Fiber Basics

What is dietary fiber?

Dietary fiber is made up of complex carbohydrates that humans lack the enzymes to digest.

Why is fiber supposed to be good for us?

Fiber does a number of things that benefit us and, with rare exceptions, has no down side other than temporary discomfort from increasing it too fast. For example, it makes eating more satisfying by increasing dietary bulk. Not only does this give more chewing satisfaction, but it also helps us feel full.

In the digestive tract, the presence of fiber continues to provide bulk -- in some cases, far more than you'd expect because it also holds water. This improves intestinal function by making it easier for your body to move foods and waste products. Further along, part of the fiber we eat is fermented by the bacteria that inhabit the large intestine and colon (don't be grossed out, these bacteria are a Good Thing!) to produce chemicals that have beneficial effects as well as additional bulk to keep things moving along comfortably.

Studies have shown that fiber prevents constipation, hemorrhoids, and diverticular disease, as well as lowering cholesterol and reducing the blood sugar fluctuations of diabetes.

Should lowcarbers deduct dietary fiber from the carb count?

Unless the plan you are on says otherwise, yes. Even Dr. Atkins eventually agreed with this concept (his original diet was developed long before our current definition of dietary fiber and for many years he was skeptical). However, since plans may account for the fiber in their carb limits, it's best to follow their rules until you are certain.

Why do the rules for fiber keep changing?

It's not just the carbohydrates that are complex, it's the entire subject.

Part of the problem is that an entire class of dietary fiber (soluble) was unknown until just a few years ago. So we are still learning about it. To make things even more complicated, we know there are other substances found in our food supply that act like fiber, but currently don't come under the rules that define fiber, at least for nutritional labels. As much as science is portrayed as knowing everything, the reality is that we are learning new things every day -- and redefining certain categories.

The bottom line: be prepared for more changes to come.

What's this about soluble fiber? There are different kinds?

There are many different kinds of fiber. With the exception of chitosan, a soluble fiber synthesized from shrimp and crab shells, all come from plants, most from edible plants. The biggest division in dietary fiber is soluble versus insoluble.

Why is the division between soluble and insoluble fiber important?

Historically, this division is important because insoluble fiber is a class that has been known for a long time -- it was "roughage" to your grandparents -- but awareness of soluble fiber is very recent, so we are still learning about it. Whenever you read something about fiber, it is important to find out when it was written, because whatever they knew at the time of writing influences what they said -- and that includes regulations for reporting fiber in foods.

Nutritionally, both soluble and insoluble fiber are found together in plant foods in varying proportions. There is also some overlap in behavior by the different types of fiber, so remember that the statements in this FAQ are general and may have some exceptions.

Both types of fiber appear to be good for us, but soluble fiber seems to have more specific health benefits (reducing cholesterol, controlling blood sugar), according to current reports.

What is "roughage" or insoluble fiber?

Insoluble fiber comes from the structural parts (cell walls, etc.) of the plant world. Insoluble dietary fiber is found in grain brans and makes up most of the fiber in vegetables and part of the fiber in fruits.

Physiologically, insoluble fiber is the roto-rooter of the intestinal tract and tends to speed up passage through the body. Anything that adds bulk to the stuff traveling though your intestines makes peristalsis easier. Just watch out for too much bran because brans are high in phytates which can bind minerals.

What is soluble fiber?

Soluble fiber in many ways is the opposite of insoluble fiber. It comes from the storage materials of the plant world used to store water. There are many different types, but they are all hydrocolloids and bind from 20-200 times their weight in water. This thickening characteristic not only gives you more bulk but it acts almost like grease in soothing and smoothing passage through the intestinal tract. They can get ver-r-ry thick and interfere with movement, so they tend to slow digestion, increase the feeling of fullness, and improve mineral absorption -- probably because they cause the nutrients to spend more time in the right zone. Some of soluble fiber's health benefits are attributed to this viscosifying effect. Others are attributed to its ability to bind things to itself.

Within the soluble dietary fiber class, there are two main divisions, fermentable and non-fermentable. Fermentable fiber goes on to feed the bacteria that inhabit our colons. Please don't be grossed out--they are extremely important for our health and comfort. It is their corpses that make up the bulk of our fecal matter -- and their by-products that help prevent colon cancer. If your intestinal bacteria are healthy, you are likely to be healthy. For example, the reason people often get diarrhea from antibiotic treatments is that the drugs can wipe out the colony of microflora. And "yeast infections" are caused by an overgrowth of the wrong kind of microorganism; a healthy colony of the right kinds can keep yeast from taking over.

Next: Fiber and Health