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Armchair nutritionists and even most genuine nutritionists might tell you that eating fiber is good for you, but most don’t know a damn thing about WHY it's good for you. They'll advise you to eat a bran muffin with your morning coffee and think they've done your bowels a great service.

Little do they know that eating wheat bran, possibly the most famous of the fibers, is equivalent to eating small pieces of plastic. In fact, researchers who study fiber's effects on the bowels actually use small pieces of plastic to mimic the effects of wheat bran in studies.

If it makes you "go," it's because the bran irritates the hell out of your bowels, prompting them to expel whatever's in them, including that SpongeBob SquarePants keychain you swallowed when you were a kid. That's hardly the way fiber – the right kind of fiber – can and should work.

When the right type of fiber is ingested, it forms a viscous gel that travels through the small and large bowels to gently make you go with the regularity of a Japanese commuter train, fix diarrhea, ameliorate irritable bowel syndrome, or in general fix whatever ails the latter half of your alimentary tract, all without causing gas.

Along the way, the RIGHT type of fiber works to increase insulin sensitivity, facilitate fat loss, reduce cholesterol, and rescue any children from burning buildings, among other great things.

So yeah, perhaps surprisingly, there's a lot of fascinating, downright sexy science to fiber, bowel movements notwithstanding. A two-part article by Johnson McRorie that appeared in a recent edition of Nutrition Today makes that abundantly clear. The wide-ranging article addressed things that could and should change the way you think about fiber, along with the way you eat.

Following is some of the stuff I learned from his article, along with some additional info from additional sources and personal experience.


The Wonderful World of Bowels

Before we can understand the relationship between fiber and the small and large bowels, we first have to understand a tiny bit about their anatomy.

The average small bowel, which is the destination of food right after it leaves the stomach, is almost exactly the same length as three Boban Marjanovics of the Dallas Mavericks laid end to end, which is about 7 meters. It's divided into three distinct regions – the duodenum, the jejunum, and the ileum – every square centimeter of which is covered with a layer called the mucosa.

This mucosa is studded with millions of small, finger-like projections called villi, every 0.1 microns of which is covered by another 1,000 microvilli. That all makes for an incredibly large area through which to absorb the nutrients introduced by the food you eat, so much so that if you flattened it all out it would approximate the size of a springy, gentle-on-the-arches tennis court.

The small bowel uses a type of muscular motor activity to approximate a slow moving paint mixer in the hardware store, thus mixing the "chyme" (the pea-soupy acidic fluid that consists of gastric juices and partially digested foods) back and forth to expose it to bile and enzymes and then feed the naturally processed food to all those hungry villi and microvilli.

After the small bowel extracts most of the usable nutrients from the food, it transits to the large intestine, which is composed of the cecum, the colon, the rectum, and the anus.

Approximately 1500 mL of liquid residue gets ferried into the large intestine every day and under normal circumstances, about 90% of the fluid and electrolytes are gradually reabsorbed, resulting in the birth of a brand spanking new baby stool.

How this stool bursts forth into the world is a wonder of nature and propulsive mechanics. About 95% of movement in the large intestine is segmental, consisting of mixing waves that facilitate the absorption of the aforementioned water and electrolytes. The rest of the movement is "propagating contractions," otherwise known as peristalsis.

The amplitude of these propagating contractions varies enormously from individual to individual, ranging from frequent, low amplitude (10 millimeters of mercury, or 10 mm Hg), rapidly propagating waves that propel gas at a rate of up to 17 centimeters per second over and around slow-moving turds, to infrequent, high-amplitude (over 100 mm Hg), slowly propagating contractions of less than 1 centimeter per second that ferry the solid stool along.

All of this is what determines your "transit time," i.e., how long it takes your Taco Supreme to work its way entirely through your system, often relatively unchanged. In general, though, gas can move rapidly through your body in less than an hour, while hardened stools can lollygag their way through your bowels and take days to vacate.

Fiber Powder

The Gritty Particulars of Fiber

Now that we can visualize the structure and function of the bowel, we can grasp how different types of fiber, each with crucial differences in characteristics, can affect them.

These crucial characteristics are as follows:

  • Solubility – This refers to whether a fiber supplement dissolves in water (making it soluble) or just floats around as particulate matter (insoluble, like wheat bran or plastic).
  • Viscosity – This is the physical property of some fibers to thicken when added to water.
  • Gel Formation – Related to viscosity, this term refers to the ability of some fibers to form cross-links when hydrated to form a "vasco-elastic" gel.
  • Fermentation – This term refers to the ability of a fiber supplement to survive enzymatic reactions in the small bowel, only to be used as a substrate by bacteria to form short-chain fatty acid and gas.

These four descriptors can be used to divide fiber supplements into four separate but important classifications:

1. Insoluble, poorly fermented

The wheat bran mentioned in the opening paragraphs is an example of an insoluble fiber. Just like plastic, it doesn't dissolve in water. It doesn't form a healthful gel and it's susceptible to some fermentation, meaning it can cause the production of a little gas. It does, however, facilitate bowel movements, albeit rudely and without much finesse.

2. Soluble, non-viscous, readily fermented

These are fibers that dissolve in water but don't form a healthful gel and are quickly fermented, leading to lots of gas and lots of flatulence without having a laxative effect. Bacteria break them down to form fermentation byproducts, some of which can then be absorbed as empty calories. Examples of this type of fiber include inulin, wheat dextrin, oligosaccharides, and resistant starches.

