Fiber is arguably our most important nutrient. (Except, by strict definitions, it isn’t actually a “nutrient.”) So, does it really prevent cancer and heart attacks? Help us lose weight? Keep us from feeling tired? Detox the liver? Make the world a better place? And what about soluble fiber vs. insoluble fiber, etc.?

Bear with me: this is going to be a multi-part article! But trust me: it’ll be worth it. Once you understand fiber, you’ll understand one of the big concepts about diet and nutrition.

Okay, before we begin to talk about fiber, we need to talk about sugars and starches first.

Most of us have an intuitive grasp of the difference between the two. Sugars are sweet, like fruit juice and honey.  Starches, on the other hand, are, well, starchy, like rice and pasta and potatoes. It turns out, however, that sugars and starches are both carbohydrates, which is to say, they’re made of the same basic stuff.

So, how, exactly, are they different? (And what does this have to do with fiber?) Let’s look at a little chemistry.

Sugars are small molecules. Starches, on the other hand, are larger molecules, made up of lots of sugars attached to each other.

As small, simple molecules, sugars are easy for the body to absorb and process. (Take in a few tablespoons of sugar or honey, and you’ll feel the spike in energy quickly enough!) Starches, on the other hand, are too big to absorb intact. Before we can absorb them, then, they need to get broken down (digested) into their component sugars. So when people say that starches turn to sugars in the body, it’s very true. A bowl of pasta won’t give you the surging sugar rush that a cup of sugar or honey will. But wait a half-hour: it’ll get you to the same place.

(For a home science experiment, chew up a piece of bread, then hold it in your mouth for 5-10 minutes. It’ll start tasting sweet. That’s how quickly the enzymes in our saliva break down starches into their component sugars).

So those are sugars and starches. Then what about fibers? Fibers are also carbohydrates. In fact, like starches, they’re also sugar polymers. The difference is, in fibers, the sugars are assembled in a such a way that our digestive enzymes can’t disassemble (digest) them easily, if at all. Fibers are indigestible starches. They seem a little starchy in our mouths (some special ones actually taste a little sugary), but they provide very few useable calories.

Obviously, this is a drawback to anyone living on the verge of starvation, unable to extract the calories from grass, or a tree. But for the rest of us, having a significant part of the diet “go in one end, and out the other” is of profound benefit.

The benefits of fiber include:

• Keeping us regular.
• Reducing the risk of heart disease.
• Reducing the risk of cancer. (Admittedly, the research here is mixed).
• Reducing the risk of type II diabetes
• Controlling diabetes and general blood sugar fluctuations
• Reducing stress and maintaining adrenal health
• Helping clear eruptive skin conditions, especially acne.
• Supporting detoxification and liver health.
• Weight management.
• Maintaining the health of the colon

And depending on the type of fiber, we might also add:

• immune system health
• increase nutrient bioavailabality
• probably lots more I’m forgetting

Soluble vs. insoluble fiber:

Fiber can be divided into two major categories: soluble, and insoluble. Soluble fiber is soluble (meaning, it dissolves) in water. Insoluble fiber does not dissolve in water.

How to tell if the fiber in a food is predominantly soluble or insoluble? Well, you could read and memorize a bunch of charts. Or you could simply imagine boiling the food. Foods high in soluble fiber will thicken water when you boil them or soak them. So think of apples and citrus, oats and okra and chia seeds. In some foods, the soluble fiber has already thickened the water inside the plant, so think of foods that feel a little “slick,” like cucumbers. On the other hand, fiber that barely effect the viscosity of water will be primarily insoluble. So think of foods like carrots and green beans, wheat and rice — no matter how long you boil these, they will barely thicken the broth. In reality, most foods contain a combination of both soluble and insoluble fibers. So think of black beans, that thicken in solution, but still hold their beany shape: beans contain large amounts of both soluble and insoluble fiber.

Some types of fiber can also be described as prebiotic. If probiotics are the healthy bacteria that live in our guts and help keep us healthy, prebiotics are the substances that feed them. Generally, the solution to temporarily lowered probiotic counts is going to be to take probiotic pills. But when probiotic counts have been low for a while, and even the good pills can’t seem to tip the balance in the right direction, a consistent course of prebiotics over weeks and sometimes even months, can usually get the job done. Especially rich sources of prebiotic fiber in the diet include Jerusalem artichoke, burdock root, barley, goji berries, agave nectar and allium-family vegetables (onions, garlic, leeks). Among fiber supplements, acacia fiber stands out as an especially rich source of prebiotics. Next Month: Why does this difference matter?

Adam Stark