Dietary fibre is defined as edible carbohydrate polymers with three or more monomeric units, including non-starch polysaccharides, oligosaccharides and resistant starch (Valdes et al., 2018; Cronin et al., 2021).
Unlike other macronutrients, fibre is resistant to the action of mammalian digestive enzymes and is fermented primarily in the large intestine by bacteria of the gastrointestinal (GI) microbiome. Fibre can have ranging physiochemical properties, meaning its physiological effects on the host can be widely variable. Therefore, fibre requires further definition to help categorise individual sources.
Fibre is often categorised based on fermentability, solubility or viscosity (Davani-Davari et al., 2019), with solubility referring to the ability to dissolve in water and fermentability referring to the rate of microbial fermentation. Many soluble fibres are also highly fermentable (and vice versa) (Linder, 2017), therefore “solubility” and “fermentability” are often used interchangeably.
How does fibre impact the microbiome?
Dietary fibre can significantly impact the composition, diversity and richness of the microbiome, acting as a substrate for specific microbes that possess the necessary enzymes for the fermentation of these complex carbohydrates (Cronin et al., 2021).
Dietary fibre can significantly impact the composition, diversity and richness of the microbiome
What are prebiotics?
Fibres that demonstrate an ability to specifically or selectively stimulate the growth of beneficial microorganisms that positively influence microbiome composition and host health are termed “prebiotics” (Davani-Davari et al., 2019). For example, fructo-oligosaccharide and acacia gum have been shown to increase levels of beneficial bacteria (eg Bifidobacteria and Lactobacillus) and reduce potential pathogens (eg Clostridium perfringens) in humans, dogs and cats (Swanson et al., 2002; Cherbut et al., 2003; Kanakupt et al., 2011; Oku and Nakamura, 2017).
What are short-chain fatty acids and why are they important for gut health?
Short-chain fatty acids (SCFAs) are metabolites (primarily butyrate, acetate and propionate) produced by the microbial fermentation of fibre. SCFAs reduce luminal intestinal pH to suppress the growth of pathogens and offer a competitive advantage to beneficial bacterial species, promoting a more favourable microbiome composition (Sun and O’Riordan, 2013; Oku and Nakamura, 2017). SCFAs also enhance mineral absorption and reduce the degradation of peptides into toxic compounds such as ammonia, amines and phenolic compounds (Slavin, 2013).
Butyrate acts as the preferred energy source for colonocytes, providing approximately 70 to 80 percent of their total energy requirement (Gasaly et al., 2021), and is vital for the maintenance of epithelial barrier integrity. This is because butyrate acts as a key regulator for normal cell colonocyte renewal, enhances intestinal mucin production, promotes epithelial tight junction formation and acts as crucial messenger molecules that help regulate local and systemic immune responses (Gasaly et al., 2021; Fernández et al., 2016; Ahmad et al., 2000; Burger-van Paassen et al., 2009; Salvi and Cowles, 2021; Pujari and Banerjee, 2020).
Short-chain fatty acids reduce luminal intestinal pH to suppress the growth of pathogens and offer a competitive advantage to beneficial bacterial species
Studies in humans with inflammatory bowel diseaseand dogs with chronic enteropathies have demonstrated lower concentrations and abnormal patterns of faecal SCFAs, reductions in important SCFA-producing bacteria, decreased bacterial diversity and a higher dysbiosis index (Parada Venegas et al., 2019; Minamoto et al., 2019). Research to help harness this metabolomic and microbiome data to develop biomarkers that can predict disease onset could offer a promising field for future diagnostic testing and therapeutic targets (Farup et al., 2016).
What are the physiological effects of fibre on gut health?
How does fibre impact gut transit and faecal consistency?
In general, soluble fibres tend to delay gastric emptying and increase transit time in the small intestine (Russell and Bass, 1985; Chandler, 2012; Moreno et al., 2022; Mudgil, 2017; McRorie and McKeown, 2017). Soluble, fermentable fibres efficiently hold water to increase stool weight and moisture, thereby acting as effective stool softeners (Mudgil, 2017). Alternatively, viscous fibres can form gels to increase the viscosity of intestinal contents (Mudgil, 2017). Fermentable fibres produce SCFAs which facilitate sodium and chloride absorption in the colon to regulate fluid homeostasis and faecal moisture (Chandler, 2012).
