable fiber e.g.: wheat dextrin, pectins, guar gum, inulin and oligofructose, fructooligosaccharides, β-glucans.
    • Viscous fiber e.g.: pectin, some gums (e.g. guar gum), β-glucans, psyllium.
  • Group 2
    • Functional fiber e.g.: fructooligosaccharides, isolated gums, polydextrose, psyllium.
    • Insoluble fiber e.g.: cellulose, lignin, some pectins.
    • Non-fermentable fiber e.g.: cellulose, lignin.
    • Non-viscous fiber e.g.: cellulose, lignin, inulin.
  • Now, it’s mix and match. For example: soluble, fermentable fibers – pectins, fructooligosaccharides. Each combination can tell what to expect as far as physiological outcome (effect in the body).

Main natural sources

  • Soluble fiber: oat products, nuts, flaxseed, legumes (lentils, beans, dried peas), fruits (apples, oranges, grapefruits) and vegetables (broccoli, spinach, celery, carrots).
  • Fermentable fiber: oat, barley, fruits (e.g. bananas) and vegetables (leeks, asparagus, chicory root, garlic, onions), soybeans.
  • Insoluble fiber: vegetables – especially dark green leafy ones, root vegetable skins, fruit skins, whole-wheat products, wheat bran, corn bran, nuts, and seeds.
  • Non-fermentable fiber: cereal fibers rich in cellulose (e.g. wheat bran).


Due to the variable effects of fiber in the body, it is important to consume fiber from a variety of sources. It is recommended to increase fiber consumption from whole foods, such as legumes, fruits, vegetables, and whole grains.

Main function

Benefit to laxation and regular stools

  • Fiber is important for normal laxation – it increases stool weight. Increased weight is due to the presence of the fiber itself, water held by the fiber, and increased bacterial mass from fermentation.
  • Large and soft stools ease defecation and reduce transit time through the intestinal tract, which may help to prevent or relieve constipation. Insoluble cereal fibers are the most effective. Also soluble fibers (inulin, oligofructose) increase faecal weight.

Benefit to risk factor for cardiovascular disease: cholesterol-lowering effect

  • High fiber intake is linked with a lower occurrence of coronary heart disease – due to the effect of certain fibers on cardiovascular risk factors. Soluble, viscous fibers (psyllium and β-glucan) have been shown to significantly lower serum cholesterol levels.

Benefit to risk of type 2 diabetes

  • Diets rich in fiber, particularly cereal fiber from whole grains (insoluble fiber), are associated with significant reductions in the risk of developing type 2 diabetes – improving glucose tolerance. The effects are more evident in individuals with type 2 diabetes.

Benefit to gut health: prebiotic effect – see: prebiotics

  • Fermentable fibers beneficially alter the composition of the intestinal flora. Inulin, oligofructose and FOS significantly increase the ‘good’ bacteria.

Benefit to immune function and inflammation

  • Role in improving immune function via production of SCFAs. Fructooligosaccharides increase T-lymphocytes (immune cells) in adults and improve antibody response to vaccines in infants. Oligofructose increases resistance to illness or infection (diarrhoea, respiratory events).
  • β-glucans stimulates the immune system directly. Soluble, non-viscous fiber may also alleviate symptoms of inflammatory conditions (irritable bowel syndrome).

Benefit to weight regulation

  • Increased intake of dietary fiber protects against development of obesity. Fiber has a beneficial influence on body weight and body fat. High fiber foods take longer to chew and have a high volume and low energy density, which helps to promote satiety. Fiber delays gastric emptying which can extend feelings of fullness. Viscous fibers (psyllium, guar gum) and insoluble fibers (cellulose, wheat bran) have shown to be satiating.

Deficiency disease

  • Diseases or medical conditions that can be linked to a diet low in fiber include constipation (less than three bowel movements in a week – dry and hard stools), risk for developing diabetes and an increased risk for coronary heart disease, and weight gain.

Recommended daily intake

Latest Dietary Reference Intakes  (DRIs) 
Institute of Medicine (IOM)

Age categoryPer day
28 g (AI)
29 g (AI)
Infants 6 – 12 monthsND
 1 – 3 years
 4 – 8 years
19 g (AI)
25 g (AI)
 9 – 13 years
14 – 18 years
19 – 30 years
31 – 50 years
50 – 70 years
> 70 years
31 g (AI)
38 g (AI)
38 g (AI)
38 g (AI)
30 g (AI)
30 g (AI)
 9 – 13 years
14 – 18 years
19 – 30 years
31 – 50 years
50 – 70 years
> 70 years
26 g (AI)
26 g (AI)
25 g (AI)
25 g (AI)
21 g (AI)
21 g (AI)

AI = Adequate Intake.
ND = Not Determined.

Synonyms: Fibres, Dietary fibre, Functional fiber, Fibre

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