In Search of the Perfect Fiber (Part 1…quantity)

Human Baby Fiber Intake

When reading through my fiber recommendations, keep in mind the amount of prebiotic fiber a newborn human baby ingests.  Human breast milk is rich in Human Milk Oligosaccharides.  These HMOs are similar in structure to inulin and resistant starch.  A man-made substance, GOS, is often used in infant formula.  GOS is made by treating cow’s milk with certain enzymes that convert it into a prebiotic fiber. HMOs are well-studied for their ability to be persorbed into the bloodstream of the baby where they can remove pathogens from the bloodstream and organs.

Human breast milk contains prebiotic HMOs at approximately 15-25 grams per liter, and babies from newborn to 6 months consume about 1 liter of milk per day.  A substance known as colostrum that is secreted into the first couple squirts of breast milk has double the HMOs as the rest.  Colostrum has approximately 22g HMOs per liter, and the rest of the breast milk has about 12g/liter.

Some studies show that breast milk available to the newborn is much richer in HMOs and it lessens as the baby grows.  All of this HMO rich breast milk is credited with keeping babies “bulletproof” as their guts fill with Bifidobacteria gorging on the HMOs.

The fact that a baby gets 15-25 grams of prebiotic fiber should put things in perspective as I recommend 25-50g of prebiotic fiber for adults!  Another way to look at this; there are about 700 calories in a liter of breast milk.  A liter of breast milk provides 15-25g of “fiber.”  An adult consuming 2100 calories per day, then, should do very well consuming 45-75g of fiber daily!

Defining a proper intake

Trying to wade through the information available about fiber is tough. Most governments around the world, at least the caring ones, have issued proclamations that we need to eat more fiber. They feel it will reduce the burden of healthcare costs by making everyone healthier.

It seems a universally accepted fact that we need to eat fiber somewhere in the 20-40g/day range. Charts such as this can be found with just a few mouse-clicks:

(chart source)

The USDA recommends “25g for women and 38g for men.” Where, exactly, these recommendations came from is a bit of a mystery. There’s a big problem with these recommendations. The food that we eat does not contain this much fiber. In many studies, the average intake of fiber around the world is less than 15g per day, most of that “insoluble.”. The problem has to do with the “fiber density” of the foods we enjoy eating.

As most people are eating between 2000 and 3000 calories per day, even eating all ‘home cooked’ food just barely meets the minimum requirements for fiber. A closer look at a common fiber-contents chart also may be enlightening.

Most people reading this probably eat ‘healthier’ than 90% of ‘civilized people.’ However, getting enough fiber from our food is hard work. To add to the problems, there is much confusion about what types of fiber we need.

Soluble vs Insoluble or Fermentable vs Non-fermentable?

The entire conversation on fiber revolves simply around the content of “soluble” and “insoluble” fiber types. In reality, these two classifiers don’t mean much. It would be better if the world scrapped all this talk of fiber and went instead to a “prebiotic potential” scale. The only reason anyone needs fiber is for its prebiotic capabilities. The “bulking” actions of conventional insoluble fibers is often claimed to “make fuller stools” and “fill you up so you aren’t hungry.” But in reality, the magic of these non-fermentable bulking fibers is their propensity to slow down the fermentation of fermentable fibers and ensure that the proper levels of prebiotic, fermentable fibers reach throughout the entire colon.

The standard classifications of fiber into soluble and insoluble rather than a meaningful, action-based naming convention has caused more harm than good. Who cares, really, if a fiber dissolves in water? It would be better to know what happens to that fiber when it reaches your colon and is fermented by the bacteria that reside there, and what species of beneficial bacteria are boosted by the fiber.

The fiber types known as “insoluble,” ie. oat bran, wheat bran, cellulose, and psyllium husk play an important role in that they can bind with cholesterol and remove it from your body as well as add some “bulk” to the stool to make it easier to pass. This seems to be a very important evolutionary advantage to eating lots of high-fiber plants and seeds. For a great recap on the differences between soluble and insoluble fiber, here’s a great 2016 review: Understanding the Physics of Functional Fibers in the Gastrointestinal Tract: An Evidence-Based Approach to Resolving Enduring Misconceptions about Insoluble and Soluble Fiber

What feeds our gut flora, and what all do they produce?