Insoluble fibres are fermented poorly, meaning they largely retain their structure throughout the GI tract and offer little nutritional value. However, they have important physiological and functional effects and tend to promote gastric emptying, decrease intestinal transit time and help to normalise colonic motility (Moreno et al., 2022; Mudgil, 2017; McRorie and McKeown, 2017). Two proposed mechanisms to explain this effect are: increased faecal bulk leading to colonic distention to stimulate peristalsis, or action of coarse fibre particles stimulating an increase in intestinal water and mucus secretion to aid the passage of faeces through the colon (McRorie and McKeown, 2017; Davenport et al., 2010).
Decreased colonic transit time and increased faecal bulk help to reduce colonocyte exposure to toxins (eg bile acids, ammonia and ingested toxins) (Davenport et al., 2010).
An animal’s response to specific fibre sources can vary and may deviate from the above general trends, therefore clinicians should be aware that not all animals will respond uniformly, and trial and error may be necessary
An animal’s response to specific fibre sources can vary and may deviate from the above general trends, therefore clinicians should be aware that not all animals will respond uniformly, and trial and error may be necessary (Linder, 2017). Potential side effects such as diarrhoea, flatulence and abdominal cramps may be seen if fibre is introduced suddenly and/or at excessive doses (Davenport et al., 2010).
Can fibre support metabolic health and weight management?
Another potential use of dietary fibre is to support metabolic health and weight management. Fibre has a low energy density which allows for increased food volume. This, in turn, induces gastric distension, supporting physical feelings of satiety and the moderation of energy intake (Pappas et al., 1989; Flanagan et al., 2017). Research suggests that the weight of food consumed, as opposed to the energy content, has a greater influence on eating patterns. Hence, satiety can be encouraged by substituting dietary nutrients with low energy density foods such as fibre (Bosch et al., 2007).
Viscous fibres appear to beneficially affect the metabolic parameters of glycaemia and lipidaemia via several proposed mechanisms:
- By eliciting a delay in gastric emptying, which encourages a more gradual delivery of nutrients into the small intestine, and improved post-prandial glucose control (Müller et al., 2018; Dikeman and Fahey, 2006; Qi et al., 2018; Jenkins et al., 2004)
- Through gelatinous fibres trapping other nutrients in their matrix, limiting nutrient-enzyme interactions (thus slowing the rate of digestion) and systemic absorption of carbohydrates and lipids (Qi et al., 2018; Jenkins et al., 2004)
- Via SCFAs which appear to interact with the neuroendocrine system to inhibit or stimulate the release of GI hormones involved in intestinal motility, satiety, and glucose and lipid metabolism (such as peptide YY, GLP-1 and leptin) (Wong et al., 2006; Kamath et al., 1987; Rondeau et al., 2003; den Besten et al., 2013; Byrne et al., 2015)
As such, emerging research suggests that the microbiome may play a role in diabetes, energy balance and obesity.
What are the common clinical uses of fibre?
Can fibre help with constipation?
In humans, the use of insoluble and soluble fibres to manage constipation has been extensively reviewed and is widely advised by numerous medical authorities (van der Schoot et al., 2022; NICE, 2022). Given the physiochemical properties of fibre form the basis of this clinical effect, results from human medicine are likely to be transferable to other species.
Fibre is documented as a dietary intervention for the management of constipation in dogs, with multiple studies demonstrating its efficacy (Davenport et al., 2010; Chandler, 2013). One study assessed the efficacy of 2 percent psyllium in dogs with conditions that predisposed to constipation, including perineal hernia, pelvic fracture, spinal disease and prostatic enlargement (Tortola et al., 2009). The authors found stool consistency improved from “dry” or “hard” to “normal” or “pasty” in 62.5 percent of patients.
Another study found that a high-fibre fig paste significantly increased faecal quantity and reduced colonic transit time in experimentally induced constipation in healthy beagles (Oh et al., 2011).
