We know that dietary fibre is good for our health. It is a special type of carbohydrate that plays a huge role for our digestive health. Dietary fibre cannot be digested by our body’s enzymes, interacting with both our digestive system and gut bacteria as it passes through to promote health and wellbeing.

Dietary fibre slows down the rate food passes through the stomach and small intestine, which keeps you fuller for longer and helps control blood sugar levels. In the large intestine, it softens our stool and speeds up transit time, assisting with regularity and laxation. Dietary fibre is also fermented by our gut bacteria, supporting their growth, keeping them in balance and releasing a number of useful by-products for our body to use. As if that’s not good enough, these effects also translate to a reduced risk of developing chronic diseases such as diabetes and bowel cancer over time (^{1, 2}).

So how does a single nutrient – dietary fibre – manage to do all of these wonderful things? It’s because dietary fibre isn’t actually just a single nutrient, it describes ANY carbohydrate that we cannot digest. This includes a massive range of different substances, such as β-glucans, cellulose and inulin, coming from a range of plant-based food sources such as cereals such as wheat and rye, as well as vegetables and legumes.

Apart from being indigestible carbohydrates, each of these dietary fibres have their own distinct properties, which determine how they behave in our digestive tract. These properties are:

• Particle size;
• Chemical structure
• Chain length (molecular size);
• Solubility (ability to dissolve);
• Viscosity (ability to thicken); and
• Fermentability (^{3, 4}).

A less soluble fibre will contribute to transit time in the large intestine to assist with regularity; soluble and viscous fibres will slow the passage of food through the stomach and small intestine and improve satiety; and fibres that are more fermentable have a greater impact on the gut bacteria.

It’s impossible for any fibre to have every single property – being soluble and insoluble at the same time just doesn’t work out! Some fibres excel at one thing in particular while offering far less in other areas. Some examples are:

• Cereal fibres (e.g. cellulose and lignins) don’t have much of an effect in the upper digestive tract but have a significant influence in the large intestine.
• Prebiotic fibres (e.g. fructansand inulin) are rapidly fermented by the gut bacteria and supports growth of beneficial species but contribute little else along the digestive tract.

No single fibre can do everything. The effect that dietary fibre has in our body depends on these properties. To get the whole spectrum of health benefits that fibre offers, we need to eat a balanced range of different dietary fibres from a range of food sources; which in essence is following national dietary guidelines.Check out these few for starters:

References:
USA FDA Dietary Fibre guidelines: https://www.accessdata.fda.gov/scripts/InteractiveNutritionFactsLabel/dietary-fiber.html
Australian dietary guidelines: https://www.eatforhealth.gov.au/guidelines/australian-guide-healthy-eating
Singapore dietary guidelines: https://www.healthhub.sg/live-healthy/15/dietary_guidelines_adults

Other References:

1. Internet: https://www.gov.uk/government/publications/sacn-carbohydrates-and-health-report (accessed October 31st 2016).
2. Stephen AM, Champ MM, Cloran SJ, Fleith M, van Lieshout L, Mejborn H, Burley VJ. Dietary fibre in Europe: current state of knowledge on definitions, sources, recommendations, intakes and relationships to health. Nutrition research reviews 2017:1-42. doi: 10.1017/s095442241700004x.
3. Holscher HD. Dietary fiber and prebiotics and the gastrointestinal microbiota. Gut Microbes 2017;8(2):172-84. doi: 10.1080/19490976.2017.1290756.
4. McRorie JW, Jr., McKeown NM. Understanding the Physics of Functional Fibers in the Gastrointestinal Tract: An Evidence-Based Approach to Resolving Enduring Misconceptions about Insoluble and Soluble Fiber. Journal of the Academy of Nutrition and Dietetics 2017;117(2):251-64. doi: 10.1016/j.jand.2016.09.021.