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Dietary Fiber – It’s Natures Broom

Years ago I remember lamenting (and writing somewhere) that I was fairly sick of reading research papers on how eating more dietary fiber was good for people, how it was time for nutritional science to move into relatively more interesting things than a topic that had literally been beaten to death.

Thankfully, soon thereafter leptin was discovered and nutritional researchers could start looking at things more interesting than why eating high-fiber vegetables were good for you (a nutritional tidbit that I file under the “Grandma was right” category).

Even so, there is still some confusion out in the world of nutrition regarding fiber.  And boring or not, it’s a topic worth clearing up.  So today I want to take a fairly comprehensive look at dietary fiber, what it is, what it does in the body, how it impacts on things like body composition (and health to a lesser degree) and finish by looking at some (admittedly vague recommendations).

What is Dietary Fiber?

.Generally speaking, fiber is included within the category of dietary carbohydrates (many athletes or bodybuilders divide carbohydrates into starchy and fibrous for example).  But fiber is distinct enough to be considered separately from other types of digestible carbohydrates.  Perhaps surprisingly, defining what is and isn’t a fiber is actually a more complicated issue than most would think.

Chemistry, botanical and physiology types all sort of want to use different definitions and spend altogether too much time arguing about what is and what isn’t a fiber.  Since I’m less interested in chemical or botanical issue than physiological ones, I won’t bore people with the details of those of those definitions and arguments.  Rather, I’m interested in the physiological effects and, hence, the physiological definitions.

Even there there are two primary definitions which are used:

  1. Soluble vs. insoluble (aka viscous vs. non-viscous)
  2. Fermentable vs. infermentable

I suspect that most are at least passingly familiar with the first definition above.  If not, here’s what it means.  Soluble fibers go into solution in liquid.  A good example is guar gum, if you put a spoonful in water and mix it, it will turn into this gel-like mass because it is soluble in fluid

Insoluble fibers, in contrast don’t do this, you can mix them until the cows come home but they won’t ever go into solution.

I suspect that readers are relatively less familiar with the fermentable vs. infermentable definitions.  I’ll come back to this below when I talk about the caloric value of fiber but, simply, some fibers can be fermented (specifically by bacteria in the intestine) into other things (e.g. short-chain fatty acids, CO2 or methane) while others are infermentable (they cannot be converted into those other things).

What Does Dietary Fiber Do in the Body?

Fiber has a number of different effects in the body which are relevant to both health and body composition.  It’s worth noting that, strictly speaking, fiber is not an essential nutrient.  That is, you won’t die if you don’t eat it regularly or at all.

Quite in fact, there are cultures such as the Alaskan Inuit and the African Masai that subsist on a diet that is essentially devoid of fiber.  However, that doesn’t mean that a sufficient intake of dietary fiber isn’t good for you or can’t provide either health or body composition benefits.

Below, I’ve listed a bunch of the major effects of dietary fibers (and note that some of these occur in the upper GI tract, others in the lower but I’m not getting into that much detail) in terms of their physiological effects.

  1. Promoting fullness/satiety
  2. Slowing gastric emptying
  3. Decreasing nutrient absorption
  4. Improved glycemic control, secondary to delayed gastric emptying and impaired nutrient absorption
  5. Decreasing blood cholesterol
  6. Decreasing mineral absorption
  7. Effects on insulin sensitivity via fermentation to short-chain fatty acids
  8. A number of effects relevant to colon cancer
  9. Helps with poopin’

I want to touch on each below although I’m going to focus more on some than others.


One of the myriad signals for fullness during or after a meal has to do with the physical stretching of the stomach.  And high-bulk foods are far more likely to do this than low-bulk foods.  In this context, meals or foods high in fiber generally contain a lot of bulk in few calories which give high-fiber foods a low energy density.

Thus they tend to make people feel fuller both in the short- and long-term.  In this context, I recall a rather “brilliant” study a few years back which found that people who ate salad first in a meal ate less total calories.  The high-bulk, high-fiber items filled them up so that they ate less of the more calorie dense foods.   Another one for the “Grandma knew best” file.

