Thursday, December 22, 2011

Dietary Fiber - The Lost Nutrient

Fiber - we have all heard of it. It seems that whenever we hear recommendations about how to achieve a “healthy well balanced diet” that getting enough fiber is always mentioned. Food companies have tried to take advantage of this and so we often find food packages touting that they have lots of fiber.

In our last couple of posts we discussed the value of fiber for both increasing the absorption of key nutrients (e.g. magnesium, calcium, and phosphorus) and also as a way to help remove toxic substances through the gastro-intestinal tract. I thought that it would be helpful to look at fiber in greater depth in order to help you choose the right type and amount of fiber for your health.

So what exactly is “dietary fiber”, why is it necessary for health, and what are the best choices to keep you healthy?


Almost every source of fiber is from plants of some kind. Cellulose is the plant fiber that provides structure to plants. It is what holds up a plant and allows it to stand tall or even take shape. Just like humans require a skeleton to provide structure for our bodies, fiber could be seen as the structure of plants. Cellulose, is used to literally hold plants together.

Fiber is a very versatile ingredient. Humans have used the fibrous parts of plants for thousands of years to make useful items like baskets, paper, or clothing (think linen which is made from the fibrous part of flax). We also happen to eat cellulose when we eat plants and it provides a source of insoluble fiber (see below for discussion on the types of fiber).

But there are other kinds of fiber in plants. Substances, like inulin, are used by the plant as a way to store energy. Inulin is a type of soluble fiber (see below for discussion). Soluble fiber behaves differently in the body and so we will examine both types to help unravel the complexities of dietary fiber.


Humans have eaten dietary fiber as long as we have eaten plants. Historically, humans ate plenty of fiber without even trying because almost every plant we foraged or farmed provided more than enough for our health. However, in the last few hundred years, humans have developed new technologies that allow whole foods to be broken into their constituent parts. With these new approaches our eating habits and tastes have changed. No longer is it customary to eat a diet that exclusively includes whole plants.

Technology allows us to strip plants of their fiber by milling grains and removing the bran and germ portions of the grain to create “white” grains (e.g. white flour, white rice, etc.). We also developed ways of making juice (basically taking the fruit of a plant and removing all of the fibrous “flesh”). Our technological advancement has taken us to a place where we now have many varieties of foods that can be wonderfully tasty and pleasant, but which also undermine the value and nutrition we once obtained from eating whole, unadulterated foods.


A diet rich in fiber can help your body to both better absorb nutrients from food (or supplements) and, at the same time, remove toxins and other waste products from your body. Some of the main advantages of consuming fiber include the production of healthful compounds during the fermentation of soluble fiber. Insoluble fiber, on the other hand, softens and increases the bulk of stool. This decreases the transit time of food through our bodies preventing constipation. The “sweeping” effect of fiber also helps to absorb and remove bile acids, which are one of the major ways your body disposes of fat soluble toxins, removing these toxins before they get reabsorbed.

But not all sources or types of fiber have the same effect so it is important to know what type is right for you. Each type of fiber is beneficial for different aspects of health. It is important to get a good mix of both types of fiber in your diet (or through supplementation) in order to get the full health benefits from fiber.


As I mentioned earlier, there are two major types of dietary fiber - soluble and insoluble. Both types are found in plant foods like fruits and vegetables, but they behave differently inside your body. Some foods have more of one type of fiber than the other, so they are listed under a category, but many of the foods listed do contain both types.


“Insoluble” fiber is metabolically inert (your body doesn’t absorb or use it) and essentially just passes through us. It absorbs water and grows in volume as it moves through the digestive system. This type of fiber is known to improve fecal bulk and helps to relieve constipation by literally sweeping the contents of the intestines along with it. Since bile acids are one of the major ways your body disposes of fat soluble toxins, the “sweeping” effect of insoluble fiber helps to remove these toxins before they get reabsorbed. Insoluble fiber also happens to bind to dangerous heavy metals, helping to remove them.

Insoluble fiber sources include:
  • cellulose
  • whole grain foods
  • wheat, rice, or corn bran (see below for more on bran)
  • nuts and seeds
  • potato skins
  • flax and hemp seed
  • lignans (compounds found in cereal grains like rye, wheat, oat, and barley; soybeans, cruciferous vegetables such as broccoli and cabbage, and some fruits, particularly apricots and strawberries)
  • vegetables such as green beans, cauliflower, zucchini, and celery
  • some fruits including avocado and bananas
  • the skins of some fruits, including kiwifruit and tomatoes

SOLUBLE FIBER (a.k.a. Viscous Fiber)

“Soluble” or “Viscous” (also called prebiotic) fiber is less straightforward in its effects on the body. Soluble fibers are fermented by bacteria in the intestines and colon into gases and physiologically helpful byproducts. Soluble fiber absorbs water to become a thick, gelatinous substance which can be fermented by bacteria in the digestive tract. These bacteria help us to digest foods and also produce energy for us.

