Fibre - Nutrition
What is Fibre?
Fibres are polysaccharides that are resistant to hydrolysis by mammalian digestive enzymes. These include substances such as cellulose, hemi-cellulose, pectin and lignin which are key structural components of plant cell walls. Although fibre is most commonly associated with plant materials, fibre like substances such as chitin are present in fungi, crustaceans and insects. Over recent years the definition of dietary fibre has been expanded to include substances such as resistant starch, inulin and non-digestible oligosaccharides. Fibre is resistant to degradation because mammalian digestive enzymes cannot break the glycoside linkages between sugar monomers. Fibres can be classified by their capacity to dissolve in water (soluble or insoluble), form a gel-like consistency (viscous or non-viscous), and/or for fermentation by colonic bacteria (fermentable or non-fermentable).
- Soluble, viscous, fermentable fibres are those that form gels within the gastrointestinal tracts, such as pectin, gums, mucilage, and some hemicelluloses.
- Insoluble, non-viscous, non-fermentable fibres remain relatively unchanged after consumption and include cellulose, lignin, and some hemicelluloses.
- Resistant starches are formed from absorbable starch during the cooking process and will act as fermentable fibre in the intestinal lumen.
Why is it Important?
Fibre is not considered an essential nutrient, but both soluble, viscous, fermentable fibres and insoluble, non-viscous, non-fermentable fibres are important for normal physiologic function of the gastrointestinal tract. Depending on the type and amount of dietary fibre present, it can help regulate gastrointestinal motility and provide a source of nutrients for commensal colonic bacteria, which in turn can improve colonic function.
Roles in the Body
Increasing viscosity of ingesta will slow transit out of the stomach and through the small intestine. This can promote a feeling of satiety, but can also delay or hinder nutrient absorption. The presence of viscous fibres can interfere with fat absorption and can slow starch digestion and glucose absorption, dampening post-prandial rises in blood sugar.
Fermentable dietary fibres are utilized by bacteria within the large intestine to produce H2 (hydrogen gas), CO2 (carbon dioxide), CH4 (methane), and the short-chain fatty acids, acetate, propionate, and butyrate. Acetate and propionate are rapidly absorbed across the large intestinal mucosa with sodium (Na2+) and are responsible for water absorption in the large intestine. Acetate and propionate can then be further utilized by the host animal, and can contribute up to 10% of energy intake. Butyrate remains in the large intestine where it is used as the preferred energy substrate by colonocytes. Increased intake of fermentable fibre has been shown to increase colonic weight and villus height in dogs. Insoluble fibres will also increase faecal bulk and dry matter content, a feature that is used in some pet foods to dilute calorie density for weight management, though efficacy at decreasing energy intake is inconsistent. Increased dietary fibre intake has also been used as an adjunct therapy to insulin administration to improve glycaemic response in diabetic cats and dogs. Decreased cholesterol and triglyceride absorption may also occur with increased intake of soluble, fermentable fibre.
Consequences of Fibre Deficiency
There are not consistent clinical signs of feeding a fibre-free diet to dogs. In otherwise healthy adult dogs, decreasing dietary fibre intake will decrease frequency of defecation and decrease faecal water content. Some dogs with recurrent idiopathic colitis may benefit from feeding higher fibre diets.
Excessive intake of non-absorbable carbohydrates (both fermentable and non-fermentable dietary fibres) may potentially result in either diarrhoea or constipation in healthy dogs and cats depending on fibre type (viscous, fermentable vs. non-fermentable, respectively). Increased dietary fibre can also decrease protein digestibility, which may be contraindicated in animals with underlying intestinal disease, and high dietary fibre intake can interfere with mineral absorption. High intake of dietary fibre also results in energy dilution and animals may be unable to consume sufficient amounts of food to meet energy needs.
Dietary fibre is found in fruits, vegetables, cereal grains, and pulses (i.e. legumes). Chitin is present in fungi and the exoskeleton of crustaceans and insects. Common pet food fibre sources, such as beet pulp and fruit (e.g. citrus, tomato, apple) pomaces are derived from the human food industry. Crude fibre as listed on a pet food label is a measure of insoluble, non-fermentable fraction of total dietary fibre and does not reflect the actual amount or distribution of fibre within a commercial diet.
Diagnosing Fibre Deficiency
Diagnosis is based on response to feeding of a higher fibre diet. For diabetic dogs and cats, improvement in glycaemic control may be seen with addition of dietary fibre. In animals experiencing colitis-like signs, the inclusion of both soluble, fermentable, and insoluble, non-fermentable fibre types may resolve clinical signs.
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Date reviewed: 18 May 2015
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