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Fiber – The misunderstood ingredient

Eat more fiber. You’ve probably heard it before. But do you know why fiber is so good for your health?

various-fiber-containing-foodsToday, fiber is one of the most talked about ingredient in the media and among nutritionists, and health aficionados. Even with this exposure, it is by far one of the most misunderstood food components in our diet. Despite the wealth of scientific and medical knowledge about fiber, most people still associate it with constipation. But when I tell them about its many benefits to health, I am almost always confronted with the same reaction; “I had no idea fiber does all that”.

So, before we appreciate why fiber is important to our health, we need to understand what it is, and how it confers its benefits. But this can only be done if we put things within the proper context of our physiology.

As simple as this seems, it is far too big of a topic to cover in a few paragraphs, so I have decided to break the subject into smaller and more digestible bites. The goal is to simplify the current clinical and scientific knowledgebase about this ‘misunderstood’ ingredient and present it in an understandable and fun way. I also plan to address some [mis]information and myths associated with dietary fiber that run contrary to conventional wisdom and our evolutionary (or design) physiology.

What is fiber?

Unlike fats, proteins or simple carbohydrates (e.g., sugars) — which your body breaks down and absorbs — fiber includes the edible parts of plants that are resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the large intestine.

Fiber is a complex carbohydrate polymer (a long string of sugar molecules attached together). Much like the bones in your body, fiber establishes the structural framework for a plant and helps to maintain its shape. There are many different types of fiber and although formed from a combination of about 10 common monosaccharides, fibers are structurally very diverse.1 Some are long strings with different number and kinds of sugars linked together, some have branches in them and some don’t (see figure below).simple-carbohydrates/complex-carbohydrates

This variability defines what fiber does in your body. What separates fiber from other simple sugars is that the enzymes responsible for breaking simple carbohydrates (e.g., sucrose and lactose) down to mono-saccharides (e.g., glucose, fructose or galactose) are not able to digest fiber into its smaller parts. And this is partly the reason behind why fiber can play such an important part in digestive health…read on

Classification and types of fiber

Fiber comes in different shapes and sizes; Dietary, functional, soluble, insoluble, viscous, non-viscous, fermentable, non-fermentable. Are you confused yet? Dietary fibers are typically classified into different groups according to various physical-chemical and physiological criteria including solubility, viscosity, and fermentability.2

Dietary and functional fiber
In 2001, a panel of experts at the Institute of Medicine developed definitions of fiber that distinguishes between fiber that occurs naturally in plant foods (dietary fiber) and isolated or synthetic fibers that may be added to foods or used as dietary supplements (functional fiber).3, 4

Viscous and non-viscous fiber
Some fibers form very viscous solutions or gels in water. This property is linked to the ability of some fibers to slow the emptying of the stomach, delay the absorption of some nutrients in the small intestine, and lower serum cholesterol.

Fermentable and non-fermentable fiber
Some fibers are readily fermented by the trillions of “good” bacteria (aka, gut flora or microbiome) in the human colon. It is the through the fermentation of dietary fiber that our gut flora is able to produce the essential by-products that we need to sustain a healthy life.

Soluble and insoluble fiber
The designation of “Soluble fiber” originated as an analytical term.5 Soluble fibers are easily dissolved in water, while insoluble fibers cannot.

The idea of fermentable fibers overshadows the soluble versus insoluble distinction, because both insoluble and soluble fibers can be fermentable. For example, insoluble resistant starch (e.g., green unripe banana, potato) is fermentable, as is soluble inulin (e.g., Apple). And most whole plant fibers contain both soluble and insoluble fiber.

It is important to note that the above classifications are somewhat arbitrary and whole foods don’t cleanly fall into a particular category. After all, how often do we visit our favorite restaurant and tell the waiter: “bring me a side dish of insoluble fiber”, or “I’ll have your fermentable fiber special”? All vegetables, grains, legumes, and fruits are a mix of insoluble fiber (the peel) and soluble fiber (the flesh underneath), some of which can be both fermentable and/or viscous.

What good is fiber if I can’t digest it?

That’s a very good question. The majority of simple sugars ingested and digested by humans are absorbed in the small intestine.6 However, more complex ones like fiber, e.g., inulin (a fructo-oligosaccharide present in large amounts in the skin of apples), cannot befiber/gut-bacteriadigested by humans and are able to reach the colon intact.7 Even though we humans can’t digest fiber (from fruits and vegetables), bacteria that live in our guts (referred to as our gut microflora) can.

