This is Part 1 of a three-part series on the fascinating topic of FIBRE! This Part 1 answers the question “what is fibre?” and explores the different categories of fibre and their health impacts. Part 2 will explore the health benefits of the different types of fibre. In Part 3 I will share my top tips for increasing dietary fibre intake!

The American Institute of Medicine recommends women consume 25g of fibre/day and men consume 38g/day. The Australian National Health and Medical Research Council (NHMRC) recommends that women consume 25g of fibre/day and men consume 30g/day.

But the average adult is only consuming around 15-20 grams of fibre/day. And that’s not a good thing. Insufficient fibre intake is linked with an array of adverse health impacts, including colon cancer, constipation, haemorrhoids, diverticulitis, irritable bowel syndrome, obesity, coronary heart disease, type 2 diabetes.

What is fibre?

Technically, fibre (or fiber for my American friends!) is a carb. But before you run to your pantry to purge it of every food that contains fibre, hold fire - fibre is a good carb. A very good carb! It doesn’t spike your blood sugar or insulin levels like “normal” carbs. In fact, fibre is VERY good for us! It was once thought that fibre’s only benefit was to keep us, ahem, regular. But as nutrition science has evolved, an array of amazing health benefits have been uncovered. More on this below.

Fibre is found in plant foods like whole unprocessed grains, legumes, fruit, vegetables, nuts and seeds. It is the part of these plant foods that cannot be digested and absorbed in our stomach or small intestine.

“Fibre” is actually an umbrella term for a diverse group of substances, all with different impacts on the human body.

In the early days of nutrition science, there were thought to be only two types of fibre - insoluble and soluble. And unfortunately, you still see quite a few nutritionists, dietitians, doctors and health bloggers who use this outdated dual classification, which doesn’t help us understand which fibre to look for to address specific health concerns.

The most helpful way to classify fibre is according to how it behaves in the human body, which in turn dictates its health impacts.

You see, fibre has three physiochemical properties:

Solubility (aka the old soluble/insoluble fibre classification): some fibres will dissolve in liquid while others do not dissolve at all. Insoluble/non-dissolving fibres travel unchanged throughout our digestive tract. But when it comes to soluble fibres, there are actually two further physiochemical properties to consider:
Viscosity: this is the degree to which a soluble fibre will thicken into a bulky gel-like substance when dissolved in liquid. Some soluble fibres will thicken into a bulky gel when added to water or other fluids, whilst others will dissolve without thickening at all. Those soluble fibres that thicken into bulky gels have been shown in numerous studies to deliver some amazing health benefits; they thicken the chyme (ie, the mix of food and digestive juices) in our stomach and small intestine, which in turn slows down the digestion and absorption of the nutrients in the foods we eat. So gelling fibres slow the rate the glucose and fats from your food enters your bloodstream, with flow-on effects to our cholesterol, blood glucose and insulin levels. More on this below!
Fermentability: Some types of fibre can be fermented (ie, digested) by the tiny microbes that live in our large intestine. Ever heard of prebiotics? Prebiotics are the substances that serve as fuel for probiotics (the healthy microbes that live in our large intestine). The fermentable fibres are prebiotics! These fermentable prebiotic fibres feed the healthy strains of bacteria in our large intestine, boosting their populations and helping them to keep other less-healthy strains in check. More importantly, the healthy inner bacteria that digest these prebiotic fibres excrete short chain fatty acids (SCFA’s) which produce a cascade of beneficial health impacts in our bowel and throughout our body. More on this below!

When you look at the different combinations of the above physiochemical properties of fibre and how each combination impacts our health, there are actually five different types of fibre we should be aware of.

There are actually FIVE discrete classifications of fibre

Each of the following classifications of fibre acts differently in the human body, and as a result delivers different health benefits:

Insoluble Fibre

Insoluble fibre does not dissolve in water. And because it does not dissolve, it won’t form a viscous gel. Insoluble fibre is also not readily fermented by the bacteria in our large intestine.

Insoluble fibre travels through our digestive system unchanged, remaining as discrete particles. I think of insoluble fibre particle as little mini “scrubbing brushes”, travelling through my digestive system, largely unchanged, all the way to the toilet.* Insoluble fibre particles increase stool size, and also cause very mild “irritation” to the lining of our large intestine, which stimulates to release of mucous and water which can help to soften our stool, and it also acts to speed up the transit of waste through our large intestine and out of our body.

