Fibre Optics Part 2
My Definition Is This…
What exactly is fibre? The official UK government definition catchily describes fibre as ‘All carbohydrates that are naturally integrated components of food that are neither absorbed nor digested in the small intestine and have a degree of polymerisation of three or more monomeric units plus lignin’. This definition was extended in 2015 to include non-digestible oligosaccharides, resistant starches and polydextrose.
What this means in practice is that the term fibre pretty much covers any components of our food that manage to get through our mouths, stomach, and small intestine without being digested and absorbed, so reaching our lower intestine intact. Mostly, this comprises of stuff contained within plant cell walls, but given the broadness of the definition, it is hardly surprising that it encompasses many different types of substance. Plant cell walls are complex structures that are not well understood, with a variety of different components, and huge variations between plant species.
Unlike any other nutrient, fibre is defined more by its function than by strict chemical composition or structure. There are several reasons why something might pass through to the large intestine undigested, meaning there are many different types of fibre, something that is never reflected in nutrition labelling and guidelines, and rarely covered in dietary surveys or research.
To make some sense of this complexity, we broadly classify fibre into four different types –
1. Non-Starch Polysaccharides
2. Resistant Oligosaccharides
3. Resistant Starch
4. Lignin
Non-Starch Polysaccharides include substances such as cellulose, pectin, glucans, gums, inulin and chitin. As a brief reminder for anyone like me who struggles to remember school chemistry and biology lessons, the term polysaccharide refers to the substances formed when monosaccharides (sugar molecules such as glucose or fructose) are stuck together in long molecular chains. Plants use polysaccharides as stores of energy, but also as structural components to build cell walls and plant bodies.
Starch is a particular type of polysaccharide formed by chains of glucose molecules stuck together in a certain way and is largely used by plants used to store energy. It has a regular crystalline structure, formed when long chains of glucose molecules are joined with a type of connection known as an alpha bond. There are two main types of starch, one called amylose, formed largely from straight chains, and one with more branched chains known as amylopectin. Both types of starch are easily digested by humans because the regular polysaccharide chains are readily broken into individual glucose molecules by our digestive enzymes, and those glucose molecules are then absorbed through the small intestine. All across the world, most of the calories that humans consume come from starch, derived from crops such as wheat, rice, potatoes, corn and casava.
Non-Starch Polysaccharides (NSPs) refer to other types of long sugar chains, made with different bonds between the sugar molecules, and often composed of sugars units other than glucose. NSPs cover a broad range of substances, from the highly insoluble cellulose chains that form structural components in many plants, to a wide range of soluble gums that form strong gels with water. Most of the fibre humans consume is composed of NSPs.
The second type of fibre molecules are known as Resistant Oligosaccharides, which are comprised of smaller chains of around 8-10 sugar units. These substances are still resistant to digestion by enzymes in the stomach and small intestine, but are quite readily fermented in the large intestine by our microbiome. Their small chains mean that are generally quite soluble in water and tend not to form strong gels. Resistant Oligosaccharides are found in legumes, artichokes, and mushrooms, but also in human milk, acting as a vital food for the infant microbiome long before the first plant foods are consumed. The distinction between resistant oligosaccharides and NSPs is a slightly arbitrary one, with some short chain inulin crossing the boundary between the two.
When it comes to the definition of fibre there is even more confusion for polysaccharide chains smaller than 10 sugar units. Codex, who produce the UN’s internationally adopted food standards, state that chains smaller that 10 units should not be classified as fibre, but does qualify this by suggesting that decisions on classification for polysaccharides with between 3 and 9 units should be left to national authorities. In the US for instance, the FDA allows food companies to classify these smaller chains substances as fibre, and there is little consensus around the world (attentive readers will have noticed that the UK definition mentioned earlier also includes these shorter chains). This may seem to be a pointless debate of semantics, but the lack of a clear definition can be an issue when it comes to nutrition research, making it difficult to compare figures for intake, and hard to make clear, comparable recommendations for consumption.
There is an equal amount of debate and uncertainty surrounding the third category of fibre, known as Resistant Starch. This term describes any starch that is not digested in the stomach and small intestine, so reaching the large intestine and providing easily fermented food for the microbiome. The reasons why starch might be resistant to digestion vary. Some is physically inaccessible, held within structures such as intact plant cells. Some forms into complex molecular structures that enzymes find hard to digest, such as the starches found in bananas, high amylose corn, or pasta that has been cooked and cooled.
