Fibre & Gut Health

Dietary fibres are a diverse group of complex carbohydrates found naturally in or isolated from plants. They are unable to be fully digested by human enzymes in the small intestine, allowing them to reach the colon largely intact. Fibres are important for our gut health; some playing a role in supporting normal intestinal transit, whilst others act as ‘prebiotics’, which can be broken down and fermented by gut bacteria when they reach the colon.[1]

Depending on the specific type of fibre and the amount consumed, they may influence gut comfort, appetite regulation, blood glucose and cholesterol concentrations.[1]

• Fruit and vegetables
• Nuts and seeds
• Pulses and legumes
• Grains and wholegrains
• Fibre-fortified foods, e.g. yogurt with added grains or fruit

There are two categories of fibre origin: dietary and functional.

• Dietary fibres naturally occur within plants e.g. cellulose, beta-glucans, lignins

• Functional fibres are carbohydrates extracted from plants by physical, chemical or enzymatic processes, or synthetically made, and added to manufactured foods and beverages that lack fibre e.g. soluble corn fibre, psyllium.

Some fibres may fall into both categories e.g. oligosaccharides, inulin, pectin, which can be both naturally occurring, or extracted/synthesised.

Fibres are often classified by their solubility, viscosity and fermentability. Additionally fibres can also be classified on their chemical structure; interactions with other cell wall compounds, digestion and food processing can also impact their nature. For example, fibre fermentability not only depends on the fibre structure, but the presence of bacteria and enzymes to break them down. Classifications can also impact the characteristics of fibres, for example, solubility typically increases fermentability, whilst viscosity decreases fermentability. This is important when identifying fibres as substrates for probiotics.[2]

Solubility: Ability to dissolve in water. Soluble fibres dissolve in water and can help lower cholesterol, slow digestion and feed gut bacteria, e.g. inulin, pectin and beta glucans found in fruit and oats. Fibres that are mainly insoluble, such as cereal fibres, help regulate the bowel and prevent constipation, e.g. cellulose[3]

Fermentability: ability to be broken down by probiotics. Fermentable fibres are broken down by gut bacteria, promoting a healthy microbiome e.g. resistant starch, pectin, inulin, oligofructose. Fibres with low fermentability can act as bulking agents and help prevent constipation e.g. cellulose[2,4]

Viscosity: ability to thicken when mixed with fluids. Viscous fibres thicken when mixed with fluids to form gels in the gut, slowing the rate of nutrient absorption and stabilising blood sugar levels e.g. pectin and beta-glucans.[2]

Due to the complex structure of fibre, each type may behave differently in the body and can trigger unique physiological responses or benefits. Also, many foods contain a mix of different types of fibres, particularly fruits and vegetables, therefore, a varied diet that includes a mix of fibres is important for well-being and digestive health.

Fibre intake recommendations are derived from clinical studies that evaluate the connections between fibre consumption and various health conditions, such as cardiovascular disease, type 2 diabetes, normal bowel activity and colorectal cancer. This evidence largely formed the Scientific Advisory Committee (SACN) recommendation of 30g fibre per day in healthy UK adults, which shows that higher fibre intake can improve bowel health and a reduced risk of cardiovascular disease, type 2 diabetes, and colorectal cancer [6]

However, UK adults are not consuming enough fibre and there is a fibre deficit: the latest National Diet and Nutrition Survey (NDNS) Years 12 to 15 data, reveals that only 4% of UK adults aged 19-65 meet these recommendations.[7]

Research is ongoing to understand the potential health benefits of regular yogurt consumption.

Common barriers to fibre consumption include:

• Many people lack of knowledge about just how much fibre is needed, which foods contain fibre and the benefits of fibre

• Perceived inconvenience of sourcing, preparing and cooking high-fibre foods

• Individual tastes and preferences

• Cost: fruits, vegetables and grains are considered costly

• Concern over potential side effects such as bloating, flatulence and abdominal discomfort, particularly if transitioning from a low fibre diet to a high fibre diet.

• Specific diets: gluten-free and low-carbohydrate diets are often low in fibre

How fibre affects gut health

A rich and diverse community of micro-organisms such as bacteria, fungi and yeast live in the gut. Within this gut microbiota, microorganisms interact with each other and with their host to modulate biological processes essential for health, such as supporting gut comfort, immunity and nutrient absorption. In the human diet, fibre is the primary source of "fuel" for gut bacterial fermentation[8] and prebiotic fibres are essential to support the growth and development of beneficial bacterial populations in the gut. Therefore, a diet rich in different fibres can greatly impact gut microbiome diversity.[9]

When fibre is fermented in the colon, it produces gases and beneficial metabolites, predominantly short-chain fatty acids (SCFAs), including acetate, butyrate and propionate. SCFAs are essential in human functions and play a part in physiological processes such as metabolism, immune support and cell proliferation, as well as being the primary fuel source for intestinal epithelial cells.[10]

A fibre deprived diet can lead to an imbalance in the gut microbiota and functional gut disorders or complaints. It has also been shown that a prolonged low fibre diet may lead to an increase in mucin-feeding bacteria which, in turn can harm the protective colonic mucosal layer, bringing pathogens closer to the epithelial cells and increasing the risk of infections.[11]

As already noted, fibre also helps prevent constipation by regulating the bowel (insoluble fibres) and acting as bulking agents (low fermentability fibres). Some types of dietary fibre, particularly soluble fibre are thought to be of interest in the management of Irritable Bowel Syndrome (IBS) symptoms.[12]

Evidence suggests that diets high in dietary fibre have a number of other health benefits due to the byproducts of fibre digestion, some of which are known as postbiotics. The evidence for the links between dietary fibre and health was reviewed in 2015 by the Scientific Advisory Committee on Nutrition (SACN), which found that there is strong evidence that diets rich in fibre, particularly cereal fibre and wholegrains, are associated with a lower risk of many health conditions, including cardiovascular disease, coronary events, stroke, type 2 diabetes and colorectal cancer.[5-6]

Fibre and heart health
Diets rich in fermentable fibres – like fruits, grains and most notably oats - have been shown to reduce low density lipoprotein (LDL)-cholesterol[5] and may play a role in cardiovascular disease prevention.[13]

Fibre and type 2 diabetes
Since fibre is linked with controlled blood sugar levels and insulin demands, it can play an essential role in reducing the risk of developing type 2 diabetes and the risk of associated certain complications.[14]

Fibre, satiation & satiety[10]
While there is currently no authorised health claim in Europe linked to fibre’s direct effect on satiation and satiety, there are several suggested ways in which eating fibre may influence feelings of fullness or hunger, such as fibre-rich foods taking longer to chew, delaying gastric emptying and producing SCFAs that stimulate enteroendocrine cells into secreting the satiety related hormones like ghrelin, glucagon- like peptide 1 and polypeptide YY.[15]

Potential issues with low fibre intake

Since high-fibre diets are associated with a lower risk of many health conditions, a low fibre diet may increase the risk of those conditions.

A low fibre diet has been linked to obesity, constipation, bloating and gas. It is also associated with diverticulitis and colorectal cancer.[10]

Diets high in animal fats and protein and low in fibre may promote ‘dysbiosis’, an imbalance of bacteria in the gut, resulting in lower production of short-chain fatty acids, a leaky gut mucosa and intestinal and systemic inflammation.[10]