In a slightly different context, it’s worth noting that individuals who have trouble meeting their energy requirements (e.g. athletes or “hardgainers”) may find it better to save salads for the end of the meal specifically so that they don’t get full too soon before eating the higher calorie part of the meal.

Slowing Gastric Emptying

As I mentioned above, soluble fibers tend to form a gel-like substance in liquids and one consequence of a high soluble fiber intake is that gastric emptying (the rate at which foods empty the stomach) is slowed when they are eaten.  Basically, they cause the chyme (the partially digested nutrients in the gut) to form this big gel which empties the stomach more slowly.  This, along with the physical stretching of the stomach tends to keep people fuller in the longer term because the food stays in the gut longer.

Impaired Nutrient Absorption

Another effect, again primarily seen with soluble fibers, is an impairment of nutrient absorption, and this holds for carbohydrates, fats and dietary protein.  Essentially, due to the gel-like mass that is formed, digestive enzymes can’t get access to the other nutrients so that more is carried out of the body.

This means that high-fiber diets will result in less total caloric absorption since more will be lost in the poo.   Basically the left-hand side of the energy balance equation will be lower when a large amount of soluble fiber is consumed.

The effect isn’t massive, fiber may reduce total fat absorption by about 3%, protein by 5%.  I can’t find a good value for carbohydrates at the moment.   Put more concretely, an increase in dietary fiber from 18 to 36 grams per day might reduce total caloric absorption by ~100 calories per day.

Now, depending on how you want to look at this, it can be seen as either a good or bad thing.  For individuals trying to lose weight, higher fiber diets will not only have positive effects on fullness and the rest but will result in less total calories being absorbed from the diet.  Again, the high-fiber nature will reduce the Energy In side of the equation (which only counts calories which are actually absorbed).

On the other hand, for athletes or bodybuilders, the impact of a high-fiber intake could be seen as detrimental, especially given that soluble fibers impact on protein absorption.   While it would be nice if fiber only impacted on carb or fat absorption, that simply isn’t the case.  There is some loss of protein with high-fiber intakes.

As well, for athletes with very high energy demands, losing digestible energy due to a high fiber intake might not be the best thing.  Again, I’d note that the total impact isn’t massive but it is worth considering.

Improved Glycemic Control

One of the most well-known and talked about effects of a high-fiber intake is improvement in blood glucose control. Between the slowing of gastric emptying and impairment of carbohydrate digestion, high soluble fiber intakes tend to improve blood sugar control.

Meaning that blood glucose levels tend to be more stabilized rather than showing larger spikes due to rapid digestion followed by crashing blood glucose.  In that crashing blood glucose can be another stimulus for hunger, this can have an additional impact on hunger control between meals (especially important when dieting).

Decreasing Blood Cholesterol

I’m actually not going to talk about the impact of fiber intake on blood cholesterol levels in great detail.  Sufficed to say that high-fiber intakes tend to improve blood lipid levels and do this through a variety of different and inter-related mechanisms.  If you want more detail than that, pick up a nutrition textbook.

Impairment of Mineral Absorption

In addition to global impacts on carbohydrate, protein and fat absorption, dietary fibers can also negatively affect mineral absorption especially calcium, magnesium, sodium and potassium.  I’d note that, in general, this isn’t really an issue for concern unless the intake of those nutrients is insufficient in the first place.

As well, when fiber intake is increased from foods (as opposed to dietary supplements), there is generally an increase in mineral intake in the first place which should help to offset any issues.  For example, the fiber intakes of our evolutionary diet is thought to be massive (some have estimated it at 100-150 grams per day) but nutrient deficiencies aren’t seen.  This is most likely due to the fact that the fiber is coming from nutrient dense fruits and vegetables.

However, when people start adding horse-doses of fiber supplements to their diet, problems can start.  Older readers may remember the bran craze in the 80’s, when bran was found to lower cholesterol, people starting eating it in massive amounts.

But they were often doing it from purified sources rather than whole foods.  While this may have improved cholesterol levels, it ended up causing issues with mineral imbalances because the massive fiber intake was not accompanied by an increase in nutrient intake.

