Dietary fiber: Eat more of it


There’s a reason why many people recommend 8-10 servings of veggies a day, much to the malaise of society in general. Apparently that’s too many servings of veggies to eat yet 8-10 of grains is okay but to the inherent tastiness of the two groups and compatibility with sandwiches. (The latter of which is too much IMO, unless your BMR is much higher than normal or you need it to fuel endurance events such as a triathalon; the majority of dieters don’t need anywhere near 8 servings of grains)

I digress already, I just hate that recommendation of Eating Well with Canada’s Food Guide (Woo, Canada!); US is similar.

Back on topic; most people only know a few facets of dietary fiber, mainly that it makes you poop and can lower cholesterol. Few people distinguish between the two overarching classes of fibers (soluble and insoluble) and even less know that the amount of different types of fiber are counted with double digits.

Also, this is the first post in which I am competent enough to actually upload pictures; I’ll put it to relatively good use.

Relatively Good use = One helpful chart on fiber types and a bunch of tasteless jokes

The anatomy you need to know

After the stomach, food passes into the small intestine. The three areas of the small intestine (duodenum, jujenum, ileum) absorb most foodstuffs that then go to systemic circulation. If not, they go to the colon (large intestine) where they undergo microbial fermentation. Food can still pass to and fro systemic circulation in the large intestine, so select nutrients (such as minerals, water, and some short chain fatty acids) that pass the main absorption centers can still be taken up by the body.

Then you poop it out.

That’s all you really need to know for this article.

Types of Fiber

The two main classes of dietary fiber that are on food labels are soluble and insoluble, classified via how they interact with water. There is also a third class called ‘Resistant starches’ which, simply, are starches that are resistant to digestion in the small intestine (where it is normally absorbed) yet can be fermented by bacteria in the colon.

Soluble fiber is a class of fibers that include Pectin, Beta-Glucans, Chitin, Inulin, Fructan, and Fructooligosaccharides/Oligosaccharides. These compounds are both soluble in water and have high fermentability in the colon.

Insoluble fiber consists of Cellulose, Hemicellulose, and Lignans. These typically have low fermentation. Although cellulse and hemicellulose are not listed on the label as insoluble fiber, they are not listed at all. (If you weight a head of broccoli, it may come to over a pound (454g), yet looking at the label you only see about 30-50 nutritional grams; after factoring out the water content, the difference not accounted for is the cellulose/hemicellulose content)

Resistant starch has low fermentation, whereas a class of similar-ish compounds (Resistance Dextrins) have high fermentability.

Resistant starch and dextrins are a unique class of compounds; they will not be covered in this article at this time.

Fermentability, by the way, means microbial health and short chain fatty acid production (via bacteria). Fermentable fibers are prebiotic by definition. [x]

Soluble Fiber: Pretty much everything good about fiber

Soluble fibers are those that are soluble in water. They absorb water when associated with chyme and soften stool while simultaneously slowing digestion. (Note: Chyme is just the word for the unrecognizable clump of nutrients that your food has become in the gut, and will eventually be called your stool)

Via this softening, soluble fiber can mechanically prevent any hard objects in the chyme from rupturing cells lining the gut.

Slowing of digestion allows more time for intestinal cells (in the small intestine) to work away on the surface area of your chyme. If the chyme goes through too fast, there is a chance that much of the inside nutrients will not be absorbed (in theory). This is the main idea of people touting soluble fiber for digestion, as it increases the overall percent of nutrients consumed via time in the small intestine, and is one of the mechanism that you can see food parts in the toilet if you had an upset stomach; they were inside the chyme and not enzyme treated in the gut.

The slower digestion goes hand in hand with a reduced uptake of the cholesterol ingested, counter-intuitively, especially when you just read the opposite above (think of cholesterol as the exception to the rule). More soluble fiber with a meal will reduce the overall amount of cholesterol taken up into the liver by that meal. This is one of the two mechanisms by which fiber can ‘lower cholesterol’ [x]

Soluble fibers have also been associated with easing digestive disorders (such as IBS and Ulceritive Colitis) and potentiating (strengthening) the intestinal walls. [x] [x] [x] And also Crohn’s Disease via the compounds discussed in the next section [x]

Short Chain Fatty Acids

Firstly, I apologize for the spike of non-human studies in this section when talking about cellular mechanisms. It’s kind of hard to get ethical approval and funding if your study cuts people open to investigate their insides. The word ‘harikiri’ and the phrase ‘gut like a fish’ are not all that easy to put in an application for approval.

