The word fiber conjures up wholesome, earthy-crunchy images of squeaky clean intestines and free-flowing coronary arteries. Yet fiber is not a nutrient at all, and is not absorbed by our bodies. What is fiber? What is the difference between soluble and insoluble fiber, is it really healthy for us, and do we even need to eat it?

PULP FICTION: The Truth about Fiber

What is fiber?

“Fiber” comes from the cell walls of plants. It provides shape and architectural support to the plant. Animals do not contain any fiber; we use bone and cartilage to support our bodies instead. Fiber is by definition indigestible by humans.

There are two types of fiber: soluble and insoluble.

All plant foods (fruits, vegetables, nuts, beans, seeds and grains) contain a combination of soluble and insoluble fiber in varying amounts. We are told that soluble fiber is good for us because it slows things down and we are told that insoluble fiber is good for us because it speeds things up. Hmmmm…

Insoluble fiber:  the tough stuff

Insoluble fiber is affectionately called “roughage.” Insoluble fiber is called insoluble because it does not dissolve in water. It is the stuff that gives tree bark, nutshells and twigs their woody texture. Foods high in insoluble fiber include grains, seeds, nuts, vegetables and certain fruits. Insoluble fibers pass through our digestive system practically untouched, because even bacteria can’t easily digest them. We are told that insoluble fiber is good for us because it adds weighty “bulk” to the contents of our intestines, helping to push things along. Why expose the smooth inner surfaces of our intestines to these abrasive indigestibles? We are told that we need them to sweep our innards clean of potential toxins. Oddly enough, I was unable to locate a single scientific article explaining what these toxins are and how insoluble fiber removes them…therefore this must simply be a common belief –an appealing image that makes sense in our minds, but that has absolutely no science behind it.

Soluble Fiber: the swell gel

The biggest difference between soluble fiber and insoluble fiber is that it is, as the name implies, soluble in water—it can partially dissolve in water. Most over-the-counter laxatives are made with soluble fiber. Soluble fiber dissolves partially in water, forming a gel–you can see this happen when you stir a soluble fiber supplement such as Metamucil® into a glass of water. The ability of soluble fiber to hold water is what allows fruits and soft vegetable parts to contain water and yet maintain their firm shape.

The soluble fiber family includes a wide variety of plant compounds such as arabinoxylan, dextrins, inulin, waxes, chitins, pectins, and beta-glucan. Soluble fiber is found in apples and pears, oats, pitted fruits, psyllium, citrus fruits, beans, berries, and brussels sprouts, among other foods. Some types of soluble fiber are more “viscous” than others, meaning they form firmer, stickier gels. We are told that this swollen gel action is good for us for three reasons:

  1. Viscous soluble fiber binds some of the LDL or so-called “bad cholesterol” we eat so that less of it enters our bloodstream.
  2. Gels move more slowly through the intestine than liquids. When we eat something sweet along with soluble fiber, the gel will slow down the absorption of sugar into the bloodstream, which may reduce blood sugar spiking.
  3. The swollen gel helps us to feel full, so we may eat less food.

These are all good things, right? Don’t we want to feel full, and lower our cholesterol and blood sugar levels? Yes, of course we do! So let’s take a look at each of these claims.

Does fiber lower cholesterol levels?

Yes. A recent review of studies on the effect of plant-based diets on cholesterol levels showed a maximum reduction in LDL cholesterol of about one-third [12]. This is why fiber is advertised as “heart healthy.” Of course, if you’ve read the Cholesterol page you know that a) LDL is not necessarily bad, and b) the most powerful way to improve your cholesterol profile is to eat a low glycemic index or low carbohydrate diet.

Can fiber lower blood sugar?

Yes. Soluble fiber has been definitively shown to reduce the “glycemic index” (the degree to which your blood sugar will spike) of sweet or starchy foods, but only by 10 to 20% [11]. This is probably because fiber slows digestion of carbohydrates by interfering with normal digestion. Of course, a much more powerful and direct way to reduce the glycemic index of the foods you eat is to…avoid high glycemic index foods.

Do high-fiber diets help with weight loss?

No. Most studies show little to no weight loss benefit, but study results were mixed and many studies were poorly designed. According to Nutrition Reviews, “the limited number of clinical trials comparing high-fiber foods with low-fiber foods have not provided consistent data indicating that these diets are more efficacious for weight loss than low-fiber control diets.”6

An interesting analysis of twenty different studies of the use of guar gum (a soluble fiber supplement) in weight loss noted that the guar gum caused abdominal pain, flatulence, diarrhea and cramps, and concluded that:

“…guar gum is not efficacious for reducing body weight. Considering the adverse events associated with its use, the risks of taking guar gum outweigh its benefits for this indication. Therefore, guar gum cannot be recommended as a treatment for lowering body weight.”10


Does fiber protect the colon from cancer, constipation, and other diseases?

