“A lie, repeated enough times, becomes reality.” – Joseph Goebbels

Nothing likes more to the food industry to make your own interpretation of the studies (often financed by themselves) and use deceptive arguments to sell more products.

The subject of fiber is a good example, but before going into detail, a bit of history.

The fiber theory

The general recommendation to increase the amount of fiber we eat is recent. The idea became popular in the 1970s, and is largely due to the effort of one person, the Doctor Burkitt. In his work in Africa, Burkitt identified that many of the common gastric diseases and conditions in developed societies (constipation, diverticulosis, colon cancer …) did not occur in Africa, and concluded that one of the protective elements in the diets of those populations was fiber.

It didn’t take long for the marketing managers of the large food companies to start promoting cereal bran (high in fiber) as a fundamental part of the diet, taking advantage of the popularity that cereals had been gaining in recent decades, especially as part of breakfast. Of course it did not matter that there were many other factors that might influence different rates of disease, nor that the individuals Burkitt studied in Africa did not eat bran or many grains (as Burkitt himself tried to explain), and that their fiber came mainly from vegetables and fruits. But the agribusiness was not going to let events ruin a good business opportunity, and its aggressive campaigns promoting the benefits of cereals, thanks to their contribution in fiber, intensified.

This coincided with two other important phenomena:

  • The demonization of fats and the cholesterol, and the mistaken idea that they are harmful to health.
  • The so-called “liberation” of women. I will not question the positive aspects that resulted from this movement (which are many), but without a doubt the incorporation of women into corporate life led to less cooking in the home in general.

And so, the ease of opening a package of sugary cereals (yes, with a lot of fiber) definitely replaced the process of cooking nutritious eggs in the breakfast. Food industry 1 – Humanity 0.

What really is fiber?

We could say that fiber is a complex carbohydrate, not hydrolyzed by our body, and therefore it is not really considered a nutrient. Despite talking about fiber in a generic way, there are many types of complex molecules with these properties (cellulose, pectic substances, resistant starch, inulin, lignin …), which come from different plant products.

Due to their chemical characteristics and their effect on the body, all these substances are generally grouped (and somewhat arbitrarily) into two large classifications:

  • Soluble fiber: This type of fiber captures a lot of water, increasing in size, and produces a viscous solution in our digestive system that slows down intestinal transit. In theory, this gives our body more time to absorb nutrients and reduces peaks in insulin in blood when dosing glucose release. This fiber is present in legumes, some cereals (oats and barley), root vegetables, some vegetables (such as beets and carrots), and many fruits.
  • Insoluble fiber: Insoluble fiber passes through the intestine unchanged, and is mainly fermented in the colon. Although it is true that in some cases it can improve intestinal motility, consumed in excess it can produce a lot of gas and abdominal distention, in addition to interfering with the absorption of minerals such as zinc, magnesium or calcium, especially if that fiber comes from cereals, high in antinutrients (such as phytates).

The current recommendation for fiber is 30-35 grams per day, which seems very elevated, considering that it is equivalent to about 7-8 bananas a day, or 1.5 Kilos of spinach.

The result of this obsession with fiber is, as usual, the use by manufacturers of processed foods, which traditionally eliminate natural fiber from their products (to extend life), and then add insoluble synthetic fiber, which which also allows them to put a claim on the container, advertising the product as ‘fiber source’ (if it has more than 3% fiber) or even ‘high fiber’ (if it has more than 6% fiber). There is little data that can lead us to believe that there is something healthy about these pseudo-foods, and in fact that extra fiber may be causing more problems than it solves.

The studies

One of the arguments that has maintained the popularity of whole grains is the large number of studies that show improvement when incorporating these foods. There are two main factors that explain these results:

  • Many studies actually compare the effect of replace refined cereals with whole cereals, so it is logical that the result is positive. After all, anything that reduces the peaks of glucose (and insulin) in the blood will help fight obesity and many of the associated diseases (diabetes, certain types of cancer, cardiovascular diseases …). If you add some additional nutrients from whole grains to this, it is clear that they are generally better than refined grains. However, what we should ask ourselves is if we would improve even more by eliminating all the fiber from cereals (refined or not). I dare say that in most cases, the answer would be yes.
  • Those who follow the general recommendations to eat a lot of fiber based on cereals, are the same ones who tend to follow other recommendations (these are good ones) such as not smoking, moderate alcohol consumption and exercising more, so in general they Those who eat more whole grains have better health than those who do not, but the reason is not the cereals. Despite the fact that statistically attempts are made to isolate the variables, there will always be a positive bias when the factors are so interrelated. These epidemiological studies can only show correlation, not causality.