Effects on Insulin Sensitivity via Fermentation to Short Chain Fatty Acids (SCFA)

As I mentioned above, another categorization of fiber is that of fermentable vs. non-fermentable, referring to whether a given fiber can be fermented (via the bacteria in the gut) to other things.  The other things that most are familiar with are hydrogen, carbon dioxide and methane; these are what cause the gassiness that can occur with high-fiber intakes.  Specifically, methane is what give farts their wonderful smell.

But fiber can also be fermented to short-chain fatty acids such as acetate, propionate and butyrate that are re-absorbed into the body and which have a variety of physiological effects.  One of those is to provide calories, a topic I’ll come back to shortly.  But the other is to impact on fuel metabolism.

The short-chain fatty acids provided by fiber fermentation impact on both fat cell metabolism and insulin sensitivity.  And while these short-chain fatty acids positively impact on insulin sensitivity, they appear to do it by blunting the release of fatty acids from the fat cell.   Yes, that says what you think it says: high-fiber intakes may be limiting fatty acid release from fat cells.  I’ll come back to this below.

A Number of Positive Effects Relevant to Colon Cancer

Again, not a topic I’m going to get into much detail on.  Sufficed to say, high-fiber intakes have a number of physiological effects that reduce the risk of colon cancer.   Quiet in fact, most of the supposed colon cancer causing effects of meat is due less to the presence of the meat and more to the absence of dietary fiber.  It’s simply that people who eat lots of fatty charred meat tend NOT to eat lots of fruits and vegetables.

Helps You Poop

And, of course, possibly the most well known effect a high-fiber intake is regularity and comfort in pooping.  That’s actually what the title of this piece refers to, I have often commented that fiber is nature’s broom.  It helps sweep stuff through the intestines and out the other end.  It does this through a number of mechanisms.

First and foremost, fiber speeds the transit time of food from one end of the intestines to the other.  So rather than sitting in the intestines, it moves towards the exit more quickly.  As well, fiber contributes to fecal bulk, essentially the size of the poo that is produced.  This increase in bulk also pulls more water into the fecal mass which makes the poop softer and easier to pass.  Both of these latter effects further contribute to the decreased transit time and all of this contributes to better regularity.

And, at the end of the day, who can argue with a good poop?

Fiber and Energy Balance

Relevant to issues of body composition, fiber can contribute in a number of ways to the energy balance equation.  As noted above, fiber impacts on caloric absorption (decreasing it, generally) along with fullness (which may cause people to spontaneously eat less) along with blood glucose control and several other mechanisms.  In general, the effect is to reduce either total food intake or caloric absorption, facilitating weight loss.

I’d mention again that the effect of fiber on fat cell metabolism via the conversion to short-chain fatty acids is perplexing, one way of looking at this is that high-fiber intakes might hurt with fat loss.  This might become more relevant when people get very lean and fatty acid mobilization is becoming more difficult (for reasons discussed in The Stubborn Fat Solution).  At the same time, real-world results call the real-world significance of this into question.  High-fiber intakes have been part of hardcore diets for decades and folks seem to be doing alright.

Depending on the goal (e.g. weight loss vs. weight gain), this can be seen as good or bad depending on the context.  For individuals trying to lose weight, most of the effects of a high-fiber diet could be seen as generally positive.  Being fuller with more stable blood sugar and absorbing fewer calories would seem a good thing.

As noted above, for individuals trying to increase their energy intake and/or gain weight, a high-fiber intake could potentially be a negative.  Between making the individual fuller at a given meal and/or keeping them fuller longer during the day, along with impairment of caloric absorption, high-fiber intakes might have a negative impact overall for some people.

Fiber Provides Calories to the Body

But there is another effect of fiber on energy balance that often goes unappreciated.  It’s often stated that fiber provides no calories to the body since humans lack the enzymes necessary to digest it.  This has often been taken even further to claim that high-fiber vegetables are “negative calorie foods”, that is they take more calories to chew and digest than they provide (assumed to be zero).

But here’s the thing: it’s not true.  Well it’s not entirely true.