No. In the World Journal of Gastroenterology in 2007, Doctors Tan and Seow-Choen published a review of medical studies conducted over the previous 35 years about fiber and colon health and concluded:

“A strong case cannot be made for a protective effect of dietary fiber against colorectal polyp or cancer. Neither has fiber been found to be useful in chronic constipation and irritable bowel syndrome. It is also not useful in the treatment of perianal conditions. The fiber deficit-diverticulosis theory should also be challenged…we often choose to believe a lie, as a lie repeated often enough by enough people becomes accepted as the truth. We urge clinicians to keep an open mind. Myths about fiber must be debunked and truth installed.”5

Ok, so if you are eating a diet that includes high glycemic and refined carbohydrates, soluble (gel) fiber will probably not help you lose weight, but it may soften your blood sugar spikes, and may lower your cholesterol a little. So, since fiber can be helpful in reducing the risks associated with eating a high carbohydrate diet, why not eat it? Is there any reason NOT to eat it?

Bacteria love soluble fiber.

We cannot digest the carbohydrates that make up soluble fiber, but the bacteria in our large intestine can, and they do. Undigested carbohydrate fibers arriving in the colon attract huge numbers of bacteria for a free lunch. Is there anything wrong with that? Why not let them enjoy themselves? Well, bacteria don’t exactly digest these carbohydrates, they ferment them, and in the process, they give off gases, like carbon dioxide, hydrogen, and methane. Not only can these gases make you unpopular at parties, but they can also cause uncomfortable cramping and bloating, both common sense signs of poor digestion. Listen to your body: good digestion should not hurt. Yes, you may feel like eating less, but it’s only because soluble fiber swells within your system, attracting a swarm of bacteria, which ferment the fiber and generate a balloon of gases, creating a feeling of fullness. In contrast, animal protein and fat are comfortably and efficiently digested by humans with virtually no gases produced.

Soluble fiber extracts are like greedy sponges.

Once trapped inside of you they require LOTS of water to form the sticky ooze that is supposed to be so good for us. A friend of mine recently told me the story of how she discovered an old container of Metamucil® in the back of her refrigerator that she innocently tried to dispose of by pouring it down the sink. Unfortunately, when she started the water running to flush it down the drain, the psyllium fibers rapidly expanded into a huge, stubborn, constipating blob that her plumber could only dislodge by physically dismantling the pipes. Word to the wise: if you do not drink lots of water along with your soluble fiber supplement, you, too, could find yourself in need of some expensive professional assistance. We were not meant to swallow concentrated extracts of plant fiber. We were designed to eat whole foods. This is why Mother Nature designed juicy, appealing fruits complete with their own water supply.

Why do experts believe that fiber is essential for health?

It is a simple misunderstanding of the research. Well-meaning, intelligent scientists looked at all the data and came to the wrong conclusion by making a single, critically flawed assumption that led them in the wrong direction. For the full description of the history and politics of the development of the false fiber theory, I recommend reading the chapter on fiber in Gary Taubes’ Good Calories Bad Calories, but allow me to summarize. About 150 years ago, the world was populated by two kinds of people: people who were still eating “traditional” diets of various types, and people who were starting to eat a “modern” diet.

The traditional diet folks were eating all sorts of things, depending on where they lived, so their diets varied tremendously from one culture to another. Some ate mostly meat. Some ate meat, and dairy products. Some ate poultry, grains, and vegetables. Some ate fish, coconuts, and fruit. All of these traditional people were incredibly healthy compared to us—all of them enjoyed extremely low rates of cancer, were virtually free of heart disease, diabetes and obesity, sported strong teeth and bones, and had normal blood pressures.

The modern diet folks were also eating all sorts of things, depending on where they lived, so their diets also varied tremendously from one culture to another. They also ate some combination of meats, fish, nuts, fruits, vegetables and grains, but what set them apart from the traditional people and what made their diets “modern” was the addition of highly-refined flours and large amounts of sugar. In the 1870’s, advances in modern milling technology saw stone grinding wheels replaced with steel rollers, capable of stripping the bran and wheat germ away from the soft center of each grain. This process resulted in the ability to produce ultra-fine, 100% fiber-free particles—the most refined flour in human history. Modern societies had also devised more efficient and less expensive ways to cultivate and distribute sugar to large numbers of people who previously could not afford to buy it.