Similarly, fibre is also recommended for the management of constipation in cats (Davenport et al., 2010; Chandler, 2013). One study demonstrated that a moderate-fibre diet enriched with psyllium significantly improved faecal consistency in 93 percent of cats (Freiche et al., 2011). This, in turn, resulted in a significant reduction in use of cisapride and lactulose.
In [patients with severe end-stage megacolon], colonic motility is absent, so the stimulatory effect of high-fibre diets or supplements is no longer effective
Constipation in cats is generally associated with other comorbidities that lead to dehydration, such as chronic kidney disease, diabetes mellitus and hyperthyroidism (Benjamin and Dobratz, 2020). While increasing dietary fibre and moisture intake is recommended in the management of mild to moderate constipation cases, highly digestible diets are recommended for animals with megacolon associated with colonic dysmotility or obstipation (severe end-stage megacolon) (Davenport et al., 2010). In these patients, colonic motility is absent, so the stimulatory effect of high-fibre diets or supplements is no longer effective. In fact, such foods can contribute to obstipation, therefore diets containing less than 5 percent dry matter crude fibre are recommended in the literature (Davenport et al., 2010).
Can fibre help with diarrhoea?
Numerous studies have been published supporting the efficacy of fibre-enhanced diets to help manage acute diarrhoea in dogs and cats. Shelter dogs presenting with acute colitis had significantly improved faecal scores when fed a high-fibre diet compared to those fed a standard diet (Lappin et al., 2022).
Similarly, dogs with acute diarrhoea supplemented with a highly digestible diet alone or in combination with psyllium demonstrated an improved time to resolution compared to the same diet with metronidazole (Rudinsky et al., 2022). Recurrence of colitis occurred at a lower rate in the psyllium-supplemented group compared to dogs on the highly digestible diet alone, suggesting increased fibre could have exerted an additional benefit to intestinal health. Furthermore, this supports the increasing rationale that antibiotic therapy is rarely indicated in the management of acute diarrhoea and that nutritional interventions can offer superior benefits.
The efficacy of high-fibre diets in dogs and cats with chronic diarrhoea (duration over three weeks) has been assessed in several studies, primarily in animals presenting with symptoms of chronic colitis (Dennis et al., 1993; Simpson et al., 1994; Livet et al., 2014; Frantz and Yamka, 2020a, 2020b). The overall findings of these studies conclude that high-fibre diets have a high success rate at resolving clinical signs (Simpson et al., 1994; Livet et al., 2014) and improving faecal consistency (Frantz and Yamka, 2020a, 2020b). Therefore, they should be considered a feasible option when selecting appropriate diet trials in these patients.
When looking at chronic idiopathic large bowel diarrhoea specifically, there are further studies to support the use of fibre-supplemented diets (Alves et al., 2021; Leib, 2000; Lecoindre and Gaschen, 2011). In a recent study, highly strung working police dogs were supplemented with psyllium (a mixed viscous fibre) for one month. Treatment response was classed as “good” or “very good” in 90 percent of patients (Alves et al., 2021). Stool consistency was scored as “normal” in 90 percent of dogs, and there was a significant reduction in defecation frequency.
Another study assessed the effect of a highly digestible diet supplemented with psyllium in dogs with an average 32-week history of chronic colitis symptoms (Leib, 2000). Following the dietary intervention, 96 percent of dogs achieved a “good” or “excellent” clinical response. Some dogs were able to taper down or discontinue adjunctive therapies, but many relapsed once psyllium supplementation ceased.
Dietary fibres can offer a varied range of physiochemical properties, reflected by the different effects the nutrient can exert on host physiology. A specific fibre source will sit on a scale of fermentability and solubility, thus may possess properties associated with multiple classifications.
The inclusion of dietary fibre appears to be integral for GI health, from a functional perspective and due to its impact on microbiome composition and short-chain fatty acid production
The inclusion of dietary fibre appears to be integral for GI health, from a functional perspective and due to its impact on microbiome composition and SCFA production. Manipulation of dietary fibre levels can offer a fundamental tool for the nutritional management of many clinical conditions in dogs and cats.