Insoluble fiber is what we usually think of when we think “fiber” or “roughage”. Wheat bran, vegetables, nuts, beans, and seeds are examples of sources of insoluble fiber. It is tough and doesn’t easily break down in our digestive tract. Insoluble fiber tends to increase the “speed of transit” through our digestive systems, adds bulk to our stools, and increases the regularity of bowel movements.
Pronunciation: in SOL yoo bel
Insoluble fiber is basically the cell walls of plants and is made of cellulose. Most whole plant foods are sources of fiber, either insoluble or soluble fiber. At least 25-30 grams of fiber is recommended per day, and most of that will be insoluble fiber.
Digestion: Insoluble fiber speeds up our digestion (as opposed to soluble fiber, which does the opposite), and adds bulk to our stool. This is what people think of as “regularity” — all fiber, but especially insoluble fiber, keeps things moving through our bowels and prevents constipation.
Colon health: Certain insoluble fiber is fermentable by the bacteria in our colons, contributing to colon health. However, soluble fibers tend to be more highly fermentable by these bacteria.
Often when we think of fiber, we think of grains, but grains aren’t as high in fiber as some other foods, and they have too much starch for most people following a low-carb diet. The following are important sources of insoluble fiber that don’t have a lot of carbohydrates:
Seeds, particularly flax and chia seeds, which are also good sources of soluble fiber.
Greens have a lot of fiber compared to the very small amount of carbohydrate. In fact, greens are sometimes considered a “free food”.
Unsweetened coconut is a great source of insoluble fiber.
Avocados are surprisingly high in fiber — 12 grams in a medium avocado.
Berries are a good source of fiber and have less sugar than most other fruit. (They are also rich in other nutrients.)
Other non-starchy vegetables are high in fiber, including asparagus, celery, cabbage, mushrooms, and eggplant.