3. Soluble viscous/gel forming, readily fermented

Examples of this type of fiber include oats, barley, raw guar gum, and glucan. They dissolve in water and form a viscous gel, thus slowing down the absorption of nutrients, which leads to better glycemic control and lowered cholesterol. They are, however, readily fermented, which leads to gas and the loss of their gel structure and its water-holding capacity, so no laxative effects or controlling of diarrhea.

4. Soluble viscous, gel forming, non-fermented

There's only one commonly used fiber in this classification and it's psyllium. This is the gold standard of fiber supplements. It dissolves in water and forms a viscous gel. It does all the things you could hope for of a fiber, lowering cholesterol, reducing absorption of calories, and providing a feeling of satiety to curb appetite.

It also helps normalize stool formation (battling both constipation and diarrhea) and quells irritable bowel syndrome (IBS), all without causing any gas to be formed.

So How Does Psyllium Do All This Magic Stuff?

When you introduce a viscous, gel forming fiber like psyllium into the digestive tract, it increases the viscosity of the chyme, which slows the rate at which the chyme is mixed with digestive enzymes.

This may sound like it's something you might not want, but you DO want it. The slowing of digestion slows the absorption of glucose, which is good. It also slows the absorption of certain nutrients, allowing them to travel farther into the distal ileum, where they're not normally found.

This unexpected delivery into the distal ileum sets off a cascade of events. Glucagon-like peptide-1 is released into the bloodstream, thus decreasing the secretion of glucagon (which slows down sugar production in the liver). Insulin production is increased, appetite is decreased, the growth of pancreatic beta-cells (cells that produce insulin) quickens, insulin sensitivity improves, and even gastric emptying is slowed.

Psyllium or psyllium-like fibers also reduce cholesterol by trapping bile, which is a substance secreted by the liver to emulsify fat particles so that they can be more easily absorbed by enzymes.

This is pretty cool because bile is usually recycled several times in a single meal, but when the bile is trapped by fiber and eliminated in the stool, the liver has to produce more bile. Cholesterol, however, is a component of bile, so by forcing the liver to make more bile, a de facto reduction of cholesterol occurs. LDL cholesterol goes down without affecting HDL cholesterol.

Onward, Into the Large Bowel

As mentioned, some fiber supplements "cure" constipation by acting as stimulants or irritants of the colonic mucosa, thus causing the large bowel to expel the stubborn, dehydrated stool.

Psyllium, however, because of its gel-forming and fermentation resisting properties, resists dehydration so that it acts as a stool "normalizer," softening hard, constipated stools while also adding firmness to liquid or loose stools.

Conversely, other supplemental fibers (e.g., wheat dextrin), often touted as having laxative benefits, can actually make the problem worse. Even some other gel-forming fibers like oat fiber, barley fiber, guar and acacia gums lose that gel-forming ability once they're fermented, which makes them duds as far as being stool normalizers.

Orange Liquid

I See London, I See France, I See a Happy Poop Dance

The American College of Gastroenterology Chronic Constipation Task Force, while no doubt fending off the romantic overtures of fanboys and fangirls, has looked at the available evidence and concluded that the only fiber supplement worth recommending/taking, at least for chronic constipation, is psyllium.

However, there's also plenty of additional research to support the use of psyllium to reduce cholesterol, induce satiety, increase insulin sensitivity, reduce blood pressure, aid the immune system, decrease chronic inflammation, and reduce body weight.

The use of fiber in general has even been found to increase the length of telomeres, which are the specialized stretches of DNA at the ends of chromosomes that play a role in determining life span.

None of this is to suggest that you shouldn't still strive to get plenty of soluble and insoluble fiber from natural foods. The trouble is, few people do. The recommendation is that men require approximately 38 grams per day while women should get about 25, but it's based on a simple formula:

Take in 14 grams for every 1,000 calories ingested.

Like I said, few people do that, so supplement with fiber and supplement wisely. That means taking psyllium in its raw, organic form, or bedazzled up a little with sweetener and orange flavoring, as found in Metamucil.

Start by mixing about 3 grams a day in water every day for the first week and gradually work up to taking 10 to 15 grams a day in two or three doses. Take it about a half hour before meals to combat cholesterol and increase satiety (so you don't pig out).

You'll definitely know it's working as few supplements announce their effects so dramatically.

Related: How to Poop Like a Champion

Related: Lose Fat With a Spoonful of This


  1. McRorie, Johnson W. Jr. "Evidence-Based Approach to Fiber Supplements and Clinically Meaningful Health Benefits, Part 1," Nutrition Today, March/April 2015 – Volume 50 – Issue 2 – p. 82-89.
  2. McRorie, Johnson W. Jr. "Evidence-Based Approach to Fiber Supplements and Clinically Meaningful Health Benefits, Part 2," Nutrition Today, March/April 2015 – Volume 50 – Issue 2 – p. 90-97.
  3. Tucker, Larry. "Dietary Fiber and Telomere Length in 5674 U.S. Adults: An NHANES Study of Biological Aging." Nutrients 2018, 10(4), 400; doi:10.3390/nu10040400.