Above I discussed the issue of fermentation of some types of fiber to short-chain fatty acids which are then reabsorbed by the body.  Well, those fatty acids provide calories to the body.  While there is still some debate in the area, researchers have generally assigned a caloric value to fiber of 1.5-2 cal/gram (depending on the specific type).

Admittedly this is an average and will depend on the specifics of the diet and the type of fiber but, simply, the idea that fiber provides no calories to the body is not true. While the caloric value of fiber is still lower than starchy carbohydrates (4 cal/g), it is not zero.

Dietary Fiber Recommendations: How Much and What Kinds?

So how much fiber do we need?  As noted above, strictly speaking fiber is not an essential nutrient; you might be healthier with it but if you never ate another gram you would not die.  You might want to die when you tried to poop but you wouldn’t actually die without it.

Due to the non-essentiality of fiber for human survival, it’s hard to make specific recommendations for daily fiber intake.

The American Dietetic Association recommends an intake of 10-13 grams of fiber per 1000 calories consumed.  This is roughly 20-30 grams per day for an average day’s diet of 2000-3000 calories per day.  It’s worth noting that the average fiber intake in the modern diet is about 10-11 grams perday which is far below this.

Most people would probably benefit from eating more fiber but they’d also generally benefit from eating more fruits and vegetables generally.  Not only do those foods provide fiber but they also provide endless other nutrients that are either essential or may improve healthy.

As I mentioned, our evolutionary diet is thought to have contained absolutely massive amounts of fiber on average, intakes of 100-150 grams/day has been thrown around in some scientific papers.  I would note again that this would have come from the intake of massive amounts of fruits and vegetables, providing numerous other nutrients (especially minerals and vitamins) that wouldn’t be found if you tried to get that much fiber from isolated supplements.

In that context, it’s worth mentioning that high-fiber foods, typically fruits and vegetables, contain tons of other nutrients important to health or what have you so looking only at the fiber content can be a bit misleading.  Getting adequate amounts of high-fiber fruits and vegetables on a daily basis has benefits far beyond just the fiber content.  Getting some at each meal would seem to be a good thing.

But how much should you really eat? And from what types of foods?

Frankly, there is very little hard and fast data on optimal fiber intakes for any goal.  Too little is bad, too much is probably bad.  Somewhere between those two extremes is about right.    People eating the modern diet get too little fiber and should increase it.  I’ve seen some meal plans that, frankly, included absurd amounts of fiber (folks with eating disorders often do this type of thing to stave off the gnawing hunger).    High fiber intakes can be problematic in some types of digestive disorders.

Find balance, people.

Depending on meal frequency, somewhere between 5-10 grams of fiber per meal would seem a decent place to start.  That should provide anywhere from 30-60 grams of fiber per day, covering average recommendations without being excessive.

I would note that if you fiber intake is currently low, do NOT try to increase it drastically in a short period of time. The body needs time to adapt to big increases in any nutrient intake and people who jump their fiber intakes massively often pay a hard price in terms of gas and such.

Finally, on the topic of types of fibers, I personally don’t get overly concerned with it.  The soluble/insoluble fibers can be further subdivided into a whole host of other categories but I consider this nutritional minutiae of little real relevance.  If you strive to consume a variety of fruits and high-fiber vegetables on a day to day basis, you’ll get a mix of fibers and cover your bases without having to worry too much about it.

In specific situations, fiber supplements may play a role (for example, soluble fibers such as guar gum can be put into yogurt/protein powder mixtures to thicken it up and/or help with fullness on a diet).  And many will use psyllium husks as a form of insoluble fiber if they are having issues with constipation or what have you.  But, for the most part, I’d rather see people increase their intakes of high-fiber whole foods rather than use purified supplements.

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16 thoughts on “Dietary Fiber – It’s Natures Broom

  1. How often should a healthy fibre full individual deficate then? If I’ve low carbed in the past I’ve sometimes gone 2-3 days without any bowel movement, can this be bad?

  2. Lyle,

    Where would things like lentils and various other legumes (along with certain grains) fit in to the picture? The current trend seems to be to include a serving or two of these but to avoid going overboard (and as you noted increase fruit and vegetable intake). I have also heard that the mthod of preparation can impact on the amount of “digestible” carbohydrates, but I am not knowledgeable in that area. Do things like soaking and fermenting legumes and grains predigest (so to speak) these foods and make more calories (along with nutrients) available for absorption?

