Fibre Optics Part 4

Plant cells before and after they are milled. Nutrients are contained in the cells on the right. After milling, nutrients are exposed and easier to digest.

Tearing Down the Walls

Most of the foods that we classify as fibre come from plant cell walls, but as we have already seen in Parts 1-3, that covers a wide variety of substances. Cell walls are complex structures with many different components, all of which have a range of properties and compositions. They differ widely between plant species, but also show huge variations within a single plant, depending on position and function. All of this can have a large impact on how these foods act in our bodies.

Type 1 or Type 2?

(Now, these next few paragraphs have a bit of complex terminology, but bear with me, it has some important implications, so is worth knowing)

Chickpeas and other pulses have a cell wall structure known as type 1, which they broadly share with other dicotyledonous plants, including most commonly eaten vegetables. Type 1 cell walls are rich in pectic polysaccharides and xyloglucans, which are non-starch polysaccharides (NSPs) with a range of gelling properties. Pectic Polysaccharides, better known as pectins, are found in large quantities in many fruits and are commonly extracted to be used as a gelling agent in jams, jellies, and desserts. Xyloglucans are also gel forming and thought to be important in the development of cell wall structures and digestive resistance.

Wheat and other monocotyledonous plants, a category which includes most grain crops, have Type 2 Cell wall structures, which are low in pectins, but tend to be rich in arabinoxylans and beta-glucans. Beta-glucans are gel forming polysaccharides with a range of interesting health properties that we shall discuss in more depth in Part 5. Arabinoxylans also form strong gels and have a range of important structural and functional properties within plant cells.

These differences appear to be important nutritionally. Laboratory modelling of human digestive processes indicates that Type 1 and Type 2 cells act very differently in the human gut. Although all intact cells are somewhat resistant to digestion, limiting access to the starches, fats and other nutrients contained within the cells, the level of accessibility varies considerably, largely depending on the type of cell wall present.

Many factors influence how resistant cell walls are to digestion, including thickness, composition and the size and number of pores, but in general, intact type 2 cells are tougher and more likely to make it through to the lower intestine intact. Type 1 cells are less resistant, but can also survive through the gut, delivering useful nutrients to the microbiome. But even when they don’t reach that far, their intact cellular structures mean that they release glucose molecules from their starch a lot more slowly than cells where the walls have been disrupted before consumption, by milling or other physical processes.

It's All About the Process

Eat more beans.

When it comes to how foods are digested, the level and type of processing is perhaps even more important than the cell type. When a grain or pulse is milled into a fine flour using a conventional milling process, most of the cell walls are ruptured, releasing the starch, so allowing it to be easily digested in the gut (see the picture at the beginning of this post). Generally speaking, the less disruption to cell walls before they are eaten, the more resistant they are to digestion, and the slower they release glucose. This means that foods with intact cell walls create a lower glycaemic response in the body, which is potentially beneficial to health.

A broad rule of thumb might be that if you want to limit glycaemic response and possibly even reduce calorie consumption, the less you process foods the better. Whole foods tend to have intact cell walls, milled flours have broken cells and easily accessible starch. There is a lot of talk in the media at the moment about whether processing of foods impacts on health independently of nutritional composition, and here is an example of a potential mechanism explaining why that could be the case. And because there are some important differences in how foods react to processing depending on the type of cell walls they contain, this could potentially provide a route to making processed foods far healthier (this is why that first slightly complex bit was important).

When you make a flour out of dried chickpeas, the cell walls get broken up, leaving the starch easily digested. But because type 1 cells are largely held together by highly soluble pectins, if they undergo a hydrothermal milling process, the individual cells become easily separated, as the pectins get broken down and dissolve. With the right sort of processing, when a plant has type 1 cell walls, it is possible to make a fine flour that still has largely intact cells, and so a high level of resistant starch. Although nutritionally identical according to nutrition labels and food composition tables, when the chickpea cells are left intact, they impact very differently on our bodies than a conventionally milled flour made out of the same ingredient.

When the same hydrothermal process is applied to type 2 cell walls, for instance from wheat or barley, because the cells are not held together by soluble pectins, they do not separate, and so the cell walls are readily ruptured, exposing the starch in the same way as conventional milling. Although some small structures might remain in any flour made from plants containing Type 2 cell walls, the starch will largely be exposed, and so will get digested in the normal way as a conventionally milled flour. It is important to remember that this is not always a bad thing. Easily digestible carbohydrates are an important source of energy, the greatest source of calories worldwide, and many consumers need easily accessible nutrients. Exposed starch is also a highly functional food ingredient, allowing us to make breads, thicken sauces and craft a wide range of delicious foods. But if you want to manage blood glucose responses, provide food for the microbiome, and deliver a range of associated health benefits, eating foods where the cells are less disrupted is probably a sensible move.

