We only became interested in the role of dietary fibre in humans towards the end of the nineteenth century. Building on the work of other scientists, a surgeon called Denis Burkitt proposed what is now known as ‘The Dietary Fibre Hypothesis’ in 1972. (1) At the time, this somewhat radical view identified that diets low in fibre increased the risk of many degenerative disorders such as cardiovascular disease, obesity, diabetes, cancer and bowel conditions, to name a few. So, public interest in fibre was put firmly on the map and the food industry went on an all-out fibre assault. Cereals were an obvious win for food manufacturers and subsequently ‘high fibre’ claims were established.
Fast forward to now and it’s not in dispute that fibre is beneficial, in fact research into fibre and digestive health particularly has evolved considerably (more on that later). There is a big market for products rich in fibre and a trend to find new sources. But what exactly is ‘high fibre’, where should we get it from and what does it do for us? I’ve written the fibre file to hopefully arm you with the facts, so that you can make an informed decision on your food choices.
WHAT IS FIBRE?
Its definition has been subject to a great deal of debate over the years. The most recent definition seems to be globally agreed as: ‘dietary ﬁbre is made up of carbohydrate polymers with three or more monomeric units (MU), which are neither digested nor absorbed in the human intestine’. (2)
In simple terms, fibre is the indigestible portion of food derived from plants. It is often described as soluble fibre (e.g. in oats and fruit) and insoluble fibre (e.g. in wholegrains and nuts), simply meaning that can or can’t be dissolved in water. Insoluble fiber is found in the cell walls of plants and is beneficial as it adds bulk to stools to assist in moving them out of the digestive tract. Soluble fiber on the other hand dissolves in water and helps to slow the passage of food through the digestive tract, lower blood sugar levels, and reduce cholesterol.
For the science brains amongst you, dietary fibre is technically a non-digestible polysaccharide. If you read my previous article on sugar ‘The Sweet Truth’, you may remember the diagram below. (3) Within the polysaccharide category, there are some types that are not digested or absorbed in the stomach or small intestine and therefore can’t be broken down into single sugar molecules. Cellulose is just one example, however there are now around 16 different types, as identified by a 2017 review of dietary fibre in Europe. (2) It’s way too confusing to talk about them all, however the following table gives a good summary of some of the main types along with their sources and health benefits. (4) Note that the fibre used to give TROO granola its sweet taste is inulin, derived from the chicory root and has benefits for both gut and immune function.
SO, WHY DO WE NEED IT?
We know that fibre is not digested or absorbed in the stomach and small intestine, so how is it useful? It’s probably best known for its role to alleviate constipation; however, we now know that it has many more important functions. These range from the reduction of glucose and cholesterol absorption in the small intestine, providing bulk for our stools (which minimises toxicity and aids hormone balance), influencing metabolic activity in the gut and the production of energy resources for the body. As a result, clinical studies suggest fibre exerts a wide range of benefits in areas such as bowel function, immunity, blood glucose control, cholesterol levels and gut health and hormone balance. (5)
We know that dietary fibre decreases the risk of bowel cancer, likely because of the effect it has in bulking our stools and moving waste through the colon quickly. As our stools contain carcinogens and toxins, we really don’t want them hanging around in there for too long! Did you know that up to 80% of our immune system is in the gut? This is because the digestive tract is lined with active immune tissue. The immune enhancing effect of dietary fibre and the link between the gut and our immune systems is an exciting and growing area of research.
In terms of regulating blood sugar levels, foods which contain fibre take longer to digest and therefore slow the absorption of glucose, in particular the soluble mucilage and beta glucan varieties from the previous table found in oats, nuts, seeds and beans. The body does not produce insulin in response to fibre, so inclusion of adequate levels can lead to lower blood glucose and decrease subsequent risk of diabetes and cardiovascular disease. Soluble fibre has also been shown to lower LDL blood cholesterol (the bad type) in a number of different ways. An all-round win for heart health.
And as if those reasons weren’t enough, eating fibre can also help with weight loss. Blood sugar balance is key this, however fibre also fills you up as it absorbs liquid in your stomach and this fullness stimulates receptors to tell your brain to stop eating. Fibre helps to eliminate excess hormones in our stools which can also aid weight loss.