  3. Hi, Lyle.

    I never like to be too much of a reductionist, but it seems like the “Grandma was right theory” basically accounts for a good chunk of eating for health and results. Whole foods + moderation/sensibility seems to cover an awful lot………………..granted it is still great when sharp people like you go into more detail, so I always appreciate a visit to you website.

    Given your comments about fiber supplements versus whole foods, what is the deal with glucomannan. I’ve noticed people (2 or 3 in particular) pimping this stuff, and while it sounds helpful in theory, it always struck me as trying to get people to spend money on something that didn’t offer much bang for your buck. Is there more than meets the eye there, or is it another one of those things that isn’t necessary and still gets touted?

  4. Hi Lyle

    You wrote: “As I mentioned, our evolutionary diet is thought to have contained absolutely massive amounts of fiber on average, intakes of 100-150 grams/day has been thrown around in some scientific papers.”

    Care to post 2 or 3 of the best links / refs to back this up?



  5. Lyle, you wrote: ‘Sufficed to say, high-fiber intakes have a number of physiological effects that reduce the risk of colon cancer. Get a textbook for more.’

    Am I correct to assume that in these textbooks I will be able to see clear biological mechanisms that show how fiber is protective of bowel cancer? Or will I just find flimsy hypotheses based on even flimsier epidemiological studies which have countless known and unknown variables?


  6. Tal: Not something I pay huge amounts of attention to. But the major textbook I was looking at at the time I wrote this had a number of direct physiological mechanisms that had been studied. Not just epidemiology.

    And I believe the 100-150 g/day fiber estimates come from the text of these papers.

    Eur J Clin Nutr. 2002 Mar;56 Suppl 1:S42-52.Click here to read Links
    The paradoxical nature of hunter-gatherer diets: meat-based, yet non-atherogenic.
    Cordain L, Eaton SB, Miller JB, Mann N, Hill K.

    Department of Health and Exercise Science, Colorado State University, Fort Collins, Colorado, USA.

    OBJECTIVE: Field studies of twentieth century hunter-gathers (HG) showed them to be generally free of the signs and symptoms of cardiovascular disease (CVD). Consequently, the characterization of HG diets may have important implications in designing therapeutic diets that reduce the risk for CVD in Westernized societies. Based upon limited ethnographic data (n=58 HG societies) and a single quantitative dietary study, it has been commonly inferred that gathered plant foods provided the dominant energy source in HG diets. METHOD AND RESULTS: In this review we have analyzed the 13 known quantitative dietary studies of HG and demonstrate that animal food actually provided the dominant (65%) energy source, while gathered plant foods comprised the remainder (35%). This data is consistent with a more recent, comprehensive review of the entire ethnographic data (n=229 HG societies) that showed the mean subsistence dependence upon gathered plant foods was 32%, whereas it was 68% for animal foods. Other evidence, including isotopic analyses of Paleolithic hominid collagen tissue, reductions in hominid gut size, low activity levels of certain enzymes, and optimal foraging data all point toward a long history of meat-based diets in our species. Because increasing meat consumption in Western diets is frequently associated with increased risk for CVD mortality, it is seemingly paradoxical that HG societies, who consume the majority of their energy from animal food, have been shown to be relatively free of the signs and symptoms of CVD. CONCLUSION: The high reliance upon animal-based foods would not have necessarily elicited unfavorable blood lipid profiles because of the hypolipidemic effects of high dietary protein (19-35% energy) and the relatively low level of dietary carbohydrate (22-40% energy). Although fat intake (28-58% energy) would have been similar to or higher than that found in Western diets, it is likely that important qualitative differences in fat intake, including relatively high levels of MUFA and PUFA and a lower omega-6/omega-3 fatty acid ratio, would have served to inhibit the development of CVD. Other dietary characteristics including high intakes of antioxidants, fiber, vitamins and phytochemicals along with a low salt intake may have operated synergistically with lifestyle characteristics (more exercise, less stress and no smoking) to further deter the development of CVD.

