(Eric Hunt/Wikimedia Commons)

When I was a kid and I would hear an adult talk about eating more fiber, I would get this mental image of someone cutting up twine and sprinkling it on their food.  Being a little older and wiser, I now know that I wasn’t imaging the right kind of fiber. But what, biologically, is dietary fiber?

As you eat a piece of celery, there are parts of that celery that you’re body can digest and part that it can’t. By and large, that indigestible part is the dietary fiber — what Google calls the “dietary material containing substances such as cellulose, lignin, and pectin, which are resistant to the action of digestive enzymes.”

Whoa. Okay, just as we break down the celery, lets break down that sentence.

Sugar chains

Fiber, or roughage, is just carbohydrates, chemical compounds composed of nothing but carbon, hydrogen, and oxygen. But unlike carbohydrates in high-fructose corn syrup or the sugar you put in your coffee — which are considered simple carbohydrates — the carbs that make up fiber (like the cellulose, lignin, and pectin noted above) are complex carbohydrates.

Here’s where we get all chemical. Just as fats are linked chains or polymers of fatty acids, carbohydrates are linked chains of carbohydrate molecules called saccharides (or, colloquially, sugars). A saccharide is basically a molecule containing a carbon atom attached to 2 hydrogens and an oxygen. With a few exceptions, that hydrogen:oxygen ratio — 2:1 — holds true for all carbs.

Simple carbs are either:

• monosaccharides: carbs consisting of only a single saccharide unit, like glucose or fructose, or
• disaccharides: carbs consisting of two linked saccharides, like sucrose (which consists of a glucose chemically bonded to a fructose),

whereas complex carbohydrates are:

• oligosaccharides, which have up to 10 linked saccharides, or
• polysaccharides, the really big guys made up of more than 10 linked saccharides.

Ball-and-stick model of cellulose. (Ben Mills/Wikimedia Commons)

We’ll put the simple carbohydrates aside now and focus on the complex ones that aren’t starches (which, being digestible, don’t come under the umbrella of fiber compounds). We’re talking about compounds or families of compounds like:

• Beta-glucans: Polysaccharides made up of glucose subunits and bound together by what chemists call beta-glycosidic bonds. Family includes cellulose (which makes up the walls of plant cells) and chitin (found in fungi — and in insects’ exoskeletons).
• Hemicelluloses: Usually found in plant cell walls alongside cellullose, hemicelluloses contain a mix of different saccharides (unlike cellulose, which only contains glucose).
• Lignin: Also found in plant cell walls (and in trees), lignin is an incredibly complex mix of three subunits called monolingols: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. (Got all that?)
• Pectins: Polysaccharides made up of a sugar called galacturonic acid plus one of a number of other sugars such as xylose, apiose, or galactose.
• Fructans: Complex carbs made of a linked chains of fructose molecules.
• Raffinoses: Trisaccharides — molecules made of three linked saccharides — made from galactose, fructose, and glucose bound together.

The structure of lignin. Now that’s what I call a complex carbohydrate. (Karol007/Wikimedia Commons)

To dissolve or not to dissolve

Taken as a whole, all of the complex carbohydrates that go into fiber can be divvied up into two groups:

• Soluble: Those that dissolve in water. In the gut, they make a kind of gel-like substance that slows the digestive process. They also ferment, releasing gases and produce byproducts that perform other physiological functions.
• Insoluble: Those that don’t dissolve in water. Instead, they absorb water and help, um, move things through. Some of the insoluble fiber compounds also ferment.

I’m not going to get into the whole discussion of the benefits of fiber or where to get the most fiber; other places have that covered.

I haven’t delved quite as deep into carbohydrate chemistry as I did for fats, but the -ins, -oses, and -ans in carbohydrate names have just as much meaning as lipo-s and omegas of fatty acids. Something for a future post, perhaps?