Sources of soluble fiber include:
  • legumes (peas, soybeans, lupins, and other beans)
  • oats, rye, chia seeds, and barley
  • some fruits and fruit juices (including prune juice, plums, berries, bananas, and the insides of apples and pears)
  • certain vegetables such as broccoli and carrots
  • root tubers and root vegetables such as sweet potatoes and onions (the skins of these are sources of insoluble fiber)
  • psyllium seed husks
  • Beta-glucan from oat bran, whole oats, or rolled oats
  • Beta-glucan from whole grain or dry-milled barley
  • Other examples of fermentable fiber sources (from plant foods or biotechnology) used in functional foods and supplements include inulin, resistant dextrins, fructans, xanthan gum, cellulose, guar gum, fructooligosaccharides (FOS), and oligo- or polysaccharides.


You may have heard of psyllium as it is one of the most common forms of soluble fiber available as a supplement. So what exactly is it?

Psyllium refers to the husk of psyllium seeds which becomes a very thick gelatinous fiber when they absorb water. This is the source of the fiber in many familiar brand name fiber supplements such as Metamucil.

Psyllium and its health benefits have been widely studied. Psyllium husk fiber helps to reduce levels of LDL (bad cholesterol) in the blood stream. In addition, consuming psyllium with a meal slows the rise of blood sugar levels and therefore lowers the levels of insulin released. This specific response lowers the glycemic index of foods with which psyllium is eaten. While psyllium has been the most studied of soluble fibers, it is safe to say that all soluble fibers carry the same health benefits.

Unfortunately, psyllium has also been found to have several drawbacks that appear to be particular to psyllium and it’s extensive use.

  • Psyllium can cause sensitivities to develop with prolonged use. Repeated exposure to the powder form (as in Metamucil and other supplements) has been shown to cause allergies to develop, eventually leading to anaphylactic reactions (similar to peanut or seafood allergies).
  • Like other soluble fibers, psyllium can also interact with certain prescription medications and supplements, making them less effective. Make sure to take medications at least 2-4 hours after taking a fiber supplement containing psyllium.
  • Many psyllium supplements are sweetened with products like Aspartame, and are not suitable for people who have a condition called phenylketonuria (for more information on this condition, check our post on artificial sweeteners). Why any dietary fiber would need to be sweetened is surprising to me, but if you prefer your fiber to be sweetened then consider using a healthier alternative sweeteners as described in our post.

Inulin, Oligofructose, and other Fructooligosaccharides

Inulin and oligofructose naturally occur in a variety of plants, where they are a means of storing energy (typically in the roots). Most of the commercially available inulin and oligofructose is either chemically synthesized from sucrose or is extracted and purified from chicory root or yakon root, although there are many other sources as well.

Inulin provides benefits very similar to psyllium, and indeed other sources of soluble fiber. It increases mineral absorption, especially that of calcium, magnesium, and phosphorus. Inulin, since it is fermentable, helps to promote the growth of helpful gut bacteria. These bacteria produce compounds that provide a protective effect to the large intestine and colon.

Inulin, however, does not suffer from the major problems associated with psyllium. It does not lead to allergic reactions and is well tolerated even at very high doses. It is estimated that, in certain parts of what is now Mexico, hunter-foragers once ate up to 135g per day of inulin fiber per day. High doses may take time for your body to adjust to, since it causes gas and bloating. Many people adjust to this after an initial period of about a week.

An exceptional qualiy of Inulin is that it has a lightly sweet taste and can be used to substitute sugars in many cooking applications. Inulin can also subsititute for fats or flour. Because of its flexibility as an ingredient and its specific benefits to nutrient absorption and waste removal, inulin is a very healthy source of fiber. For a more detailed look at inulin, read our post on artificial sweeteners.

Soluble fibers improve the quality of digestion

Soluble fibers cause the stomach to delay releasing food into the small intestine. This allows your stomach to stay full with food longer, resulting in a stronger sensation of fullness, which may help with weight control. This delay also helps to reduce the sharp rise of blood sugar (absorbed mainly by the small intestine) following a carbohydrate heavy meal. This slower rise in blood sugar means that less insulin needs to be released in response, which reduces the amount of sugar turned into fat. This helps prevent a cascade of changes that ultimately may lead to insulin resistance.