Cellulose, hemicellulose and lignin are insoluble fibres. They are all components of the structural cell walls in plants. Insoluble fibre is found in wheat bran, whole, unprocessed or minimally processed grains like brown rice, bulgur wheat, and (genuinely) wholegrain flour, bread and pasta, beans and legumes (lentils, chickpeas), root vegetables, nuts and seeds.

The Soluble Fibre Categoriess

All of the soluble fibres dissolve in water (or your digestive juices). But that is where the similarity ends. That is because different types of soluble fibre behave very differently - and have very different health impacts - according to:

the degree to which they thicken into a viscous gel; and/or
the degree to which they serve as food for the tiny bacteria in our large intestine.

So in reality there are actually four distinct categories of soluble fibres, all with their own unique health benefits:

Soluble + Thickening/Gel-Forming + Non-Fermentable

This category of soluble fibre thickens into a bulky, gel-like substance in our stomach and small intestine, and it stays that way all the way through our digestive tract, as the tiny bacteria in our large intestine don’t ferment (ie, eat) it. These kinds of fibre thicken the chyme (ie, the mix of food and digestive juices) in our stomach and small intestine (which is usually quite thin), into a bulky gel. A thickened, gel-like chyme in our stomach and small intestine slows down the rate at which we digest and absorb the nutrients in the foods we eat. So it will, for example, slow the rate the glucose and fats from your food enters your bloodstream.

For this reason, these kinds of thickening/gel-forming fibres are recommended to people who want to improve their glycemic control, or lower their LDL cholesterol.

And because this category of fibre are not fermented (digested) by the bacteria in our large intestine, they stay in this viscous gel form all the way through our bowel. This viscous gel has a “stool normalising” in the bowel - if your stool is too hard due to constipation, it will soften it. If your stool is too loose due to diarrhoea, it will firm it.

Psyllium is a soluble fibre that dissolves in fluid and thickens into a bulky gel, but which does not ferment in our large intestine.

Soluble + NON-thickening/gel-forming + NON-Fermentable

These fibres are technically described as “viscous” as they do mix into and bind water, but they do not thicken into a gel like psyllium, above. They don’t slow digestion and absorbtion of nutrients in the small intestine, and therefore don’t deliver the same kinds of health benefits of the gelling/thickening fibres, like improved blood glucose regulation and lowered LDL cholesterol.

The benefit of these kinds of fibres arises from their water mixing/binding capacity. Because they are not fermented/digested in our large intestine, they keep their stool softening function all the way through your large intestine.

For this reason, these kinds of fibres are principally used as a bulk-forming laxative, increasing the water content of your stools, softening them and making them easier to pass.

The fibre supplements methylcellulose and calcium polycarbophil fall into this category.

Soluble + Thickening/Gel-Forming + Fermentable/Prebiotic

These kinds of soluble fibres thicken into bulky, gel-like substance in our stomach and small intestine which thickens the chyme in our stomach and small intestine, and slows the absorption of nutrients like glucose and fat into our bloodstream. So they are recommended for people who want to improve their glycemic control and/or lower their LDL cholesterol.

But these kinds of fibres are also “fermentable” - they serve as food for the healthy bacteria in our large intestine. Fermentable fibres are also called prebiotics. “Prebiotics” are substances that serve as food for “probiotics” - the tiny microbes that reside in our large intestine and which yield positive health benefits.

When the probiotic bacteria in our large intestine start digesting these types of fibres, theylose their thick, gel-like consistency, and as that thick gel-like consistency is lost, so is the stool normalising effect. So unfortunately, fermentable/prebiotic fibres won’t help keep you regular.

But while one benefit is lost, a LOT of others are gained. These fermentable/prebiotic fibres feed healthy strains of inner bacteria, boosting their populations which in turn helps to keep other less-healthy strains of bacteria like e.coli and other gram negative bacteria in check.

More importantly, when these healthy inner bacteria digest these fermentable/prebiotic fibres, they excrete short chain fatty acids (SCFA’s). SCFA’s are amazing substances which yield an array beneficial health impacts in our bowel and throughout our body. More on this below!

Β-glucans (in oats, barley and mushrooms - ever notice how porridge gels as you cook it or how barley thickens a soup?), pectin (in apples, citrus peels, beetroot, peaches, cherries and pears), guar gum (used as a commercial food thickener), glucomannan and xanthan gum are all soluble thickening/gelling fibres that are fermentable.