Keeping It All Together

A typical human gut being attacked by some giant biochemistry. Improved gut health would probably help resist this sort of attack.

Human studies have provided evidence that plant cells from beans and legumes survive chewing and digestion, remaining intact in the human gut. But the human gut is largely a black box, and exactly how cells change during these early stages of digestion is still not well known. Even less is understood about how cell walls eventually get broken down by the microbiome, although it is thought that most of the structures are eventually digested in some way, providing a range of valuable nutrients to the lower reaches of the gut.

Cell walls of either type are highly complex, with structures that we do not yet fully understand. They contain a range of components, and exactly how these components interact is not completely clear, especially as they move through human digestion. Almost all of the fibre we consume originates within plant cell walls, and these complex structures often encapsulate other nutrients, including fat, protein, and carbohydrate. They also contain a selection of intrinsic micronutrients such as polyphenols, phytosterols and phytoestrogens, which will reach the lower gut if they are sufficiently bound up in cell wall structures. To what extent the known gut health benefits of fibre are due to these components is certainly an area that needs far more research. But if there are benefits from allowing these bound up nutrients to reach the lower reaches of the gut, those benefits will almost certainly be lost if the fibre is highly processed. Right now, however, if you simply look at a nutrition label to determine how much fibre something contains, there is no way of telling how processed that fibre is, or to what extent the cell walls are intact.

The way that we traditionally measure how healthy a food is leans heavily on its chemical composition, but with fibre, it increasingly seems that the complex macrostructures that it is contained within might have a considerable influence on how our body interacts with it. The truth is that even in 2023, with all the millions spent on nutrition, gut health and obesity research over the years, our understanding of the importance of these structures is still in its infancy.

What Do We Know?

A picture of heart on a stick, as I wanted to represent cardiovascular benefits and can only afford free stock images.

Evidence is building. The sort of hydro-thermically milled legume flours with intact cells described earlier produce a significantly lower glycaemic response than conventionally milled equivalents when cooked up in similar processed foods, with a degree of difference that would usually result in a large reduction in cardiometabolic disease risk. There is also experimental evidence that suggests legume flours with intact cells stimulate satiety hormones more effectively, perhaps providing a mechanism for why some highly processed foods can promote higher calorie consumption. And there is widely thought to be a benefit of delivering bound up nutrients to the microbiome, although this area still requires a considerable amount of research before it is well understood.

It is certainly the case that not all cell walls are equal, even within similar foods. Chickpeas and haricot beans both have type 1 cell walls, but behave very differently under processing and digestion. There is probably a need for more than two cell wall categories, and certainly a requirement for a deeper understanding of how different types of cells are digested once they are consumed.

Perhaps though, even before we fully understand what is going on, we should start to distinguish between intrinsic sources of fibre and purified, extracted and processed forms, to account for the clear impact of cell wall structures. Although, as we discovered in Part 3, testing for fibre is difficult enough, there is certainly a case for some consumer labelling visibility of intrinsic vs extracted fibre, along with communication that intrinsic is the preferred option. Surely that would help people navigate this complex world and make more informed decisions.

And Now, The Nuance

Perhaps. Certainly, if you want to modify glycaemic response, eating foods with plenty of intact plant cells is probably a good idea, and these new flours could make a difference. In truth however, the notion that foods with added processed fibre are somehow worse than foods with intrinsic fibre is a difficult one to pin down. Instinct would say that this should be the case, but once again, we are probably over-simplifying a complex reality. Although intact cells are almost certainly good for a range of reasons, some soluble fibres might also have an important role to play, and will do that more effectively if they are freed from their cells.

Our health associations with fibre are largely connected to bulking stools or feeding friendly gut bacteria, but there are a whole range of other benefits that are also vital to consider. Some NSPs have benefits linked to their potential to form gels as they pass through the gut. Perhaps the most important of these are known as beta-glucans, which arguably benefit from being freed as much as possible from their cellular structures, somewhat countering the ‘intrinsic is better’ narrative. And importantly, when we look more closely at what these gel forming fibres can do, as we shall in Part 5, they are probably as close to a miracle food as it is possible to be.