For science!

Pig guts are similar to humans for the most part, so if you just take the studies with a grain of salt they should be fine. Also, to reiterate, there are still a lot of human studies here; just the applied ones, not the gutting ones.

With that being said, onwards!

Short chain fatty acids (hence SCFAs) are as the name implies. They are fatty acids, but with shorter carbon chains. They are produced in the colon by bacteria from any carbohydrate source that is highly fermentable. Fibers are the main source of interest here (as well as resistant starches/dextrins) since they do not get taken up by the human’s small intestine and pass on to the colon, and insoluble fibers are not highly fermentable, hence the reason this appears in the soluble fiber section. [x]

Short chain fatty acids can, in general, modulate pH in the colon. This indirect modulation of pH can increase absorption of select minerals such as Calcium, Magnesium, and Iron (remember how I said nutrients can still interact with the colon? Prime example) and reduces ammonia uptake from the body. [x] [x] [x]

They also contribute calories, it differs between SCFAs, but an approximate amount is 1.7-2kcal per gram.

It’s pretty much insignificant and not worth people to worry about though. Aside from a myriad of health benefits, an entire 100g of soluble fiber would be 170-200kcal. And if you eat 100g soluble fiber, you ain’t eating much else.

  • Butyrate/Butyric Acid

One of the breakdown products when bacteria eat Acetyl-CoA from fermentable fiber sources (the other breakdown product being Acetate)

Butyrate seems to have anti-inflammatory properties as well by interacting with Cytokines (which are, aside from an awesome sounding word, modulators of inflammation in the body). [x]

Butyric acid has also been shown to increase insulin sensitivity (via PPAR activation, it’s a gene thing) and energy expenditure (via themogenin, or heat production) in mice. The dose was pretty high, so don’t put too much hope into increasing your energy expenditure unless you really down the fermentable fibers. [x] However, this study also found increased glucose uptake and activity of glucose transporters (GLUTs, they’re a pretty big deal) via gene expression. The theory of which follows [x]

(Note: These are ass-GLUTs by the way (glucose uptake in colonocytes), the above study did not try to note any GLUT activation in peripheral tissues. The above study also noted that the benefits may be shared amongst SCFAs in general, and not just butyrate.)

A simple over of how genes work is thus, personified as a business. The genes (printers) exist in the cell’s nucleus (office) and get transcribed by proteins (mail room clerks), transcription just tells the genes what to do. Certain messaging molecules (office workers) tell the proteins (mail room clerks) what to do via receptors (e-mail?).

Enter histones. They’re contract workers who don’t care about their job and just want to shoot the shit with the mail room clerks; they’re kinda pushy too, so they usually get their way. They interfere with transcription.

Butyrate is one of the security guys, and he inhibits a class of enzymes called Histone deacetylases (HDACs). Acetylated histones seem to interfere less with transcription, so it is favorable to have more histones in the acetylated form rather than the deacytelated form. (So, for our scenario, he stops the contract workers from getting their security passes into the office area; thus reducing interference)

Not sure the significant of this reaction, but butyrate and HDAC are under research right now for possible anti-artherogenic effects as well [x] [x]

Increased levels of histone aceylation are also correlated with longevity, but whether or not this applied to soluble fiber intake has yet to be answered. (Although fiber intake is correlated with longevity as well, we do not yet know the causative factor) [x]

Given the HDAC inhibitor mechanism works for GLUT2 receptors in colonocytes, helps insulin sensitivity via PPAR upregulation, and is under research for anti-artherogenic effects via transcription as well; it may be reasonable to suggest that butyrate and HDAC inhibitors in general could have beneficial transcription effects over the entire body (as histones are ubiquitous). Butyrate is also released into systemic circulation, so it has a way to act on all cells.

  • Acetate

Acetate is the most proliferate SCFA. It can be taken up by myocytes (muscle cells) and used as fuel. It has the ability to be used as a substrate for butyric acid due to it’s simple molecular structure.

Don’t think it’s going to make a difference in exercise capacity though, 50-70% of it is taken up by the liver, and this is after the colonocytes (butt cells) get their share, which admittedly is not overly significant. [x]

It is also a substrate for cholesterol synthesis, and can increase hepatic cholesterol production.