Soluble + NON-Thickening/Gel-Forming + Fermentable/Probiotic

These kinds of fibre are not very “fibrous” at all! But they are classified as fibre and have amazing health benefits so let’s just roll with it. These fibres dissolve in water but they do not thicken into a gel. They therefore don’t thicken the chyme in our stomach and small intestine, so they won’t slow down absorption of nutrients like glucose or fats into the bloodstream. They also won’t bulk your stool or bind water to it, which means they don’t provide any laxative or “stool normalising” effects in the bowel.

So, no glycemic regulation, non LDL cholesterol lowering and no laxative or stool normalising effect. But don’t write off this category of fibre yet…

The health benefits of this kind of fibre derive from the fact that they are prebiotics - food for the tiny micro-organisms that live in our large intestine. Fermentable prebiotic fibres feed healthy strains of inner bacteria, boosting their populations and helping them to keep other less-healthy strains in check. More importantly, the healthy inner bacteria that digest these fibres excrete short chain fatty acids (SCFA’s) which in turn have beneficial health impacts in our bowel and throughout our body (more on this below).

When the tiny microbes in our large intestine “ferment” (digest) these prebiotic fibres, in addition to excreting SCFA’s, gas can also be produced. This gas can cause discomfort for those who have IBS, or for those who have consumed a low fibre standard Western diet and who are not used to consuming prebiotics.

Inulin, wheat dextrin, oligosaccharides, maltodextrins, resistant starch, polydextrose, soluble corn fibre and resistant starches are all soluble fibres that are fermentable and non-viscous. Inulin is found in onions, Jerusalem artichokes and chicory root. It is also available in supplement form. Resistant starch is found in oats, legumes, green bananas and (my fave!) “carby” foods like potatoes, pasta and rice that have been cooked and then cooled in the fridge - seriously, you can read about it HERE and HERE.

To summarise the four classifications of soluble fibre, I created this table:

THE FOUR CATEGORIES OF SOLUBE FIBRE, ACCORDING TO DEGREE OF THICKENING/GELLING AND FERMENTABILITY

So the four categories of soluble fibre, plus insoluble fibre, gives us FIVE distinct categories of fibre, each with its own unique physiochemical behaviour in the digestive system, which flows on to distinct health impacts throughout the human body.

Whole Food v Fibre Supplements

We can consume fibre:

In whole-food form: in whole unprocessed grains, legumes, fruit, vegetables, nuts and seeds).
As a supplement: There are many different types of fibre supplements;
As an additive to processed and packaged foods.

I strongly recommend you consume your fibre in wholefood form wherever you can, and if you are consciously seeking to boost your fibre intake, think “food first!”. That’s because:

Most plant foods contain more than one type of fibre, and different fibres can yield synergistic health benefits (more on that below); and
Fibre-rich, whole plant foods like fruit, vegetables, whole grains, legumes, nuts and seeds come with an amazing “travelling companions” like vitamins, minerals and super-healthy phytonutrients. Science is increasingly uncovering an array of amazing health benefits - for an example, check out THIS POST on one variety of phytonutrient, the anthocyanins found in purple plant foods. One family of phytonutrients, the polyphenols, are also a preferred food source for our “good guy” gut bacteria.

Eat all of the different types of fibre!

So we now know that there are actually FIVE different classes of fibre, and that each of these five classes behaves differently in the human body, and delivers its own unique set of health benefits!

Don’t miss out on a fantastic health benefit - make sure you diet includes all of the five different types of fibre. How do you do this? Put down the processed food and enjoy a wholefood diet that is predominately made up of a wide variety of all types of plant foods - vegetables, fruit, nuts, seeds, whole grains, pulses and legumes. Go for as much variety as possible in all the colours of the rainbow.

Consuming different types of fibre together can yield synergistic benefits

For example, fermentable/prebiotic fibres like resistant starch are usually quickly fermented in the proximal colon (ie. the start of our large intestine). This means that most of the excretion of the beneficial short chain fatty acids produced by fermentation of these prebiotic fibres is concentrated there, and they aren’t necessarily distributed all the way through our large intestine.

But when we combine insoluble fibre with easily fermented probiotic fibres like resistant starch (from cooked-then-cooled carbs), the release of SCFA’s and the pH lowering effect gets spread all the way through your large intestne. This means more healthy bacteria, and greater excretion of beneficial short chain fatty acids in your distal colon (on your left side) which is where abnormal cells and tumours are more likely to be found.

***

So we know that the American Institute of Medicine recommends women consume 25g of fibre/day and men consume 38g/day. And the Australian National Health and Medical Research Council (NHMRC) recommends that women consume 25g of fibre/day and men consume 30g/day.