    Mayo Clin Proc. 2004 Jan;79(1):101-8.Click here to read Links

    Comment in:
    Mayo Clin Proc. 2004 May;79(5):703; author reply 703-4, 707.

    Cardiovascular disease resulting from a diet and lifestyle at odds with our Paleolithic genome: how to become a 21st-century hunter-gatherer.
    O’Keefe JH Jr, Cordain L.

    Mid America Heart Institute, Cardiovascular Consultants, Kansas City, MO 64111, USA.

    Our genetic make-up, shaped through millions of years of evolution, determines our nutritional and activity needs. Although the human genome has remained primarily unchanged since the agricultural revolution 10,000 years ago, our diet and lifestyle have become progressively more divergent from those of our ancient ancestors. Accumulating evidence suggests that this mismatch between our modern diet and lifestyle and our Paleolithic genome is playing a substantial role in the ongoing epidemics of obesity, hypertension, diabetes, and atherosclerotic cardiovascular disease. Until 500 generations ago, all humans consumed only wild and unprocessed food foraged and hunted from their environment. These circumstances provided a diet high in lean protein, polyunsaturated fats (especially omega-3 [omega-3] fatty acids), monounsaturated fats, fiber, vitamins, minerals, antioxidants, and other beneficial phytochemicals. Historical and anthropological studies show hunter-gatherers generally to be healthy, fit, and largely free of the degenerative cardiovascular diseases common in modern societies. This review outlines the essence of our hunter-gatherer genetic legacy and suggests practical steps to re-align our modern milieu with our ancient genome in an effort to improve cardiovascular health.

    Eur J Nutr. 2000 Apr;39(2):71-9.Click here to read Links
    Dietary lean red meat and human evolution.
    Mann N.

    Department of Food Science, RMIT University, Melbourne, VIC, Australia.

    Scientific evidence is accumulating that meat itself is not a risk factor for Western lifestyle diseases such as cardiovascular disease, but rather the risk stems from the excessive fat and particularly saturated fat associated with the meat of modern domesticated animals. In our own studies, we have shown evidence that diets high in lean red meat can actually lower plasma cholesterol, contribute significantly to tissue omega-3 fatty acid and provide a good source of iron, zinc and vitamin B12. A study of human and pre-human diet history shows that for a period of at least 2 million years the human ancestral line had been consuming increasing quantities of meat. During that time, evolutionary selection was in action, adapting our genetic make up and hence our physiological features to a diet high in lean meat. This meat was wild game meat, low in total and saturated fat and relatively rich in polyunsaturated fatty acids (PUFA). The evidence presented in this review looks at various lines of study which indicate the reliance on meat intake as a major energy source by pre-agricultural humans. The distinct fields briefly reviewed include: fossil isotope studies, human gut morphology, human encephalisation and energy requirements, optimal foraging theory, insulin resistance and studies on hunter-gatherer societies. In conclusion, lean meat is a healthy and beneficial component of any well-balanced diet as long as it is fat trimmed and consumed as part of a varied diet.

  7. Lucas: since there tends to be less residue on low-carb diets (protein and fat being digested with extreme effiiency), there is often a reduction in pooping frequency. This is even true with fairly high-fiber intakes. I’m not sure why.

    Arthur: Certainly cooking tends to increase digestible carbohydrate and caloric absorption. There’s actually a very interesting book I just read called Catching Fire by Stephen Wrangham that deals with this. Finding exacting numbers is more difficult than you might expect.

  8. Lyle,
    I hope you see this message, and I apologize for bringing up an old topic – then again, I hope it will please you that I looked for this article first and read it before asking you my question.

    I would appreciate it if you could clear this up for me.

    My question is this: is it not insoluble fiber (ISF) that impairs nutrient digestibility?

    Following are 3 studies (1 using humans, 1 using sows, and 1 using dogs) I have found that state that it is ISF that lowers the metabolizable energy (ME) content of food, not soluble fiber (SF).