Fiber consumption has traditionally been associated with improving intestinal transit, reducing various diseases of the digestive system, and even cancer. Many recent studies seriously question these beliefs.

Fiber and ‘regularity’

If I got a coin for every ad from some constipation sufferer whose life has been improved by eating All-Bran, I’d be a billionaire. This great propaganda effort has made many people with toilet problems try to solve it with more fiber, making the problem bigger.

This interesting study, which analyzes the effect of reducing fiber intake for two weeks in people with constipation, concludes that the people who improved the most were those who reduced fiber the most, and those who eliminated it improved even more. Hmmm, curious …

In the words of the study itself «… The role of fiber in constipation is analogous to that of cars in traffic. The only way to improve slow traffic is to reduce the number of cars. If we increase the cars, the traffic worsens …«.

This other study, this time in children, concludes that there was no significant benefit in increasing fiber, and this other he also does not find constipation problems due to a low fiber intake.

What many are unaware of is that an increase in fat intake can help improve constipation. Instead of whole grains, eat more fatty fish, avocados, some nuts, olive or coconut oil, eggs, and even a supplement of fish oil. Constipated vegetarians are not uncommon, and the problem is clearly not a lack of fiber, but a lack of fat.

On the other hand, some doctors recommend fiber supplements only when you want to stop diarrhea (analysis), which seems quite logical to me.

Fiber and diverticulosis

Another of the ‘miracles’ attributed to fiber is the prevention of a fairly common disease, diverticulosis, or the appearance of lumps (called diverticula) in the colon, and associated diverticulitis, when these lumps become inflamed.

Many studies, such as this, find a higher incidence of diverticulosis in individuals with higher fiber consumption.

Fiber and cancer

Despite the fact that the ‘fiber hypothesis’ promulgated in the 1970s placed great emphasis on the protective factor of fiber in colorectal cancer, studies in recent decades tend to dismiss this idea, especially with regard to the fiber from cereals.

In 1999, one of the studies broader on the subject, where almost 90,000 women participated, during 16 years. Conclution: “Our data do not demonstrate the existence of a protective effector of fiber against colorectal cancer or adenoma.«.

An intervention study also concluded shortly after that a low-fat, high-fiber diet had no positive effect on colorectal adenoma recurrence.

It also seems pretty clear that supplementing with wheat bran (high in fiber) does not prevent the recurrence of colorectal cancer (study). Other study similar concludes «… Neither the fiber intake of a wheat bran supplement nor the total fiber consumed affects the recurrence of colorectal adenoma, adding more evidence to the existing literature that indicates that a diet high in fiber, especially rich in cereal fiber, does not reduce the risk of colorectal adenoma«.

As a curiosity, one of the theories that Burkitt himself proposed to explain the higher incidence of colorectal cancer in modern societies was our unnatural way of ‘going to the bathroom’. The modern insistence on spend all day sitting, even for such… personal issues, it does not facilitate a complete evacuation of the colon and increases the pressure in the intestine. This idea, esoteric to some, has increasing scientific support, and societies that squat (as in many areas of Asia) have a much lower prevalence of digestive diseases, regardless of fiber consumption. It seems that the posture of squat It is useful for much more than developing strong legs :).


I want to make it clear that I am not attacking the fiber, and I do believe that maintaining a reasonable intake of fiber helps to maintain a good microbiota, which for me is much more important than the amount of fiber itself.

Some people, following diets very low in carbohydrates, and without consuming fermented products, for long periods, can develop problems in the intestinal flora.

But you will have all the fiber you need, and a healthy microbiota, following a diet rich in vegetables, fruits and tubers (better carbohydrates). If you are on a diet ketogenic or you are taking antibiotics, it may be advisable to take some prebiotic, which is basically a type of fermentable fiber (food for our friendly bacteria).

Obsessing over disproportionate traditional recommendations for fiber (drilled to death by doctors and industry), mostly sourced from grains, makes little sense from an evolutionary perspective, and very little scientific backing.

More detail on the best types of fiber.