Marks Daily Apple
Serving up health and fitness insights (daily, of course) with a side of irreverence.
9 Sep

Dear Mark: What’s the Deal with Fiber?

Fruits and VegetablesLast week’s guest post from Konstantin Monastyrsky, author of Fiber Menacegenerated 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:

1. Because the health (and composition) of the gut flora helps determine the health of the human host (that’s us!). It’s difficult to name a physiological function or health parameter that is not impacted by the gut microbiome, including but not limited to digestive, cognitive, immune, emotional, psychological, metabolic, and liver health. Our microbiota depend on fermentable fibers for food. It’s not clear what exactly constitutes “healthy gut flora,” and we’re still teasing out exactly how it affects the various physiological functions, but we know we need them and we know they need to eat something to even have a chance at helping us.

2. Because the short chain fatty acids that are byproducts of fiber fermentation, including butyrate, propionate, and acetate, improve our health in many ways. Butyrate in particular has been shown to have beneficial effects on insulin sensitivitycolonic transportinflammation, and symptoms of Crohn’s disease. It’s also the preferred fuel source for our native colonic cells. Basically, without enough butyrate (and, by extension, fermentable prebiotic fiber to make it), our colons don’t work as well as they should. This can lead to digestive impairments and perhaps even cancer. Mucin-degrading bacteria predominate in colorectal cancer patients, for example, while butyrate-producing bacteria rule the roost in healthy patients without cancer. Populations with lower rates of colorectal cancer also tend to have higher levels of butyratePropionate is helpful, too, though not to the extent of butyrate.

3. Because by feeding and bolstering the populations of “good bacteria,” we reduce the amount of available real estate for “bad bacteria” to set up shop. Gut bacteria don’t just float around in there. They cling to surfaces, nooks, crannies, and crevasses. They’re impossibly small, but they do take up space. After antibiotic treatment where both good and bad gut flora are indiscriminately targeted and wiped out, pathogenic obesity-promoting bacteria take advantage of the open space. That’s a worst-case scenario, but it shows what can happen when the harmony of the gut is disturbed (whether by antibiotics or lack of fermentable fibers).

Overall, because the health of our gut community is inextricably tied to the health of our minds and bodies, I think attaining fermentable fiber through the fruits and vegetables we eat is incredibly important. Heck, even the only food that’s actually expressly “designed” to feed humans – breast milk – contains prebiotic compounds whose main purpose is to feed and cultivate healthy gut flora in infants, which suggests that the need for prebiotics is innate. Coprolite (read: ancient fossilized stool) studies certainly indicate that our ancestors may have consumed a significant amount of prebiotics, and we even have receptors and transporters built-in to handle and accept the butyrate produced from fermentable fiber. Or, if you want to say that humans haven’t evolved a dietary requirement for fiber, that’s fine. But we have evolved to rely on gut flora to help our bodies work best, and that gut flora has evolved to require a steady, varied source of fermentable, prebiotic fiber. That can’t be denied.

have been suspicious of fiber in the past, though. Like Konstantin, I’ve discussed the folly of loading up on the kind of fiber whose only purpose is to rend the intestinal walls, doing enough damage to induce mucus secretion which acts as lubricant. I’m talking about insoluble fiber, of course.

Insoluble fiber is a bulking agent. You know how weight lifters swear by whole milk and beef for adding mass? Insoluble fiber is like that, only for poop. It makes for extremely impressive toilet bowl displays and potentially expensive plumber fees, I’ll admit. And some people “need” to feel like they’ve done something down there. They like to take a peek after a bowel movement and let the distinct sensation of accomplishment wash over them. But for digestive health? I’m unconvinced, and there’s not much evidence in favor of it. Optimally, stool is made up of mostly water and bacteria – not undigested food.

The health claims just don’t add up.

For one, insoluble fiber doesn’t ferment very well. That’s why neither we nor our gut microbes can digest, say, cellulose-rich grass – we don’t have the hardware, and neither do our gut flora. No fermentation, no short chain fatty acid production.

How about constipation? Bulking up your stool is supposed to improve symptoms of constipation, right? That’s why almost every doctor will tell you to “eat more fiber” upon hearing that you’re constipated. It’s gotta be evidence-based advice! Well, the actual evidence is rather weak. A recent meta-analysis concluded that while increasing dietary fiber does increase the frequency of bowel movements, it does nothing for stool consistency, treatment success, laxative use, and painful defecation. So it will make you poop more often, sure, but each bowel movement is going to hurt and you’re still going to need laxatives to do it. Another recent study found that stopping or reducing dietary fiber intake reduced constipation.