Time after time, when the health of a traditional culture was compared to the health of a modern culture, the traditional culture came out on top, so it was clear to researchers that modern diets were unhealthy. The problem was that some scientists looked at these two types of diets and jumped to the wrong conclusion about why modern diets were inferior to traditional diets. They assumed it was because modern diets were lacking in fiber, not because modern diets were loaded with refined carbohydrates.

Now, it is absolutely true that modern diets contained far less fiber than traditional diets did, so they were not wrong about that. Traditional peoples who ate grains ate them whole, stone-ground, or cultured—all forms of grains that have not had their fiber stripped away. However, study after study has shown that adding fiber back to our modern diet does not restore us to the excellent health our ancestors enjoyed, so simply adding fiber back in to our diets is not the answer.

I have yet to see a single scientific study demonstrating that fiber solves any of our problems. At worst, fiber causes constipation, irritation and damage to the inner lining of the intestine, flatulence and pain. At best, fiber reduces blood sugar spikes by ten to twenty percent, reduces LDL cholesterol by about one-third, and promotes a “feeling of fullness”. If you are eating a diet that contains more than a small percentage of sugar and refined carbohydrates, you could eat a truckload of oat bran and still never see your blood sugar, cholesterol level or appetite come down to normal and stay there.

What I found especially fascinating in my research was this: I am not aware of a single study designed to evaluate the health effects of fiber that compares a diet containing fiber to a fiber-free diet. When you think about it logically, if you wanted to figure out whether or not a particular food is good for humans, you would want to design the study exactly that way—one diet WITH the food and one diet WITHOUT the food. Very simple, but it’s never been done! The closest thing we have to this kind of a study is the historical observation of traditional peoples like the Eskimos.

Here is an example of the kinds of studies that have been done to show that fiber is healthy:

TITLE: “Whole-Grain Ready-to-Eat Oat Cereal, as Part of a Dietary Program for Weight Loss, Reduces Low-Density Lipoprotein Cholesterol in Adults with Overweight and Obesity More than a Dietary Program Including Low-Fiber Control Foods.”8

Sounds promising, until you read the study and learn that there was no difference in weight between the two groups after twelve weeks, that LDL only decreased by about five percent, and waist circumference only decreased by about a half-inch. You read further and discover that the “low-fiber control foods” were refined-carbohydrate junk foods:

“…participants were randomly assigned to consume either two portions/day (3 c/day) of whole-grain ready-to-eat oat cereal (providing 3 g/day b-glucan) or low-fiber breakfast/snack foods (eg, ready-to-eat corn cereals, white toast, plain bagels and English muffins, pretzels, soda crackers, or rice cakes) with a similar energy and macronutrient content (control group).”8

Of COURSE oat cereal is (perhaps, a tiny bit) healthier than white toast, plain bagels, English muffins and pretzels! However, it is impossible to conclude that the oat cereal was beneficial—it is just as likely that the reason for the very modest benefits seen in the oat cereal eaters was not that they were eating more fiber than the comparison group, but that they were eating fewer refined carbohydrates (sugars and flours) than the comparison group. Or it may be that oats are not as bad for you as wheat and corn. To make matters worse, three of the study’s authors were scientists who worked for General Mills, the manufacturer of Cheerios, the oat cereal used in the experiment! Coming soon to a commercial near you…

Cardboard: yes. Delicious: no.

If you’re still not convinced that we are not designed to require fiber, I have no choice but to appeal to your good judgment and common sense. Pour yourself a nice big bowl of wheat bran, take a spoon out of your kitchen drawer, and dig in. I don’t mean a bowl of sweetened bran cereal with milk or yogurt or berries on top, I mean a bowl of 100% unsweetened wheat bran. Does it look or smell or taste good to you? Do you like it? Do your kids like it? Can you swallow it?

We want to believe in the power of fiber. It is so much easier and so much more appealing to contemplate adding fiber, which is tasteless and indigestible, than to contemplate subtracting refined carbohydrates, which are addictively delicious and fun to eat.

The Bottom Line about Fiber:

Fiber is not essential for human life or health.

Adding fiber to your diet cannot cure any health problem, because it doesn’t get to the root of the problem.

If you eat risky refined and high glycemic index carbohydrates regularly, soluble fiber may soften your blood sugar (and insulin) spikes and may reduce your cholesterol a little by interfering with their digestion.

If you find soluble fiber supplements useful, take care to drink plenty of water with them.

If fiber bothers your digestive system, or you don’t like eating it, you can safely avoid it, since it is not essential to your health.

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Last Modified: Sep 3, 2015