There is debate about the definition of resistant starch because the level of digestive resistance varies considerably. For instance, the level of ripeness of bananas, or the way that foods have been processed can make a huge difference to how resistant the starch is to digestion. But these substances do appear to provide a variety of interesting health benefits, contributing to a healthy microbiome and perhaps even making us feel fuller for longer. Again, it is highly unlikely that all types of resistant starch act in exactly the same way, with intact chickpea cells likely to behave very differently in our guts to bananas or cooled pasta.
Cynically perhaps, it could be suggested that the food industry has an interest in more and more stuff being included under the definition of fibre to help them make health claims on products, which perhaps explains why resistant starches and smaller chain oligosaccharides have been added into the definition in recent years. The alternative however is for these substances to be lumped under the heading of carbohydrates, which would provide little incentive to increase the amount of them in manufactured products and would not reflect our understanding of how differently they act in the body. It is increasingly clear that resistant starches and shorter chain oligosaccharides can be of great benefit to health, particularly when they reach the large intestine and provide food for the microbiome.
The fourth category of fibre is Lignin, which is the second most abundant substance in nature (cellulose being the first) and is largely contained within plant cell walls. It is the structural component of most woody plants, and as such is extremely insoluble and resistant to digestion. Lignin is not a carbohydrate, being made from phenolic compounds, and although it remains largely undigested, even after passing through the human colon, because of its known positive impact on digestion and faecal bulking, it is still considered a dietary fibre.
Despite the complexity of the definitions, there is a very high level of consensus around the world that consuming fibre is beneficial to health, and also an awareness that very few people actually eat enough, particularly in developed countries with a high consumption of processed foods. There is reasonable agreement that most of us should be eating 25-30g a day of total fibre, but in the UK, less than 1 in 10 people achieve that target.
Because of a wealth of evidence that fibre is good for us, there have been Government recommendations to consume more of it in the UK since the 1970s. But increasingly it seems that whilst recommending more is no bad thing, the types of fibre consumed may actually be of greater importance that the headline amount. It is relatively easy to consume 10g of inulin in a drink without really noticing, and whilst that may be useful for a number of reasons, it is unlikely to have the same physiological impact as 10g of insoluble cellulose, 10g of a gel forming pectin, 10g of lignin, or 10g of the resistant starch found in whole chickpeas. Is that good or bad? Well, that largely depends on what outcome you are interested in, but for the vast majority of people, it would almost certainly be more beneficial to concentrate on getting a variety of different types of fibre, rather than focusing on just one.
Sadly, nuance is the enemy of marketing, and it is extremely difficult to persuade consumers to behave in a nuanced way. This is perhaps why public health drives over the past 40 years or so have tended to focus on less nuanced nutrition advice like cutting down on sugar, calories or fat, with fibre being left behind as the unfashionable healthy choice. The 2016 SACN Report on Carbohydrates produced for the UK Government contained a wealth of information on the considerable health benefits of eating more fibre and recommended raising the guideline daily amounts. But when it came to converting recommendations from the report into policy, the focus largely fell on the far shorter and less well-evidenced section on reducing sugar consumption, which resulted in sugar taxes and industry wide initiatives to reformulate. Fibre, once again, was left behind.
Even if there was enough will, providing the public with advice on how to consume different types of fibre would admittedly be difficult. There is very little information available on the quantities of different types of fibre people currently consume, and food composition tables rarely provide any detailed analysis of the levels of fibre types contained within common foodstuffs. Even basic details about the solubility of fibre are rarely available, let alone information about cell structure, sugar type, bonds, molecular weight, or gel formation. Researchers can test food stuffs to produce their own information, but this is expensive and difficult, meaning that very little work gets done in this area.
What is certain is that a range of different fibres will have a range of different effects. As we shall explore, the impact of fibre is way more than just bulking out stools. The impact is also a lot wider than just feeding our microbiome, important though that may be. The complex gelling and water binding activities of many types of fibre have important impacts on how food is absorbed, and in many cases, this may well create some of the most exciting health benefits of all.
The term dietary fibre refers to a vastly complex set of plant materials, with impacts that will never be possible to fully communicate on food packaging. If we simply add an arbitrary amount of a particular fibre to meet a nutrition target, then we may help make food a little healthier. But if we can regularly introduce a host of different fibre containing ingredients into formulations, then we stand a chance of making real change, transforming health outcomes, and vastly improving the dietary quality of processed foods.
To do this effectively, we need to fully understand fibre in all its forms. We also need to find more ways of incorporating it into delicious, compelling food products, creating a host of new fibre containing ingredients and recipes. This work is not easy and is not something that consumers or regulators are currently demanding. But it is important, and if we want a food system that works for everyone, it is a challenge the industry should start to face.