Or cancer? One recent study compared the fiber intakes and gut flora composition of advanced colorectal cancer patients to healthy controls. Healthy controls who ate high-fiber tended to have more butyrate-producing microbes than low-fiber healthy controls and high-fiber cancer patients, suggesting that it’s not “fiber” that protects against cancer but “fermentable fiber.” The cancer patients who ate high-fiber were likely eating insoluble, cereal-based fiber, which was not protective. This jibes with an older study’s results: while both fruit and vegetable fiber were associated with lower risks of cancer, cereal fiber – which is mostly insoluble – was associated with a slightly higher risk. Another study found similarly protective links between fruit and vegetable fiber and stomach cancer, but not grain fiber.

If you desperately need to execute a double decker at the home of a sworn enemy, load up on insoluble fiber beforehand. Otherwise, stick to what insoluble fiber you’ll get as a byproduct of eating real fruits and vegetables.

How many fruits and vegetables should I eat, you might be wondering? And does that mean soluble, fermentable fiber, and lots of it, is fair game?

It depends. I know that’s not a sexy, easy answer, but it’s the right one. Allow me to explain.

The Sub-Saharan farmer who’s spent his life handling and milking goats, picking up and distributing manure, working the fields, plunging his bare hands into fresh loamy soil to plant a seed or pull a weed, taking his meals with soil still underneath his fingernails, eating lots of fiber-rich vegetation (often without washing it), and encountering not a single dose of antibiotics is going to have a more robust, varied gut microbiome and greater capacity to handle fiber than the suburban pencil pusher (perhaps that term needs updating – let’s go with keyboard rattler or desk jockey) who’s spent his childhood mostly indoors wearing a perma-sheen of hand sanitizer and sunscreen while eating a diet of peanut butter and jelly on white bread, mac and cheese, hot dogs, and pizza, and his later years wracked by chronic low level stress that disrupts his gut flora and alters his digestion.

Should you fear fruits and vegetables because of the fiber? Has your modern upbringing ruined your digestive capability forever? No, I don’t think so. It may have temporarily impaired your ability to handle fermentable fibers – increasing numbers of people are reporting trouble with the class of fermentable carbohydrates known as FODMAPs (check the FODMAP list at this PDF) largely because they don’t have the right levels/populations of gut flora – but it isn’t permanent. You just need to be aware of the complex, delicate interplay between the food fiber we eat, the composition and health of our gut flora, and our digestion. You should pay attention to your own digestion and how fiber affects it. You should introduce foods rich in soluble, fermentable fiber gradually and even cautiously. Allow time for your gut flora to adjust to the new food source. Expect flatulence.

But you should definitely introduce them.

Eventually, you’ll be able to fish a sample out of the toilet, snap a shot of it with your iPhone camera, and have your entire gut microbiome analyzed on the spot, complete with dietary fiber recommendations for optimal butyrate production and minimal flatulence, but that’s a long while off. Scientists are still figuring out which gut flora are best, which species are good and which are bad, what kind of fiber source they like, and how often and how much we should feed them. In other words, we’ve reached the stage of knowing enough to know that we know very little. In the meantime, we know “gut flora are important.” We have vague ideas of which populations are “good” based on correlative studies that link certain species with diseases. We know we need gut flora because of their endless interactions with the host, and that they need food. We know that the plants (and breast milk) we eat provide that food.

And that’s about it.

It’s enough to get started, though. I’d say between 75 and 100 grams of carbs from mostly vegetables and some fruits, plus the occasional emphasis on plants particularly heavy on the prebiotic fiber – stuff like raw onion and garlic, leeks, jerusalem artichokes, dandelion greens, raw plantains and green bananas – should provide sufficient food for your gut. If you have too much, you’ll know it.

Oh, and the Big Ass Salad is definitely here to stay.

That’s what I’ve got, folks. What say you?

You want comments? We got comments:

Imagine you’re George Clooney. Take a moment to admire your grooming and wit. Okay, now imagine someone walks up to you and asks, “What’s your name?” You say, “I’m George Clooney.” Or maybe you say, “I’m the Clooninator!” You don’t say “I’m George of George Clooney Sells Movies Blog” and you certainly don’t say, “I’m Clooney Weight Loss Plan”. So while spam is technically meat, it ain’t anywhere near Primal. Please nickname yourself something your friends would call you.

