While some keto or low-carb proponents claim fiber is useless at best and actively harmful at worst, I come down on the side that says fiber is probably helpful for most people. Some folks have persistently better responses to low- or no-fiber keto diets, and I won’t argue with that—I’ve seen it happen and I’ve read the studies where de-emphasizing fiber can actually improve constipation, for example.

I’ll just say that I have an opposite reaction, and, most importantly, I love eating a variety of plant foods that also happen to contain a ton of great nutrients in addition to fiber.

Do I buy into the idea that fiber is important because it is every human being’s responsibility to produce as much colonic bulk as humanly possible? No.

Do I think we should be consistently pushing the limits of our digestive tracts, performing feats of bathroom heroism so momentous they border on Herculean, and making sure the toilet bowl buckles beneath us? No.

The real value of fiber lies not in its coarseness, its tendency to form colonic bulk, to keep us topped off. The true value lies in its fermentability. A fermentable fiber is a prebiotic fiber—fiber that feeds our gut bacteria.

I won’t get into the many roles our gut bacteria play in our health today (I’ve covered that before. 1, 2, 3).

I will, however, explain why we need to be feeding our gut bacteria. Our gut bacteria form a physical barrier against incursions and colonization by pathogenic bacteria; they take up room along the gut lining so pathogens can’t. If we don’t feed our gut bacteria with prebiotics, it won’t be around to protect us. 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 by antibiotics or, to a less extent, a lack of fermentable prebiotic fibers.

When our gut bacteria eat prebiotics, they also give off metabolites like butyric acid—a short chain fatty acid that our colonic cells use as an energy source and which improves metabolic health.

Gut bacteria also convert antinutrients like phytic acid into nutrients like inositol. The almond meal-obsessed keto eater would do well to have a powerful gut biome set up to convert all that phytic acid to inositol.

Now, some writers will come up with specific blends of fibers, powders and gums to create the “optimal” prebiotic diet for your gut bacteria, but that’s pretty silly. The gut is a complicated place. We’ve barely begun to even identify all its inhabitants. To think we know the precise blend of isolated fiber that will make them flourish, and then act on that, is a mistake.

A better option is to eat foods that contain fiber. Some of the prebiotic fibrous foods with the best nutrient profiles also happen to be extremely keto-friendly.

1) Almonds and Pistachios

Nuts are usually favored in health-conscious circles for a few reasons. They like the monounsaturated fat. They like the mineral profile, or the complete protein, or their ability to dissemble into nut meals and form baked goods. But what gets short shrift is the fiber content. Now, I can’t speak for other nuts, but almonds and pistachios in particular contain fiber with potent prebiotic effects. People who eat almonds and to an even greater extent pistachios end up with improved gut bacteria profiles.

2) Green Bananas

Ripe bananas are difficult to squeeze into a ketogenic diet. The green banana—an unripe one—is mostly resistant starch, a type of starch that cannot be digested and travels untouched until colonic bacteria metabolize it. It’s one of the best stimulators we know of butyric acid production. And sure, you could do a spoonful of raw potato starch to get your resistant starch, but the beauty of the green banana is that it also provides potassium, another nutrient that some find difficult to obtain and stay keto.

3) Wild Blueberries

Blackberries, boysenberries, raspberries, and strawberries are all loaded with fiber, and you should eat them. They’re lower carb than you think, they’re loaded with polyphenols, and topped with some real whipped cream they make a fantastic dessert. But wild blueberries are special. They’re smaller than other berries, which increases the amount of skin per ounce you get, and skin is where all the polyphenols and fiber lie. Heck, even the blueberry’s polyphenols have prebiotic effects on the gut biome.

4) Mushrooms

A few years ago, I wrote a whole post on mushrooms. Suffice it to say, they’re quite wonderful, bordering on magical. I did not discuss the fiber they contain. It turns out that all the various mushroom polysaccharides/fibers, including beta-glucans, mannans, chitin, xylans, and galactans also act as potent prebiotics that improve the health of the host.

5) Avocado

Your standard avocado has about 12-15 grams of fiber, if you eat the whole thing. I

6) Jicama

Great with chili powder, salt, and lime juice, jicama is about 11 grams of carbs per cup, but half of those are inulin, a potent prebiotic fiber with a tendency to really ramp up butyrate production.

7) Onions

Onions are another fantastic source of inulin. They go into almost every dish of every cuisine, so there’s no excuse not to be eating onions.

8) Garlic

I’ve been known to treat garlic like a vegetable, roasting an entire cast iron pan full until brown and sweet and chewy. They’re another great source of prebiotic fiber.

9) Leeks

Leeks have more inulin than onions. Try them crispy in egg scrambles.

10) Broccoli

Broccolini is a major part of my favorite meal of the day—my Big-Ass Keto Salad. Broccoli (and cruciferous vegetables in general) has been shown to have modulatory effects on the gut biome.