  • Propionate

Seems mostly to be taken up by the liver. [x] It has the ability to suppress (in part) cholesterol synthesis, which is the second way dietary fiber can ‘lower cholesterol’ that I alluded to earlier. It’s possible propionate can be used as a substrate for gluconeogenesis (production of glucose from non-glucose sources). Similar to acetate above, it is unlikely to show much salient effects due to the amount of it, you need a lot of it to turn into a minimal amount of glucose. [x]

Much research on propionate in humans is preliminary. Research on it in the past has been on cattle (ruminants) and is not exactly able to be extrapolated to humans due to digestive and fermentation differences in both species metabolisms.

Insoluble Fiber: Y U NO useful?

Awesome list of the benefits of soluble fibers and their derivative SCFAs, huh? Let’s look at the extensive list for insoluble:

  • Eases constipation and makes you poop

For real? Just one?

Okay, maybe not just one; the second one is pertaining to fat-soluble toxins in the body, so it may not be applicative to everybody; just those undergoing drastic fat loss or have low enough body fat so that serum levels of these toxins become a concern when losing body fat further.

Fat soluble toxins (assumed that they are stored in adipose tissue) can undergo two fates when the adipose tissue no longer becomes full of triglycerides and they get ejected. They can either:

  • Go to the liver, get metabolized into a more water soluble metabolite, and then get excreted from the body via urine.
  • If the toxin cannot be metabolized by the liver, it gets ejected into the small intestine, and await an insoluble transport out of the body via fecal lipid content.

If there is no insoluble transport, the fat-soluble toxins is taken back up by the liver and redistributed to systemic circulation. Over time, serum levels could theoretically increase if it’s not excreted. (Maybe a reason that dieters at low body fat levels feel sick when losing a lot of fat really fast? Who knows).

The above is theoretical when applied to insoluble fiber though. Science has only elucidated this transport out of the body with nutraceutical intervention, aka, feeding people Orlistat, which inhibits fat uptake and makes an abundance of insolube transports. Administering Orlistat has been shown to decrease levels of organochlorine compounds in the body (fat-soluble pesticides) [x]

I would imagine that some fatty acids are required, though, for insoluble fiber to work in the same manner. Toxin adheres to fatty acid, fatty acid adheres to insoluble fiber, insoluble fiber makes a mad dash for the exit. Just my conjecture. (Thankfully, flax seed has a high fat content as well; and is known for turning one’s colon into a shotgun)

So in sum, aside from two conditional benefits; insoluble fiber really doesn’t show much else. There’s even some talk about how insoluble fiber could mechanically damage cells if it goes through the body too fast with no soluble fiber to slow it down, but so far this is just a logically sound theory with no evidence behind it (to my knowledge). It may be wise to always pair soluble fiber with the insoluble though; just to be on the safe side.

Sources of Fiber

Most, if not all, natural fiber sources have a blend of both soluble and insoluble fibers. The following list mentions food which predominately contain one of the types of fibers.


  • Legumes
  • Oats, ryes, chia, barley
  • Fruits such as Prunes, Plums, Berries, Apples, and Pears.
  • Veggies such as Brocolli, Carrots, and especially Jerusalem Artichoke.
  • The bulk component of root tubers (Sweet potatoes, onions, etc.)


  • Whole grain foods
  • Wheat and corn bran
  • Nuts and Seeds
  • Root Tuber skins (Potato skins, Onion skins, etc.)
  • Flax seed, otherwise known as the fuse for the colonic bomb
  • Select fruits such as Avocado
  • Veggies such as Green beans, Cauliflower, Nopal, Zucchini, and Celery.

Pictured above: Nutritional Gunpowder

It should be noted that the above classifications are based on relative percent of fiber and not total fiber amount. Celery is mostly insoluble but overall has a pretty shit total of dietary fibers; it’s almost all water content.

Many of the above studies on soluble fibers and their benefits were on Inulin and Fructooligosaccharides, which are thought to be the two best sources of soluble fiber when it comes to microbial health. (So if looking for a fiber supplement, look for these two; although Pectin is turning out to be pretty nice)

Fructooligosaccharides, as well as oligosaccharides in general, are very high in legumes. Although Wikipedia has a comprehensive list of the sources of Inulin, the ones that you are likely to eat include Wild Yam, Jerusalem Artichoke, Onions, Agave, and Jicama. (Yes, those are the ones you are likely to eat, ever want to munch down on a pound of dandlion or coneflower?). Pectin is most known for being in apples, although it is fairly ubiquitous.

That’s an Jerusalem artichoke, note the lack of resemblance to Artichoke

So, yeah; what the hell do I do with this info?