  1. Fat is the new fiber! Lol. :)

    Nomad wrote on September 10th, 2013
  2. Amazing to read this just when my husband had started complaining that his ‘toilet habits’ weren’t like they used to be and he thought he was constipated! Now I realise that what we are both experiencing is normal after a few weeks of eating in a primal style.

    Loving this life style change :)

    Kim R wrote on September 10th, 2013
  3. Really interesting!

    When I first was trying to change my gut health (GAPS diet), I couldn’t get enough onions. I would make a bunch of veggies and onions and I’d end up picking out all the onions because that was what tasted good to me. I wonder if my body knew I needed them?

    Debbie wrote on September 10th, 2013
  4. Totaly agree with the good and bad bacteria theory!

    Romzzz wrote on September 11th, 2013
  5. The replies to this post tell me that most people do not understand the concept of prebiotic fiber.

    Using a nutrition label to get adequate fiber for prebiotic purposes will do the exact opposite of what you are trying to do…it will get you loads of non-fermentable fiber and fiber that is fermentable by the wrong species of gut microbes.

    As Mark said in the blog above:

    “You should introduce foods rich in soluble, fermentable fiber gradually and even cautiously. Allow time for your gut flora to adjust to the new food source. Expect flatulence….But you should definitely introduce them.”

    Studies show that an intake of 10-30g/day of prebiotics is optimum, and it takes 3 weeks to even out the microbial populations. Plan on a 3 week phase-in period, start at lower amounts. Too much too soon will result in major gas and bloating.

    And Mark said:

    “Eventually, you’ll be able to fish a sample out of the toilet, snap a shot of it with your iPhone camera, and have your entire gut microbiome analyzed on the spot, complete with dietary fiber recommendations for optimal butyrate production and minimal flatulence, but that’s a long while off.”

    Actually, it’s not that far off–the American Gut Project will mail you a swab kit and analyze your gut flora and compare you to the rest of the world based on eating habits, $99.

    and Mark said:

    “Scientists are still figuring out which gut flora are best, which species are good and which are bad, what kind of fiber source they like, and how often and how much we should feed them. In other words, we’ve reached the stage of knowing enough to know that we know very little. In the meantime, we know “gut flora are important.” We have vague ideas of which populations are “good” based on correlative studies that link certain species with diseases.”

    There are thousands of species in our guts–some we will never understand. But we do know, without a doubt, that healthy people have guts dominated by a strain known as ‘Bifidobacteria’ and that unhealthy guts are associated with ‘Enterobacteria’. These microbial populations are easily reversed with diet. A diet low in prebiotics and high in SAD foods causes a gut filled with Enterobacteria, leads to leaky gut, auto-immune conditions, lactose intolerance, poor glucose regulation (insulin resistance), hypercholesterolemia, and poor absorption of minerals. A diet high in whole foods with minimal sugar and loads of plant prebiotics ensures a gut dominated by Bifidobacteria and better glucose regulation, lipid oxidation, robust immune response and healthier colons.

    And Mark said:

    “We know we need gut flora because of their endless interactions with the host, and that they need food. We know that the plants (and breast milk) we eat provide that food.”

    Here’s my thoughts on how to easily take charge of this issue:
    1. Eat your daily BAS
    2. Eat fermented foods (yogurt, kefir, sauerkraut, etc..) often.
    3. Avoid sugar, flour, and vegetable oils
    4. Specifically target foods high in resistant starch…greenish bananas, raw/dried green plantains, cooked and cooled potatoes and rice, and properly prepared legumes. RS is easier to get in larger amounts than inulin and other prebiotics!
    5. They sell all kinds of prebiotic supplements. Most contain 1-3g of inulin, pectin, or gums. Some are resistant starches made of special corn or barley. If you want to supplement your prebiotic intake, I like unmodified potato starch (not flour), tapioca starch (AKA tapioca flour), and plantain flour. These need to be added to a smoothie or a cold drink–can’t be heated. 1TBS contains about 8g of prebiotics, and will not cause blood glucose to spike in the least. These real-food items are cheap and last a long time, prepackaged supplements contain fillers (often wheat) and are expensive.