Since foods rich in fiber tend to be low in energy, fiber consumption may also help with weight loss and maintenance. In addition, viscous fibers tend to reduce spikes in blood sugar and can help reduce the development of insulin resistance, so less sugar is converted to fat in the first place. This is an important consideration since obesity and diabetes is such a widespread problem and contributes to the risk of many other diseases. In previous posts, we discussed the some of the most common problems associated with obesity, such as chronic inflammation, diabetes, and heart disease.

Soluble fibers from fruit, vegetable, and legumes (but not cereal fiber) are also associated with a reduced risk of duodenal ulcer. Although the mechanism behind this proposed positive effect of viscous fibers on duodenal ulcer is not known, one hypothesis is that the delay in gastric emptying, known to result from the ingestion of viscous fibers, may play a role.



You’ve probably heard of bran being high in fiber, but what exactly is it? Bran is the hard fibrous outer shell of the seeds in cereal grains. Bran is found in any cereal grain, including rice, corn, wheat, oats, barley, and millet. For example, when brown rice is refined into white rice the part that is removed is the rice bran. The same is true of whole wheat milled into white flour - they remove the wheat bran.

Bran is a very good natural source of fiber, vitamins, and minerals. The increased vitamin and mineral content makes bran a particularly healthy source of dietary fiber. Bran is particularly rich in dietary fiber and essential fatty acids and contains significant quantities of starch, protein, vitamins (especially Bs), and minerals like iron.

Bran is used in breads, cereals, and can be used in almost any application where added fiber is desired. Bran is made up of both soluble and insoluble fibers, so it is a good choice if you are trying to get both. The only significant problem with bran is that it contains a lot of oil, so it can go rancid; bran is often heat treated to improve its shelf-life and help prevent the oils from going rancid.

While bran can be found in many cereals (like All-Bran) and other food products, beware of choosing sources of bran that are filled with sugar or unhealthy sugar substitutes. Any fiber, including bran, can simply be added to water and swallowed easily. There is simply no reason to undermine the health benefits of the fiber by adding sugar.


Most of the research on fiber suggests that around 20g per day of fiber is a good minimum level to get the health benefits discussed above. This might not be easy to gauge, but many foods have dietary fiber listed on the packaging to give you a good idea of how much you are getting. Of course, if you were to eat only whole foods, you probably would not need to supplement your diet with fiber. Unfortunately, it is difficult to eat a diet of exclusively whole foods these days, so a dietary supplement usually makes sense.

While there is no specific upper limit to how much fiber you can eat, your body can only tolerate so much before you start to experience some side effects. Soluble fiber, since it is prebiotic (feeds good bacteria in your gut), which can cause gas or bloating when eaten in high amounts. Insoluble fiber in high amounts (and soluble, to a lesser extent) may cause constipation, especially if you don’t drink enough water with it. This is due to the way that fiber absorbs water; so if there isn’t enough water, the fiber can clump up and lead to constipation.

To avoid issues with high-fiber diets, there are a few tips and tricks that you may find helpful. If you take a fiber supplement, make sure to take it with a meal and drink at least a glass of water with it. Taking fiber with a meal enhances the nutrient absorption of the meal, while also lowering the meal’s impact on blood sugar levels. Since we tend to drink during meals anyway, this makes it easier to get enough fluids with your fiber.


With all the different types and sources of fiber, it is important to choose the one right for you. But no matter which type you choose, it almost certainly comes from a plant, so it is a good idea to make sure that it comes from an organic source. You just don’t need a side of pesticides with your fiber.

One of the easiest ways to add fiber to your diet is to simply add a fiber supplement to foods you cook. You could add ground organic flax seeds or an organic source of inulin or bran to baked goods like breads. Inulin is especially well suited to being added to food, since it can effectively replace fat, sugar, or flour. I use organic oat bran, an inulin made from organic agave, and an organic flax seed meal to supplement the fiber in my diet. I rotate the fiber I take daily to get the benefits of different fibers in my diet. No matter what source you choose, adding fiber to your diet is a healthy choice. Fiber is truly a lost nutrient in our modern diet that we need to stay healthy.


an amazing resource on all the major types of fiber and their effects on the body.

Cereal grains, legumes, and Diabetes

Water-soluble dietary fibers and cardiovascular disease

Supplementation with fiber-rich multimixtures yields a higher dietary concentration and apparent absorption of minerals in rats

Researched and written by Dr. Rebecca Malamed, M.D. with assistance from Mr. Malcolm Potter.

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