I would recommend partitioning your fiber intake more towards soluble sources than insoluble. Insoluble isn’t bad in any way, but it’s not that good either when compared to the fermentable sources. Maybe look for the Fructooligosaccharide and Inulin sources above, but don’t stress too much about those.

Essentially, a higher fiber intake may be overall ‘better’ than a low fiber intake not because of fiber itself, but because a higher intake inherently means more soluble fiber.

Given how many benefits of soluble fiber come from SCFA production, and although the type of fiber influences which SCFAs are produced they are still produced. Supplementing with a soluble fiber source such as Metamucil, whether or not you need to, may provide added benefits via gastrointestinal health and gene transcription maintenance (think of this as just preserving health).

With this, I see no reason why anybody in a healthy state should be consuming less than 30g of fiber a day. It’s easy to do with high fiber foods and even easier with Metamucil. (Some exception given to those on ketosis diets, but still, Metamucil can be bought carb-free)

If you don’t want to eat a bunch of raw veggies or buy soluble fiber supplements, you have two options:

  1. Suck it up
  2. Make chili with a lot of beans and throw in 2 whole onions, or eat some Borscht with a few onions and a whole head of cabbage, or stews in general. The soupy medium ensures that all water-soluble nutrients are kept in the broth and the veggies are no longer a course on their own, but compliments to the meat in the dish.

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  1. Hyakiss says:

    Thank you for the article. I was not even aware of the distinction between insoluble and soluble fiber, though I think it’ll take a few more read-throughs to really absorb all the information here. In the meantime, can you give a tl;dr version of what the end benefits of soluble fiber are? Theoretically it makes it easier to absorb nutrients from your food and also lower cholesterol?

    Are there any implications from this for fitness as apposed to general health?

    When you recommend 30g of fiber a day, what percentage of soluble fiber do you have in mind?


  2. Silverhydra says:


    Soluble fiber enhances nutrient absorption (except cholesterol, which it reduces the amount you get from food) and softens stool, protects and repairs intestinal cells, and ferments into fatty acids. These fatty acids can help modulate insulin sensitivity and help regulate every cell in the body to a small degree.

    The only implications for fitness are the benefits vicariously through health (you can’t squat with an upset stomach or the runs, fiber intake may promote feeling of well-being which would then give you better workouts), and then butyrate (fatty acid) may increase metabolic rate by increasing heat production and fat loss (although it’s not miraculously significant, just a nice side effect of fiber)

    Don’t think there are any significant implications on muscle gain/preservation though.

    As for >30g fiber a day, don’t try to micromanage the fiber content; your head would explode really fast if you tried to micromanage everything. Just try to get more soluble fiber sources in your diet, the insolubles will work their way in there somehow.

  3. pierce inverarity says:

    Hi Silverhydra — Thanks for the great site. I’ve really been enjoying it since I stumbled across it a few days ago.

    I’ve read in multiple places that fiber (especially a fiber supplement) can bind with different vitamins and minerals and prevent their absorption. Thus, “they” recommend to take vitamins and fiber at different times. But it sounds from your article like the opposite is true — that fiber helps to absorb nutrients. Any idea where this discrepancy comes from? Would you recommend taking a fiber supplement with other vitamins or supplements, or at different times?


    • Silverhydra says:

      Fiber *itself* doesn’t impede too much nutrient absorption, but a nutrient commonly consumed in veggies alongside fiber called phytate does. Phytate basically is a shell of Phosphorus molecules that love to grab minerals and bind to them, and prevent their absorption. Some fiber supplements do also contain phytate.

      I wouldn’t worry about it too much, to be honest. I have never come across a study where the inhibition was 100%. Usually if there is inhibition, the phytate or whatever is preventing about 20% of the vitamin uptake; the other 80% is still quite sufficient.

      I don’t see a need to consume fiber supplements and multivitamins at different times.


  1. […] Fiber is easy to get! Just eat some low carb/green veggies and supplement with some kind of fiber additive. My prefered foods are broccoli and leafy green salads. The greener the better.  I supplement with simple psyllium husks (aka metamucil). Just mix it in with some water, let it expand and gel, and gulp it down! About a spoonful is all you need. For a more scientific explanation for why fiber is so important, check out Silverhydra’s page about it. […]

  2. […]  Aus dem Englischen von N.D. “Furor Germanicus” M. / copyright by & Kurtis Frank […]

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