The gut microflora feed on this undigested fiber (from fruits and vegetables) in order to break it down through a process called fermentation and produce a wide range of compounds that have both positive (and sometimes negative) effects on gut physiology.8-10 In fact, we can salvage about 10% — 30% more energy out of our foods thanks to these microbes.11,12 How’s this for boosting our food’s effective mileage?

It is through the fermentation of dietary fiber that our
gut bacteria produce the essential by-products
that we need to sustain a healthy life.

How much fiber should I eat?

Guidelines from the Food and Drug Administration (FDA) recommend eating a minimum of between 25 – 38 grams of dietary fiber per day,1 but most Americans don’t even get half the necessary dietary-fiber amount from the fruits and vegetables they consume. And when we do, they’re mainly potatoes and tomatoes — in the forms of french fries and pizza, according to a recent report from the U.S. Department of Agriculture. The study finds that more than half our veggie intake comes from potatoes and tomatoes, and only 10 percent comes from dark green and orange veggies like spinach, kale, carrots and sweet potatoes.

Let’s not even talk about kids. Very few of them would say they crave a good fiber-rich meal and the thought of fruits and veggies may even bring a few gags and groans at the dinner table. Those of you who are a parent will undoubtedly appreciate this. Regardless of the challenge, each of us needs to figure out a way to up the fiber intake in ourselves as well as our children.

The benefits of fiber

Fiber is one of those interesting ingredients. It’s a “Jack of all traits” if you will and some its benefits are realized either directly (mostly as a result of its structural and physio-chemical characteristics), indirectly (by-products of fermentation) or combination. Below (no particular order) are some of these benefits (just to give you a taste):

  • Weight loss management & appetite control
  • Blood sugar regulation Absorption of minerals
  • Detox & waste management
  • Protect phytonutrient degradation
  • Support gut microflora
  • Inhibits pathogenic bacterial growth
  • Immune function regulation
  • …and much more

In future blog articles, I intend to take you a bit deeper in each of the above topics and explain how fiber achieves all that. We will also explore some of the misconceptions surrounding the role of fiber and gut ailments such as Irritable Bowel Syndrome.

Happy digestion,



  1. Obel, N., Porchia, A.C. & Scheller, H.V. Dynamic changes in cell wall polysaccharides during wheat seedling development. Phytochemistry 60, 603-610 (2002).
  2. James, S.L., Muir, J.G., Curtis, S.L. & Gibson, P.R. Dietary fibre: a roughage guide. Internal medicine journal 33, 291-296 (2003).
  3. Trumbo, P., Schlicker, S., Yates, A.A. et al. Dietary reference intakes for energy, carbohydrate, fiber, fat, fatty acids, cholesterol, protein and amino acids. Journal of the American Dietetic Association 102, 1621-1630 (2002).
  4. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients). (The National Academies Press, 2005).
  5. Marlett, J.A. Content and composition of dietary fiber in 117 frequently consumed foods. Journal of the American Dietetic Association 92, 175-186 (1992).
  6. Bond, J.H., Currier, B.E., Buchwald, H. & Levitt, M.D. Colonic conservation of malabsorbed carbohydrate. Gastroenterology 78, 444-447 (1980).
  7. Hudson, M.J. & Marsh, P. in Human Colonic Bacteria. Role in Physiology, Pathology and Nutrition. (eds. G.R. Gibson & G.T. Macfarlane) 61-73 (CRC Press, 1995).
  8. Cummings, J.H. Dietary fibre. British medical bulletin 37, 65-70 (1981).
  9. Cummings, J.H. Dietary fibre and large bowel cancer. The Proceedings of the Nutrition Society 40, 7-14 (1981).
  10. Cummings, J.H. & Macfarlane, G.T. The control and consequences of bacterial fermentation in the human colon. The Journal of applied bacteriology 70, 443-459 (1991).
  11. Parker, D.S. The measurement of production rates of volatile fatty acids in the caecum of the conscious rabbit. The British journal of nutrition 36, 61-70 (1976).
  12. Bergman, E.N. Energy contributions of volatile fatty acids from the gastrointestinal tract in various species. Physiological reviews 70, 567-590 (1990).


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