People often ask, “What happens if we don’t eat fiber?  What do our gut microbes do then?”

“Fiber” is foods that we eat that aren’t digested in the small intestine, byproducts of digestion that spill over into the large intestine, cells shed during normal cell repair, and mucin produced in the large intestine.  Some microbes (bacteria/fungi) prey on other microbes.  Some microbes eat the dead bodies of other microbes.

Exogenous sources of fiber that come from the food we eat are resistant starch, non-starch polysaccharides, oligosaccharides, sugars, fats, and proteins.  There is an entire field of nutritional science devoted to glycans (as wikipedia defines: “compounds consisting of a large number of monosaccharides linked glycosidically”). These glycans are generally indigestible in the small intestine and can be found in just about everything we eat.

In nature, I believe you will find that resistant starch is the most commonly found fiber source with NSP and other oligosaccharides close behind.  The sugars, proteins, and fats that end up in the large intestine are leftovers from digestion in the small intestine.

If there is no fiber present in the diet, the most obvious effect is on stool formation.  The gut will be populated with bacteria and other microbes that enjoy eating the other “inputs.” This results in a stool commonly referred to as “rabbit poop,” but can also lead to impacted, hard-to-wipe stools and constipation.

Our gut bacteria ferment this fiber. Preferably the largest “output” will be short- and branched-chain fatty acids.  Other natural outputs are the gas and odoriferous chemicals produced.  In a fiber-free diet, less SCFA will be produced. Butyrate is particularly important and special cells that line the colon (colonocytes) rely on butyrate to fuel their metabolism.  When butyrate is not present, they can switch over to using glucose from their blood supply.  When fueled by glucose, colonocytes begin to behave differently and do not regenerate and repair themselves as the do when fueled by butyrate.

You can see this all in action with a couple of simple experiments.  Eat a diet of steak and eggs for two days and you’ll see profound changes in your stool.  Eat nothing but leafy greens and other non-starchy, non-fermentable fiber veggies and you’ll see another drastic shift.  Eat nothing for two days and you’ll see the stool produced by only endogenous “fibers.” If there happens to be no, or little bacteria present, as after a round of antibiotics, stools will not be formed and diarrhea results.  The colon can also flood itself with liquids in an attempt to quickly clear perceived threats.

I think a good judge of fiber intake is how your stool looks.  The Bristol Stool Chart is a great reference:

Picture credit

The ideal stool is in the Type 4 and Type 5 range.  They should be easy to pass, non-noxious, and easy to clean-up after.

The Fiber Menace

The book Fiber Menace is a great example of fiber misinformation.  The author, Konstantin Monastyrsky, wrote a very compelling book on the dangers of fiber.

This entire chapter in itself is a summary of the most prominent problems caused by fiber. Hence, the list of key points is brief:

  • Fiber from plants wasn‘t consumed by humans during most of evolution because until very recently there was no means to process fiber.
  • Sugars and starches are broken down in the small intestine, but the small intestine can‘t break down fiber because the human body lacks the necessary enzyme.
  • There are two types of fiber—soluble and insoluble. Soluble fiber causes osmotic diarrhea, because it retains water inside the large intestine. Insoluble fiber absorbs digestive juices and expands four to five times its original size. The expansion of insoluble fiber may cause esophageal, gastric, and intestinal obstruction.
  • Fiber interferes with gastric (stomach) digestion, and is the leading cause of indigestion, GERD, heartburn, gastritis, and ulcers.
  • Fiber obstructs the small intestines throughout their entire length, and is the primary cause of intestinal disorders. Because the intestines are responsible for the assimilation of nutrients, fiber-related inflammatory disease causes malnutrition, and an acute deficiency of vitamins and minerals.
  • Children are particularly vulnerable to fiber, because their digestive organs are smaller than adults.
  • Fiber is a primary cause of flatulence. These gases are formed during fiber‘s fermentation inside the large intestine.
  • Fiber increases stool’s weight and size, and causes mechanical damage to colorectal organs. Even minor damage leads to constipation. When more fiber is added to combat constipation, more damage is incurred.
  • Fiber‘s impact on the small and large intestines affects male and female genitourinary organs because of their proximity. Women are particularly vulnerable because female reproductive organs occupy a large space in the abdominal cavity, and because of the specifics of menstruation.
  • Fiber has no measurable affect on heart disease. If anything, it worsens the outcome because of the excessive carbohydrate consumption that comes with fiber.
  • Patients who try taking supplemental fiber to reduce cholesterol levels develop a “myriad” of digestive disorders
Mr. Monastyrsky’s demonization of “fiber” changed the way many people feel about fiber, including several well-known Paleo Diet leaders.
Mark Sisson wrote a nice piece on “Fiber” back 2010, many thousands of people read this:

CW says Americans need serious fiber in their diets. And by “fiber” CW often means bran buds, whole wheat, psyllium husks – you know, sticks and twigs roughage. We’re talking that 1980’s Saturday Night Live bit about Super Colon Blow cereal. Let’s just say that the more sensitive among us, in particular, want to broach the question: “Is this really the best way?”

What is the point of fiber anyway? What does it do? Well, on one hand, soluble fiber (vegetables, fruit, oatmeal, and legumes that partially dissolve in water) enhances the thickness of the stomach’s contents. This slows stomach emptying. While this can give the body more time to absorb nutrients, it can also “trap” minerals like calcium or zinc, binding them up in such a way that they don’t have the opportunity to be absorbed. Insoluble fiber (like whole grains, seeds and fruit skins) increases the mass of the stool, which actually moves the stool more quickly through the intestines. Insoluble fibers pass through the digestive system relatively intact.

Soluble fiber slows stomach emptying, which prevents the body from being flooded with glucose at the same rate as it would be with a low fiber meal (assuming a high glycemic load in the meal). But therein lies the pertinent question: if you maintain a diet with low glycemic load, do you really need to slow the digestion process with fiber? Hmm. If that fiber were adding a plethora of nutrients, as found in vegetables, then the answer would be yes. But as for a fiber source without all those nutrients? Not so convincing.

— I’ve read Monastyrsky’s book and am a big fan of his work. Thanks for sharing your story!
Mark Sisson wrote on February 22nd, 2010

Read more:

It was a great time in the world of dieting! People were turning their noses up at decades of advice from the government, their doctors, and ” old wives’ tales.” Fiber was just one of the dominoes to fall.

It was great to see Mark Sisson change his stance on fiber in 2013 when he realized that by downplaying the importance of fiber, he was removing nearly all vestiges of prebiotics from his dietary advice.

Last week’s guest post from Konstantin Monastyrsky, author of Fiber Menace, generated a lively, boisterous, and at times combative comment section. I use these descriptors in the best sense possible, mind you; debate is healthy and necessary, even – nay, especially – if it’s impassioned. So right off the bat, I want to thank everyone who wrote in. I also want to thank Konstantin, whose views on fiber forced me to reconsider my own way back when I first encountered him over five years ago. Without his input last week, we wouldn’t be having this discussion, and many people would still be sitting on whatever side of the fiber fence they prefer, never even considering that another side exists. I know I might still be catching up if I’d never read his book all those years.

Many of you asked whether I endorsed the views espoused in the guest post. You wondered whether I’d shifted my stance on the Big Ass Salad. You wanted my take on the whole fiber thing, basically. So without further ado, let’s discuss fiber.

It’s often said that fiber is indigestible, that it serves no nutritive purpose – and that’s partially true. Humans can’t digest fiber. Our digestive enzymes and endogenous pancreatic secretions simply have no effect on roughage. Our gut flora, though? Those trillions of “foreign” cells residing along our digestive tract that actually outnumber our native human cells? To those guys, certain types of fiber are food to be fermented, or digested. That we feed our gut flora these prebiotic fibers is important for three main reasons:

Read more:

Since Mark Sisson wrote the above, he has written many, many great posts on prebiotic fibers.  Mark’s Daily Apple continues to keep people well-educated and Mark Sisson is not afraid to change course when he sees that his advice may be lacking. Many other paleo bloggers and others have recently began seeing the folly of their ways and are now all discussing prebiotics instead of just “soluble and insoluble fibers.”

