What You Need to Know About Dietary Fiber

closeup of a bunch of high fiber cauliflower

Dietary fiber might be one of the most overlooked, but important, part of our diet. It is absolutely essential for a healthy digestive system.There are two kinds of dietary fiber you should know about, soluble and insoluble. Soluble fiber will dissolve in water and in your gut is turned into a type of gel that is fermented and used by the beneficial bacteria living in our gut. It slows digestion of food and helps lower cholesterol and blood sugar. You will often see soluble fiber called a prebiotic for this very reason. The other kind of fiber, insoluble, is basically roughage, helping food move smoothly through the digestive tract.

Where it Comes From Naturally

We get dietary fiber naturally from the foods we eat. Foods like beans, particularly navy beans, lentils and white beans, most berries (not so high in strawberries), peas, barley and bran are all near the top of the list of foods containing fiber. These are not the only source. Avocados, artichokes, whole wheat pasta, and most every fruit and vegetable to some extent.

We need both soluble and insoluble fiber. While much attention has been given to foods with insoluble fiber due to their ability to move food through our systems. Less attention has been paid to the prebiotic effect of soluble fiber. Essentially, it serves as food for the probiotic bacteria in our gut.

Prebiotic, soluble, fiber is high in foods like raw chicory root. That’s not something you normally find on the shelf unless you are drinking coffee alternatives. It is, however, added to many breakfast cereals. Jerusalem artichokes, not the big green artichokes, but the ones that look a bit like ginger, are super high in fiber and can be eaten raw by shredding them in salads, or cooked into something that resembles potatoes. If you are diabetic, Jerusalem artichokes have a low glycemic index and therefore provide a good alternative to potatoes. Other things like leaks and onions, both raw and cooked are high in fiber. Also, potatoes are a funny source of fiber. Raw potatoes are high in soluble fiber that is lost in cooking. But let a potato cool and some of that fiber returns. So, given the choice between a steaming hot baked potato and a cold potato salad, the potato salad will have more fiber. Just watch for all the other potentially nasty things that get added to potato salad.

How much fiber do we need?

People in the US eat about 15 grams of fiber a day, which is just under half the recommended amount of fiber for adult males, and far less than is needed for females. The Institute of Medicine recommends that males under the age of 50 get 38 grams a day and women under 50 get 25 grams. Over that age, men should get 30 grams a day and women 21 grams. So, a cup of beans and a cup of berries a day will get you almost all the way there.


Supplementing Fiber

Because we no longer eat the hunter-gatherer diet of our distant ancestors, and eat a lot of processed foods deplete of fiber, we have the option of supplementing. When supplementing remember that there are two kinds of dietary fiber and supplementing both kinds would be helpful. Supplement with prebiotic fiber to feed your intestinal flora, and supplement with insoluble fiber to stay regular, or overcome problems like constipation. Just a note here, prune juice, which has little or no fiber, and raw prunes which have a decent amount of fiber actually help with constipation because of the high sorbitol content. This sugar alcohol can act as a laxative. Remember that next time you buy sugar-free candy sweetened with sorbitol.

There are all kinds of fiber supplements that range in price from $10 to $20. While Consumer Reports rated psyllium husk supplements as the best. The soluble fiber in psyllium is arabinoxylan, a hemicellulose. It’s a very weak prebiotic. A good prebiotic fiber will contain inulin and/or oligofructose. Inulin, is extracted from chicory, which you will remember is a great natural source of soluble fiber. Oligofructose is a fiber similar to inulin. Acacia fiber is also considered a good prebiotic.

Supplements that contain methylcellulose are some of the best for relieving constipation.

Warning: When taking fiber supplements it is always best to start off with lower doses and build up to higher doses. Everyone has a different tolerance so it’s best to err on the side of caution. It’s not dangerous but can cause an overgrowth of gut flora, as well as stomach aches. This is known as SIBO or Small Intestine Bacterial Overgrowth. If this happens, one thing people have done is cut back on their carb intake, which will kill off some of the bacterial flora.

As always, I am not a medical professional so take what I say as informational only and consult a medical professional about any of your health concerns.


I will cover probiotics in great detail in another article. But since I’ve talked so much about prebiotics it only makes sense to touch on probiotics, the symbiotic bacteria that live within our guts. Scientists are still trying to figure out these bacteria but we do know that they serve as a defense against harmful bacteria by regulating immune response. They suppress inflammation and work within the lining of our intestines to act as a barrier to harmful organisms. Without this balance of good and bad bacteria things can go very wrong, not just in our gut, but can lead to cancer and cardiovascular disease.

We get probiotics naturally by eating some fermented foods like sauerkraut, brine pickles (not the ones with vinegar, kim chi, and you’re probably wondering why I didn’t start with yogurt. Most dairy products that claim to have probiotics have very little after they go through the pasteurization process (cooking). But, by all means, make your own at home. Additionally, there are lots of probiotic supplements on the market. We’ve listed some in the products portion of the Live A Longer Life website.

Summing it all up

We need dietary fiber to live. We get it naturally through the foods we eat but most Americans eat half of the recommended amount of fiber each day. The two kinds of dietary fiber, soluble and insoluble serve different purposes with the soluble fiber forming a gel and providing food for good bacteria in our gut (prebiotic effect) and insoluble fiber acting as the roughage we need to move food easily through our intestines. Eat the right foods and if you need to, supplement with the kind of fiber your body or situation calls for.


  1. J Clin Gastroenterol. 2008 Sep;42 Suppl 3 Pt 2:S174-6. doi: 10.1097/MCG.0b013e31817f183a. Evaluation of prebiotic potential of refined psyllium (Plantago ovata) fiber in healthy women.
  2. https://en.wikipedia.org/wiki/Dietary_fiber
  3. http://paleoleap.com/much-good-thing-gut-flora-overgrowth/
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Posted by Ted Coombs

Ted Coombs
Ted Coombs is a medical anthropologist, futurist and author who is passionate about health through knowledge.