My bugbear is that these recommendations just refer to “fibre” - there is no acknowledgement of the five different types of fibre and their different physiochemical actions and attendant health impacts in the human body.

For example, the Australian NHMRC has made a specific fibre intake recommendation for people wishing to reduce coronary heart disease risk - it recommends 28g of fibre/day for women and 38g/day for men. But it doesn’t tell us which types of fibre will actually have this effect. And we know (thanks to my research!) that not all types of fibre will lower our LDL cholesterol or have any effect on CVD risk!

It concerns me that people might load up on the wrong types of fibre like wheat bran or inulin to try to lower their cholesterol. I think in years to come, public health fibre recommendations will become a lot more specific. But for now, at least my clients know about the different types of fibres.

Make sure you check out Part 2 in this three-part series to understand the health benefits of the different types of fibre. And Part 3 for my top tips for increasing dietary fibre intake!

* apologies for that unedifying visual!

REFERENCES

Gidley MJ, Yakubov GE. Functional categorisation of dietary fibre in foods: Beyond ‘soluble’vs ‘insoluble’. Trends in Food Science & Technology. 2019 Apr 1;86:563-8.

McRorie Jr JW. Evidence-based approach to fiber supplements and clinically meaningful health benefits, part 1: What to look for and how to recommend an effective fiber therapy. Nutrition today. 2015 Mar;50(2):82.

McRorie Jr JW. Evidence-based approach to fiber supplements and clinically meaningful health benefits, part 2: what to look for and how to recommend an effective fiber therapy. Nutrition today. 2015 Mar;50(2):90.

McRorie Jr JW, 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 Feb 1;117(2):251-64.

Makki K, Deehan EC, Walter J, Bäckhed F. The impact of dietary fiber on gut microbiota in host health and disease. Cell host & microbe. 2018 Jun 13;23(6):705-15.

Zeng H, Umar S, Rust B, Lazarova D, Bordonaro M. Secondary bile acids and short chain fatty acids in the colon: a focus on colonic microbiome, cell proliferation, inflammation, and cancer. International journal of molecular sciences. 2019 Jan;20(5):1214.

Song M, Chan AT, Sun J. Influence of the gut microbiome, diet, and environment on risk of colorectal cancer. Gastroenterology. 2020 Jan 1;158(2):322-40.

Chen JP, Chen GC, Wang XP, Qin L, Bai Y. Dietary fiber and metabolic syndrome: a meta-analysis and review of related mechanisms. Nutrients. 2018 Jan;10(1):24.

Goff HD, Repin N, Fabek H, El Khoury D, Gidley MJ. Dietary fibre for glycaemia control: Towards a mechanistic understanding. Bioactive carbohydrates and dietary fibre. 2018 Apr 1;14:39-53.

Brum J, Ramsey D, McRorie J, Bauer B, Kopecky SL. Meta-analysis of usefulness of psyllium fiber as adjuvant antilipid therapy to enhance cholesterol lowering efficacy of statins. The American journal of cardiology. 2018 Oct 1;122(7):1169-74.

Soliman GA. Dietary fiber, atherosclerosis, and cardiovascular disease. Nutrients. 2019 May;11(5):1155.

Gibb RD, McRorie Jr JW, Russell DA, Hasselblad V, D’Alessio DA. Psyllium fiber improves glycemic control proportional to loss of glycemic control: a meta-analysis of data in euglycemic subjects, patients at risk of type 2 diabetes mellitus, and patients being treated for type 2 diabetes mellitus. The American journal of clinical Nutrition. 2015 Dec 1;102(6):1604-14.

Hills RD, Pontefract BA, Mishcon HR, Black CA, Sutton SC, Theberge CR. Gut microbiome: profound implications for diet and disease. Nutrients. 2019 Jul;11(7):1613.

Lambeau KV, McRorie Jr JW. Fiber supplements and clinically proven health benefits: How to recognize and recommend an effective fiber therapy. Journal of the American Association of Nurse Practitioners. 2017 Apr;29(4):216-23.

Jovanovski E, Khayyat R, Zurbau A, Komishon A, Mazhar N, Sievenpiper JL, Mejia SB, Ho HV, Li D, Jenkins AL, Duvnjak L. Should viscous fiber supplements be considered in diabetes control? Results from a systematic review and meta-analysis of randomized controlled trials. Diabetes Care. 2019 May 1;42(5):755-66.