Short-Chain Fatty Acid Requirements

There is ample research on SCFA production in the gut and its effects. There is little research on exactly how much SCFA we need to enjoy all of its health benefits. The benefits of “fiber” are mainly all related to SCFA production and a healthy gut environment. While butyrate gets all the attention, other byproducts are just as important. Butyrate seems to be the key factor in maintaining a healthy gut environment, though.

Astute blogger, Melissa McEwan, was one of the first paleo bloggers to see that fiber was an important piece of the gut health puzzle, in her five part series, The Human Colon in Evolution, she showed this chart:

And said of it:

Interestingly, one of the top producers is something known as “resistant starch.” Resistant starch represents the growing nuance in understanding of fiber, since it is a starch that acts like a fiber in terms of acting as a bacterial substrate. It first showed up on the scientific radar when scientists found that low rates of colon cancer were not just found in populations with high-fiber diets, but those with high-starch diets (O’Keefe, Kidd, Espitalier-Noel, & Owira, 1999)1. Researchers found that a particular starch resisted digestion and ended up being fermented by colonic flora. They called this resistant starch and it is found mostly in cooked starches, some raw starches like green bananas, and some rough unprocessed grains and seeds. The former is termed type III and is a major part of the diets of many foraging populations who consume pounded and cooked starches like cassava, taro, true yam, and sago palm.

Also interested in the fermentation products of dietary fiber types was a researcher named J. H. Cummins, who was characterizing the byproducts of fiber fermentation in 2001. His paper, Prebiotic digestion and fermentation, contains this figure which is very useful in determining the types and ratios of fibers we may want to consume:

As you can see, different fibers create different proportions of short-chain fatty acids. If nothing else, this just proves that these fibers are causing bacterial activity with an end-point of increased SCFA over a diet lacking in these fibers.

Hopefully one day researchers will delve into the exact amounts and types of fibers we need to produce the amounts and types of short chain fatty acids we require. Until then, we are winging it using the best information we can find.

In Search of the Perfect Fiber:  Quantity Recommendations

I would like to offer a range of 20-50g per day as the amount of fermentable (classic ‘soluble’) fiber one should strive to ingest on a near-daily basis. I don’t think there is any need to target a specific amount on non-fermentable (classic ‘non-soluble’) fibers if you are eating several servings of fruit and vegetables most days. This amount applies to everyone, irregardless of age or sex.

While I’m at it…what’s with “old people” needing less fiber?  Every official chart I’ve seen says that people over 50 need about 1/3 less fiber.  What is going to happen to me when I turn 50?  Maybe they think that when you are 50 you are over-the-hill and about to die, so no sense in having decrepit old people hog all the good fiber from the young whippersnappers.

And women?  Why do women need less fiber?

In my world, old people and women will get MORE fiber.  Babies?  Kids?  Sure, they eat less than adults (maybe) so they will be getting less fiber.  I guess if I was to make a formula, it would be something like “Fermentable fiber should be consumed at the rate of 15-25g per 1000kcal of food.”
Modern diets give us approximately 15g of total fiber per day. Judging by the current state of the world’s guts, this is an inadequate amount, especially if it is nearly 2/3 “insoluble” non-fermenting fiber.

Archeological evidence estimates that certain populations of hunter-gatherers may have eaten as much as 135g of fiber per day. Looking at food/fiber charts, you can see that this would be a difficult feat to match today unless you ate like a gorilla.

There is probably no dangerous upper limit to fiber consumption as long as one does not replace all dietary carbohydrates with fiber.

I am also convinced that eating this amount of fermentable, prebiotic fiber can become second-nature to us, though it may require supplementing. I hate to think that I am adding to worries about what we eat and adding another layer of orthorexia to your day. In that regard, let’s not get too wrapped up in counting fiber grams, but let’s do examine our daily diets for fiber sources and see where we stand. I think everyone will find they fall short in fermentable fiber.

In the next post, we’ll discuss real-food options for obtaining this amount of fermentable fiber.

Tim Steele



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