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Pet Food Ingredients Provenance / Fibre
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Although usually the smallest percentage of the major nutrients, fibre is critical for normal digestive function in dogs. It is defined as “substances that cannot be broken down into absorbable parts by the body’s enzymes”, and is mostly indigestible, non-starch polysaccharide from plant origins.
Very little research has focussed solely on the role of this nutrient however, to date fibre has been linked with several essential functions, including:
Nearly all food ingredients contain some fibre, however the main sources are by-products from food production (milling, sugar production, potato processing), e.g. wheat bran, sugar beet pulp, etc. Refined, pure fibres are also available, e.g. cellulose. Some of the most common sources used in pet food recipes are listed below (Table 1).
| Cellulose | Wheat Bran | Oat Fibre | Pea Fibree | Tomato Pomace | Apple Fibre | Citrus Fibre | Sugar Beet Pulp | |
| Origin | wood | hull | husk | hull | pulp | dried apple | peel | root |
| Total fibre content (%) |
99.5 | 32 | 90 | 88 | 14 | 60 | 65 | 75 |
| Insoluble (%) |
96.5 | 29 | 88 | 80 | 9 | 47 | 37 | 50 |
| Soluble (%) |
3 | 3 | 2 | 8 | 5 | 13 | 28 | 25 |
| Fat (%) |
0 | 4 | 1 | 0.1 | 0.5 | 2 | 0.5 | 0.5 |
| Carbohydrates (%) |
0 | 35 | 1 | 0.5 | 55 | 30 | 16 | 4 |
| Calorific value (kCal/100g) |
0 | 150 | 30 | 16 | 170 | 160 | 90 | 60 |
Total, or crude, fibre can be separated into soluble (fermentable) and insoluble components (Table 2). The insoluble fractions pass through the gut with little physical change, whilst the soluble fractions pass through the stomach and small intestine intact, but are digested (fermented) by the bacteria naturally occurring in the large intestine. Sugar beet pulp is commonly used in many recipes since it has a good proportion of both soluble and insoluble fibres, and is consistent in both quality and supply.
| SOLUBLE FIBRES | INSOLUBLE FIBRES |
| (Fermentable) | (Un-fermentable) |
| Beta-glucans | Cellulose |
| Some xylo poly saccharides (XPS) | Lignin |
| Xylo oligo saccharides (XOS) | Some xylo poly saccharides (XPS) |
| Mannan-oligo saccharides (MOS) | |
| Fructo-oligo saccharides (FOS) |
In general, the insoluble fibres stimulate the inside of the gut wall, thereby drawing moisture into the lumen, causing the gut contents to swell. This in turn gives a feeling of fullness (satiation) and increases the rate of passage of the food by increasing peristalsis. It also provides texture to the food, which can assist teeth cleaning. The soluble fraction provides a food source for the bacteria in the large intestine and slows the rate of food passage through the gut.
Fibre may be used in three ways to control bodyweight in dogs. Firstly, insoluble fibre binds fat in the diet, preventing its absorption into the body. It also heightens the body’s sensitivity to insulin, resulting in a lower insulin peak post-feeding. The second method is by virtue of the low energy content of most fibrous ingredients. Since, by definition, fibre cannot be digested by the body’s enzymes, it is unable to release energy, and so is commonly used in “Light” diets, or in food for breeds that are susceptible to obesity. Thirdly, insoluble fibre binds water within the gut causing the contents to swell and fill the lumen. This triggers receptors in the gut wall to signal to the brain that it is full, and hence inhibit the hunger reflex. In this way, overall food intake can be controlled. However, care must be taken since over-supply of insoluble fibre in the diet can cause excessive distension, which can lead to blockage and/or cessation of peristalsis.
As already mentioned, fibre is essential for the maintenance of bacteria in the large intestine. These bacteria may be divided into three subgroups: those that are pathogenic (“harmful”), those that are beneficial to the host (“good”), and those that are always present but are neither pathogenic nor beneficial (“neutral”). Harmful bacteria, which include staphylococcus and clostridia, can cause diarrhoea, infections and intestinal degradation. Neutral bacteria, which include enterococci, E. coli, and streptococcus, are not particularly harmful or beneficial when present in normal quantities. Good bacteria include bifidobacteria and lactobacilli, and have been shown to promote effects such as stimulation of the immune system, improved digestion, and improved absorption of essential nutrients. Bacteria colonise the small intestine within 3 hours, and the large intestine within 12 hours of birth, and all three types battle for nutrients and space within the gut environment throughout life.
Two of the main soluble fibres that promote the growth of “good” bacteria are mannan-oligo saccharides (MOS) and fructo-oligo saccharides (FOS), and are collectively known as prebiotics. In general, MOS inhibits the replication of harmful bacteria by blocking their attachment to the gut wall, whilst FOS promotes the growth of the beneficial strains by providing a food source. Purified sources of both of these fibres are now routinely added to dog foods, since most ingredients contain low and variable amounts.
An additional method of promoting the growth of “good” bacteria is to add these strains directly into the food. These are known as probiotics. This is now widely accepted in human nutrition with the growth of yoghurt drinks containing specific cultures such as enterococcus and saccharamices. However, it is more difficult to achieve this in dog foods since the cooking process destroys many of these sensitive, living organisms. Techniques to protect these bacteria through cooking, and methods to apply them post-processing are being developed, and may allow application into dog foods in the near future.
In summary, fibre is an essential nutrient in the dog’s diet however, thought should be given to the proportion of soluble and insoluble components, as well as how to maximise the growth of beneficial bacteria.
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