McRorie JW, Chey WD. Fermented fiber supplements are no better than placebo for a laxative effect. Digestive diseases and sciences. 2016 Nov 1;61(11):3140-6.

Brownlee IA. The physiological roles of dietary fibre. Food hydrocolloids. 2011 Mar 1;25(2):238-50.

Dhingra D, Michael M, Rajput H, Patil RT. Dietary fibre in foods: a review. Journal of food science and technology. 2012 Jun 1;49(3):255-66.

Threapleton DE, Greenwood DC, Evans CE, Cleghorn CL, Nykjaer C, Woodhead C, Cade JE, Gale CP, Burley VJ. Dietary fibre intake and risk of cardiovascular disease: systematic review and meta-analysis. Bmj. 2013 Dec 19;347:f6879.

Evans CE, Greenwood DC, Threapleton DE, Cleghorn CL, Nykjaer C, Woodhead CE, Gale CP, Burley VJ. Effects of dietary fibre type on blood pressure: a systematic review and meta-analysis of randomized controlled trials of healthy individuals. Journal of hypertension. 2015 May 1;33(5):897-911.

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 Dec;30(2):149-90.

Gill PA, Van Zelm MC, Muir JG, Gibson PR. Short chain fatty acids as potential therapeutic agents in human gastrointestinal and inflammatory disorders. Alimentary pharmacology & therapeutics. 2018 Jul;48(1):15-34.

O’Grady J, O’Connor EM, Shanahan F. Dietary fibre in the era of microbiome science. Alimentary Pharmacology & Therapeutics. 2019 Mar;49(5):506-15.

Lockyer S, Stanner S. Prebiotics–an added benefit of some fibre types. Nutrition Bulletin. 2019 Mar;44(1):74-91.

Aleixandre A, Miguel M. Dietary fiber and blood pressure control. Food & function. 2016;7(4):1864-71.

Ötles S, Ozgoz S. Health effects of dietary fiber. Acta scientiarum polonorum Technologia alimentaria. 2014 Jun 30;13(2):191-202.

Veronese N, Solmi M, Caruso MG, Giannelli G, Osella AR, Evangelou E, Maggi S, Fontana L, Stubbs B, Tzoulaki I. Dietary fiber and health outcomes: an umbrella review of systematic reviews and meta-analyses. The American journal of clinical nutrition. 2018 Mar 1;107(3):436-44.

Weickert MO, Pfeiffer AF. Impact of dietary fiber consumption on insulin resistance and the prevention of type 2 diabetes. The Journal of nutrition. 2018 Jan 1;148(1):7-12.

Singh J, Metrani R, Shivanagoudra SR, Jayaprakasha GK, Patil BS. Review on bile acids: Effects of the gut microbiome, interactions with dietary fiber, and alterations in the bioaccessibility of bioactive compounds. Journal of agricultural and food chemistry. 2019 Apr 10;67(33):9124-38.

Prado SB, Castro-Alves VC, Ferreira GF, Fabi JP. Ingestion of non-digestible carbohydrates from plant-source foods and decreased risk of colorectal cancer: A review on the biological effects and the mechanisms of action. Frontiers in nutrition. 2019 May 15;6:72.

Gianfredi V, Nucci D, Salvatori T, Dallagiacoma G, Fatigoni C, Moretti M, Realdon S. Rectal cancer: 20% risk reduction thanks to dietary fibre intake. Systematic review and meta-analysis. Nutrients. 2019 Jul;11(7):1579.

Evans CE. Dietary fibre and cardiovascular health: a review of current evidence and policy. Proceedings of the Nutrition Society. 2020 Feb;79(1):61-7.

Jovanovski E, Yashpal S, Komishon A, Zurbau A, Blanco Mejia S, Ho HV, Li D, Sievenpiper J, Duvnjak L, Vuksan V. Effect of psyllium (Plantago ovata) fiber on LDL cholesterol and alternative lipid targets, non-HDL cholesterol and apolipoprotein B: a systematic review and meta-analysis of randomized controlled trials. The American Journal of Clinical Nutrition. 2018 Nov 1;108(5):922-32.

Li YO, Komarek AR. Dietary fibre basics: Health, nutrition, analysis, and applications. Food quality and safety. 2017 Mar 1;1(1):47-59.

Kaczmarczyk MM, Miller MJ, Freund GG. The health benefits of dietary fiber: beyond the usual suspects of type 2 diabetes mellitus, cardiovascular disease and colon cancer. Metabolism. 2012 Aug 1;61(8):1058-66.