FIBRE AND GUT HEALTH
The emerging research in this area is fascinating. A recent research paper identified 188 relevant studies in the area of the human gut ‘microbiota’ (or gut bacteria) and the consumption of dietary fibre. (6) It seems that the amount and type of fibre consumed can have a huge effect on the number of beneficial bacteria in our gut. We used to think that fibre did not provide any energy to the body, however it is now known that some types can be fermented in the large intestine by our gut bacteria. This process feeds the good bacteria and helps them to grow.
It also produces short chain fatty acids (SCFAs) which can be absorbed into the blood stream and used as energy, as well as providing food for the cells of the colon. In the last few decades, it became apparent that SCFAs may play a key role in the prevention and treatment of metabolic syndrome, bowel disorders, and certain types of cancer. (7) When bacteria in the lower intestine break down fiber, a substance called butyrate is produced which may inhibit the growth of tumors of the colon and rectum (8)
This fermentation, leading to beneficial changes in gut bacteria comes from fibre termed as ‘prebiotic’. Prebiotics are defined as “a selectively fermented ingredient that allows specific changes, both in the composition and/or activity of the gastrointestinal microbiota that confers benefits.” (9) Prebiotics fall into the oligosaccharide and inulin category and sources include raw leeks, onions (cooked or raw), green bananas, chicory root, raw asparagus, raw garlic, jerusalem artichoke and dandelion greens. (10)
Before you all start loading up on the foods above, there should be a word of caution for anyone suffering from digestive issues such as IBS. There has been research to suggest that these fermentable fibre sources can aggravate the symptoms of IBS such as bloating, discomfort and flatulence. (11) It is not the fibre itself that is causing the issue, but rather the fermentation process, as sufferers from IBS usually have abnormal bacterial balance and therefore undesirable, or ‘bad’ bacteria may be feeding from the fibre. (12)
A specific dietary approach has been identified in helping symptoms of those with IBS, called FODMAP, which stands for Fermentable Oligosaccharides, Disaccharides, Monosaccharides and Polyols (bit of a mouthful hence the need for an acronym!) It relates to the dietary restriction of poorly digestible carbohydrates that are easily fermented in the gut and includes some of the fibre sources we have been discussing. Although this approach can be successful in relieving symptoms, it is important to note that it may not be addressing the underlying causes of IBS. The hypersensitivity of the gut in IBS sufferers is something that should firstly be assessed by a trained nutritional professional, as this dietary regime is restrictive and excludes many beneficial foods. (13)
HOW MUCH IS ENOUGH?
So, hopefully you now have a better understanding of fibre, how much do we need to eat? In 2015 the Government announced new guidelines, following the SACN Report on Carbohydrates and Health stating that intake should be as follows: (14)
|Age (years)||Recommended intake of fibre|
|2-5||15g per day|
|5-11||20g per day|
|11-16||25g per day|
|17 and over||30g per day|
The new level represents an increase of 7g per day for adults and the report suggests that to meet this, it would be necessary to consume around 3-5 servings of fibrous vegetables (for example peas, broccoli and carrots), 2-4 servings of fruit (for example an apple with skin, a portion of strawberries and an orange) plus 3 servings of wholegrain bread/pasta/beans/pulses (for example, 2 slices of wholemeal bread, lentils and wholemeal spaghetti). (14) The link between higher consumption of dietary fibre and a reduced incidence of the health issues already discussed was the driver for this change.
It is estimated that the current adult intake in the UK is 12g below the new recommendation, at only 18g of fibre daily. The sources are estimated to come from cereals (39%), vegetables and potatoes (31%), fruit (9%) and meat products (11%). (15) I have to say that I found it shocking that meat products, which contain little, to no fibre showed a higher consumption percentage than fruit! One can only assume that this figure is driven by processed meats which have fibre added for both technological reasons and for benefits to health. (16)
For a manufacturer to claim that a food is high in fibre, the product must contain at least 6g of fibre per 100g or at least 3g of fibre per 100 kcal. (17) But is 6g per 100g really a ‘high’ level? I would urge you to check out any packaged goods you buy to make the comparisons. Did you know that two of the leading ‘healthy’ granola brands on the market contain only 6.9g and 10.6g of fibre per 100g, compared to TROO granola at a massive 16.7g-18.3g, depending on which variety you choose.