    Tim wrote on September 11th, 2013
  6. I think guy flora is the basis for a procedure that takes healthy weight people’s stools and puts it in obese people. I think the theory is the healthy person’s flora will help reset the balance of good vs bad in the over weight person’s gut.

    ngyoung wrote on September 11th, 2013
  7. There is an excellent list for Paleo FODMAPs in by Aglaée the Paleo dietitian.
    My flatulence and bloating are absent when I stick to that list.

    Paleo FODMAPs wrote on September 11th, 2013
  8. I never had constipation issues before I went Primal over a year ago, but after about a month or so my bowel movements became very difficult. My poop is hard and painful to pass, and when I wipe there is only a bit of blood – It’s almost like I don’t really need to wipe at all!

    Not sure what to do about this; I’ve just been tolerating it ever since it started.

    Egglet wrote on September 13th, 2013
  9. Okay, I guess this is the best post for me to ask my very basic question:
    Do I have to eat vegetables?
    I really don’t like them and have only ate them because I was trying to be a good boy. The ones I do eat since going Paleo are sweet potatoes, onions and garlic (just because they season my meat nicely).

    I eat fruits rarely, maybe 2 or 3 times a week.

    In the past I always ate salads, broccoli, blah, blah, blah,… but I rather not. I know that they’re are rich in minerals and stuff, but am I okay without them?

    I am taking a good Probiotic supplement, Vitamins D3 and C as well as Omega 3’s.

    It’s kind of obvious the answer I’m looking for, but I’m willing to what’s best regardless.

    Thanks to all :)

    Joey wrote on September 17th, 2013
  10. Help! Impacted bowel in 85 year old…Been reading all these comments avidly waiting for some comments about the value of chia seeds but no mention! How come? I was hoping that I might find some insight for helping with my elderly Mum’s issue of impacted bowel for which she was hospitalised for afew weeks ago. She has some narrowing of the colon. She has been prescribed Movicol sachets which look just like chia gel when made up. But she’s not taking them regularly as had afew issues with overflow. The consultant (we are in the UK here) had never heard of chia and dismissed them as non evidence based mumbo jumbo

    gillyd wrote on October 10th, 2013
  11. I have always known that I need to either eat a salad or a big plate of fresh fruits for lunch every day that’s the only way I stay regular. I have Hashimoto’s Hypothyroidism and one of the side effects is constipation but if I eat my fruits or salad I don’t have that problem.

    Fancee wrote on June 27th, 2014
  12. Super disappointing in Mark for missing the mark and addressing a bigger picture. Everything about fiber and what it should be doing for your body and what it fails to do… Nothing in the article about Fiber being ESSENTIAL for DETOXification. NOWHERE in this article is the word Detox even used! C’mon Mark, dig a little deeper and look at the next level or what fiber is doing for us not just surface idiocy about how big your stools are. Your system needs to flush and cleanse toxins through which fiber, in addition to water, does. I was inadvertently poisoned by pesticides and had to heal myself and did extensive research into detoxing and thank God, came back from that horrendous experience.. Fiber is key to your body ridding itself of waste buildup in the intestines. Additionally, I was having major issues from lack of fiber in my rectum area, dealing with fissures… After multiple issues and two doctors visits and having surgery there, not a single MD said fiber would help prevent these issues…After increasing my fiber, not a single issue there in an area that MD’s said would be chronic my whole life. We must be our own doctors and do our own research to heal ourselves. TRUST ME, Get your proper fiber intake!

    Katie C. wrote on September 12th, 2014
  13. Hi, i’m doing a low carb, healthy fat diet, but I have constipation, very hard stools and I don’t what to take for it, I’m so confused one expert says this the other one says something else, what do you recommend?.

    Kelly wrote on March 5th, 2016
  14. So… What about FODMAPs? You mentioned garlic and onions are a great way to feed your gut bacteria, but what about FODMAP-sensitive individuals (like me)? I have been observing myself for quite a while, been on an extremely low FODMAP diet and every time I try to reintroduce for example garlic or onions – I can’t. Even in reasonable quantities, I get extreme bloating and gas that prevents me from leaving the house and having any social life… Not to mention the discomfort and the constipation that most FODMAPS cause for me. The literature on FODMAP-sensitive guts is very limited, so it would be great if you could elaborate on it. I mean the correlation of the kind of fiber you recommend and the FODMAP sensitivity. In my case, it seems that it’s only harming me and my gut…

    Sylwia wrote on August 6th, 2016

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