    Despite the human study being the most relevant, the type of fiber seems to have consistent effects across the board. I don’t know that you have access to the full texts of these studies, as I am quoting from these texts and not the abstracts to which I linked you. For your ease of reading, I have removed the parenthetical citations to older references in the quotations.

    The human study:
    “The overall effect of increasing mixed fiber sources, particularly those that contain a substantial amount of insoluble fiber, is to decrease the digestibility of energy-yielding nutrients and to decrease the amount of ME available from the diet. ”

    The sow study:
    “Two possible mechanisms may explain increases in energy digestibility when SF, but not ISF, is included in the diet for sows. First, SF delays gastric emptying due to its ability to swell and form a viscous material). Delayed gastric emptying improves digestive and absorptive efficiency. Second, increased SF intake increases bacterial populations in the large intestine as evidenced by increased excretion of bacteria in feces. The large populations of intestinal microbes likely increased fermentation and utilization of SF to a greater extent than ISF. Microbial fermentation in the large intestine produces VFA* that can satisfy up to 30% of the maintenance energy requirement of the pig.

    In contrast to SF, ISF decreased energy digestibility of diets. Insoluble dietary fiber decreases intestinal transit time, which limits nutrient digestion and absorption. The primary components of ISF are cellulose, hemicellulose, and lignin, which are less likely to be utilized and fermented by gastrointestinal flora compared with SF. Low digestibility caused by ISF components explains why greater intakes of ISF increase the amount of plant material excreted in the feces and increases fecal bulk.”

    *I could not find what VFA stood for either in the study itself or the cross references.

    The dog study:
    “Among the SH-containing diets, there was a linear increase in corrected fecal output as I:S increased. Insoluble fiber is associated with increased fecal bulk. The soluble fiber is available for fermentation by hindgut microflora and does not increase fecal bulk as substantially as insoluble fiber. Therefore, the higher the I:S in the SH-containing diet, the more total feces excreted.”

    I apologize in advance if I am behind in the research. I am still searching the NCBI database for more studies.


  9. Hi Lyle, Thanks for the great article.

    The one question I had was about the 100-150 g/day figure. I’ve looked at the papers you listed and they don’t list a exact number, one has a table comparing the different diets and just says that fiber intake was High in Palaeolithic diets compared to other diets,

    The closest thing to a number I can see is a quote

    “The typical
    Paleolithic diet compared with the average modern Ameri-
    can diet contained 2 to 3 times more fiber,”

    Cordian defines a typical modern American diet intake as being 15grams in another paper:

    Origins and evolution of the Western diet: health implications for the
    21st century1,2
    Loren Cordain, S Boyd Eaton, Anthony Sebastian, Neil Mann, Staffan Lindeberg, Bruce A Watkins, James H O’Keefe,
    and Janette Brand-Miller

    “The fiber content (15.1 g/d) (23) of the typical US diet is
    onsiderably lower than recommended values (25–30 g) (116).”

  10. Going from memory, the values I threw out are in the full text of the paper but I don’t have copies handy any more to check.

  11. Methane as a gas doesn’t have any smell. The farts smell is from hydrogen sulfide.

  12. “I’d note that the effect isn’t massive, fiber may reduce total fat absorption by about 3%, protein by 5%. I can’t find a good value for carbohydrates at the moment. Put more concretely, an increase in dietary fiber from 18 to 36 grams per day might reduce total caloric absorption by 100 calories per day.”

    Can you post the studies that reference these values? This seems really interesting. Thanks!

  13. They are from some nutrition textbook or another.

  14. Oh okay, don’t worry. I found something by the FAO that seems to back it up… sort of:

    “There is also a certain amount of evidence that fibre in the diet may reduce the apparent available energy from fats and proteins. When moderate amounts of fibre are introduced into the diet as wholemeal bread, fruit, and vegetables, small reductions in the energy value of all the nutrients occur, amounting to an overall fall of 2–3% in the availability of dietary energy. Further increases in the fibre content of the diet, with fruit and vegetables at the levels consumed by vegetarians, result in an additional 2–3% reduction in available energy (5).”

    5. Southgate, D.A.T. & Durnin, J.V.G.A. Brit. J. Nutr., 24: 517–535 (1970).

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