WHAT SHOULD I EAT?
There’s a sensible approach to fibre intake in my opinion, in fact, it’s an approach to healthy eating more than anything. Think about food sources being the least processed and as close to the ‘whole’ food as possible. The advice to consume 8-10 portions of fruit and veg (with only 2-3 as fruit) is a good one. I prefer blending to juicing, as the fibre is retained and soup with added vegetables and grains is another way to boost fibre levels. Vegetables as crudités make a quick high fibre snack option too.
In terms of grains, stick with wholemeal varieties and steer clear of white refined foods. Be experimental with the wide range of fibre rich grains on the market that are also high in protein, vitamins and minerals, such as quinoa, buckwheat, millet and amaranth. Include some of the fermentable fibre sources that are known to be beneficial for gut health (assuming that you do not have any digestive issues). Eat your bananas green, or chop and freeze them green to add to smoothies.
Be mindful of being seduced by ‘high’ fibre claims when there are alternatives with higher levels. If these foods are also high in sugar, then it’s a no-no. Cereal brands and snack bars are the worst offenders here. For example, a popular brand of cereal bar that I looked at, had high fibre (at 7.7g) but included a whopping 32.5g of sugar per 100g! There are countless cereals on the market that have incredibly high levels of sugar, so always check the pack before you buy. Oats and oat bran are heart healthy fibre rich options if you like to alternate your TROO granola with porridge or overnight oats.
Do include legumes – that’s beans and pulses, which are high in fibre and don’t forget that nuts and seeds make a great snack with added fibre too. Flax or linseed which you can buy milled as a powder is really versatile for adding to baking and cereals and has 27g of fibre per 100g. Chia seeds have become popular and can be included in lots of different recipes too and have 34g of fibre per 100g.
Eat as wide a range of food from as many different good sources as you can. And remember that not all fibre is created equal, so a couple of slices of toast each day isn’t going to suffice!
- Cummings, JH & Engineer, A: Denis Burkitt and the origins of the dietary fibre hypothesis. Nutr Res Rev. 2017;6:1-15
- Stephen, A et al: Dietary fibre in Europe: current state of knowledge on definitions, sources, recommendations, intakes and relationships to health. Nutr Res Rev. 2017; 30 (2) 149-190
- Klosterbuer A, Roughead ZF, Slavin J: Benefits of dietary fiber in clinical nutrition. Nutr Clin Pract 2011;26:625-635
- Caleigh M. Sawicki, et al: Dietary Fiber and the Human Gut Microbiota: Application of Evidence Mapping Methodology. Nutrients. 2017 Feb; 9(2): 125.
- Besten, G et al: The role of short-chain fatty acids in the interplay between diet, gut microbiota, and host energy metabolism. J Lipid Res. 2013;54(9): 2325–2340.
- Avivi-Green C, Polak-Charcon S, Madar Z, et al. Apoptosis cascade proteins are regulated in vivo by high intracolonic butyrate concentration: correlation with colon cancer inhibition. Oncol Res. 2000;12:83-95
- Gibson GR, et al: Dietary prebiotics: current status and new definition. Food Sci Technol Bull Funct Foods 2010; 7:1-19;
- Bijkerk CJ1, Muris JW, Knottnerus JA, Hoes AW, de Wit NJ. Systematic review: the role of different types of fibre in the treatment of irritable bowel syndrome. Aliment Pharmacol Ther. 2004 Feb 1;19(3):245-51.
- King TS1, Elia M, Hunter JO. Abnormal colonic fermentation in irritable bowel syndrome. Lancet. 1998 Oct 10;352(9135):1187-9.
- Brown, B. 2015. The Digestive Health Solution, edn 2. Exisle Publishing, Australia.
- Arun Kumar Verma and Rituparna Banerjee. Dietary fibre as functional ingredient in meat products: a novel approach for healthy living — a review. J Food Sci Technol. 2010 Jun; 47(3): 247–257.