How fibre works, part one
The word “fibre” may have had you thinking of a thread-like substance. Dietary fibre is important, but there don’t seem to be any threads in your food. What gives?
For one, the name is a misnomer. Not all dietary fibre is fibrous, but some of it is. Dietary fibre (a term used to distinguish from fibrous fibre) is the indigestible part of food that offers no nutrition but still has a role to play in health. There are two broad classifications based on whether the fibre dissolves in water (soluble) or not (insoluble).
Cellulose is an insoluble fibre. At the molecular level, it looks like this. Each of those rings is a glucose molecule (yup, sugar), and the rings are joined by oxygen atoms (called glycosidic bonds).
You may recall this is exactly what starch is (or specifically, amylose): glucose molecules joined by oxygen. Notice, however, that every second glucose ring here is flipped over. In amylose, in contrast, they are all the same side up. This seemingly minor difference has a huge impact. Starch dissolves in water. Cellulose does not.
Glucose is a three-dimensional molecule—some of the atoms on the molecule can be spatially oriented in a different direction, affecting the molecule’s chemical properties. This is called stereoisomerism. Glucose’s two isomers are named α (alpha) and β (beta). Starch is made from α-glucose and cellulose from β-glucose. α-glucose molecules link to each other the same side up, with a natural bend to the chain that makes them a tightly packed spiral. β-glucose molecules link by flipping over every other molecule. Here’s a page explaining this in really simple language. It’s worth a few minutes of your time.
If you want to be even more pedantic, there is another type of isomer called an “optical isomer” or “enantiomer”. Two molecules may be exact mirror images of each other, a bit like your right vs left hand. They’re usually identified by D- and L- prefixes respectively. They’re chemically incompatible—again a bit like putting your right foot in your left shoe. D-glucose is the common type, but L-glucose has the same sweetness and was once proposed as a replacement sweetener because it’s not an energy source inside your body (it’s also why it’s almost nowhere to be found in nature). Starch (amylose) is made from α-D-glucose and cellulose from β-D-glucose. (This is still not sufficiently pedantic, there are even more details.)
Strands of cellulose form hydrogen bonds, binding together into fibre. These bonds are tight enough to prevent water molecules passing through, which is why cellulose is excellent material for cell walls (and wood).
Cotton is 90% cellulose and one of the purest sources in nature. Your cotton t-shirt is essentially made of sugar. Cellulose is also the primary component of cellophane, the plastic film that almost everything comes wrapped in.
Plants make both starch, for energy storage, and cellulose, for structural material. Our digestive systems cannot digest cellulose. Grass is mostly cellulose. Some mammals (cows, sheep, etc) have special gut bacteria (missing in humans) that help break down cellulose into β-glucose.
In the human body, cellulose behaves a bit like hair clogging a drain, clumping up with dietary waste (“bulking”) to deliver a solid motion at the far end of the digestive tract. Along the way it facilitates interactions with your own gut flora, the details of which merit a separate post.
Chitin is a fibre similar to cellulose but made of N-acetylglucosamine, a derivative of glucose, which makes it stronger than cellulose. It’s found in the cell walls of fungi and in the exoskeletons (“skin”) of insects. Chitin is part of the dietary fibre in mushrooms.
Dietary fibres have three primary mechanisms by which they aid human health: bulking (like cellulose and chitin), viscosity, and fermentation. We’ll cover the other two in subsequent posts. A good diet requires a good balance of all three types. The standard nutrition facts label doesn’t provide this break-up, so you’ll need other sources of information.
Kilter is HasGeek’s humble attempt to provide a space for reasoned debate on how your body actually works, and how you can find your own path to good health via better nutrition, fitness and habits.
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