A closer look at fibre in grain-free diets

Perception of grain-free

Owners often perceive grain-free diets as a healthier option than pet foods containing grains as a source of starch. A study by Banton et al. (2020) found that owners of dogs with a presumed food allergy were four times more likely to select ‘no grain’. Pet owners are often influenced by claims such as ‘limited ingredient diet’, ‘sensitive skin/stomach’ and ‘exotic protein’.

Digestibility

Grain-free diets replace grain ingredients with tubers and legumes to achieve the starch minimum for successful extrusion. Nevertheless, each class of alternative starch sources has a unique nutritional composition that will affect diet processing and digestibility. Moreover, most grain-free diets are formulated with a combination of carbohydrate sources, making it challenging to estimate the digestibility of the final product.

In terms of protein sources, it is important to consider the amino acid composition in pulses (peas, lentils, chickpeas, dry beans) as they are often high in lysine and low in methionine, plus they have a different fibre content and different characteristics. Research by Carciofi et al. (2008) found a higher large intestinal fermentation for legume-based diets as they contain a greater concentration of soluble fibres (legume oligosaccharides) compared to cereal grains. The overproduction of gases and short-chain fatty acids can attract water-increasing faecal bulk.

Benefits and risks of fibre content

Dietary fibre content has multiple beneficial effects on postprandial glucose and insulin responses, intestinal integrity and microbiota. Additionally, it affects
gastric emptying and gastrointestinal transit time. Nevertheless, it has been hypothesised by Pezzali et al. (2020) that high-fibre diets can increase susceptibility to taurine deficiency in dogs linked to obligatory bile acid conjugation and enterohepatic circulation for the reabsorption of taurine. They propose that high-fibre diets may increase faecal output and losses of taurine-conjugated bile. The higher rate of microbial fermentation would lead to more efficient taurine degradation and consequently to a decrease in enterohepatic circulation and recycling of taurine, resulting in the risk of taurine insufficiency and dilated cardiomyopathy (DCM).

Formulating a grain-free diet

Multiple factors should be considered when formulating a grain-free diet, including differences in nutrient digestibility and bioavailability, fermentability, quantity and characteristics of fibre in the final product. In addition, possible interactions between nutritional components of the diet, especially when using novel ingredients, should be evaluated.

Caution is advised when the inclusion of legumes and tubers exceeds the concentration previously investigated in research studies. Additionally, plant- based ingredients tend to have lower small intestine digestibility for protein compared with animal protein. To avoid amino acid deficiencies, pet foods that contain substantial amounts of legumes and tubers and lower proportions of animal protein should address amino acid imbalances through the addition of alternate ingredients or fortification. Moreover, as proposed by Mansilla et al. (2019), for high-fibre diets containing high levels of oligosaccharides, fortification of taurine and other sulphur amino acids should be considered.