11) Sauerkraut

Kraut gives you two in one. It’s a fermented food, which is great for the gut biome. And it’s cabbage, which is very fibrous. Even pasteurized kraut improves gut health.

12) Dark Chocolate

Dark chocolate, the good stuff with a high cacao content (85%+) and low sugar content, is an incredible source of prebiotic fiber. Eat more of it.

13) Animal Fiber

Obligate carnivores like cheetahs who don’t eat any plants (willingly) still have gut bacteria. These gut bacteria thrive on “animal fiber,” the gristle and cartilage and other bits of connective tissue that comprise a good 20-30% of the walking weight of a prey animal. Humans are not obligate carnivores, but eating the entire animal has been a mainstay of advanced hominid existence for millions of years. I find it very likely that something, someone, somewhere inside our guts is breaking down the animal fiber we eat—so you’d better be eating some!

Not so tough, is it? It’s not like I’m suggesting you load up on bran muffins, psyllium smoothies. I don’t want you dumping flax meal into everything or munching on those awful fiber gummies. Just eat some basic, healthy, low-carb plant matter—foods that don’t really scream “fiber”—and the rest will take care of itself.

What’s your favorite low-carb source of fiber? Let me know down below.

Thanks for reading, everyone.

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References: 

Hernández E, Bargiela R, Diez MS, et al. Functional consequences of microbial shifts in the human gastrointestinal tract linked to antibiotic treatment and obesity. Gut Microbes. 2013;4(4):306-15.

Ukhanova M, Wang X, Baer DJ, Novotny JA, Fredborg M, Mai V. Effects of almond and pistachio consumption on gut microbiota composition in a randomised cross-over human feeding study. Br J Nutr. 2014;111(12):2146-52.

Jiao X, Wang Y, Lin Y, et al. Blueberry polyphenols extract as a potential prebiotic with anti-obesity effects on C57BL/6 J mice by modulating the gut microbiota. J Nutr Biochem. 2019;64:88-100.

Jayachandran M, Xiao J, Xu B. A Critical Review on Health Promoting Benefits of Edible Mushrooms through Gut Microbiota. Int J Mol Sci. 2017;18(9)

Nielsen ES, Garnås E, Jensen KJ, et al. Lacto-fermented sauerkraut improves symptoms in IBS patients independent of product pasteurisation – a pilot study. Food Funct. 2018;9(10):5323-5335.

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For today’s edition of Dear Mark, I’m answering three questions. First up, what can a person do to help their gut recover its barrier function after too many antibiotics? Are there any foods, supplements, or dietary strategies? Second, what can explain rapid fatigue during sprint sessions on a keto diet? Is this simply part of the deal, or are there modifications you can make? And finally, what do I do when I know I’m going to get a bad night’s sleep?

Let’s go:

Mark – any idea how to cure leaky gut caused by overuse of antibiotics. Tried raw dairy for a month to no avail.

First of all, check out my post on leaky gut. Read through it and follow my suggestions for preventing and treating intestinal permeability. It’s a great place to start.

Then, let’s look at some other interventions that have been shown to improve recovery from antibiotic therapy. While most of the studies referenced don’t explicitly describe antibiotic-induced leaky gut, anything that improves gut function and restores healthy gut bacteria will also normalize leaky gut—since it’s the eradication of native gut bacteria that causes antibiotic-induced leaky gut.

Fermented dairy. You tried raw dairy. What about fermented dairy? While raw dairy has its merits, it’s fermented dairy that just works for recovery from antibiotics. Yogurt is a good option to try, although the evidence is a bit inconsistentKefir is probably better; it’s been shown to improve patients’ tolerance to triple antibiotic therapy during treatment for H. pylori infection. This is even worth consuming during antibiotic therapy, as many of the probiotic bacteria found in fermented dairy show resistance to common antibiotics.

Fermented vegetables like sauerkraut are also must-eats. The fermented cabbage contains ample amounts of L. plantarum, a bacteria strain that’s been shown to prevent antibiotic-related diarrhea in piglets (another omnivorous mammal). Good options exist in stores (check the refrigerated section; shelf-stable pickles and kraut aren’t lactofermented), and even more are available in farmer’s markets, but the best way to get the most bacteria-rich vegetable ferments is to make your own.

Supplemental probiotics are fantastic here, too: large doses of the desired microorganisms delivered directly to your gut. Some of the strains used in Primal Probiotics, like B. clausii and S. boulardii, have been shown to be effective against antibiotic-related diarrhea, so that could be a good choice.

Don’t forget the food for your gut bugs: prebiotics. You need to eat fermentable fibers and other prebiotics like resistant starch to support the growth and maintenance of the helpful bacteria that improve gut barrier function. Consider eating cooked and cooled potatoes, unheated potato starch, leeks, garlic, onions, green bananas, apples, pears, berries, and pretty much any fruit or vegetable you can get your hands on. Plenty of them are low-carb enough to work on a keto diet, if that’s your desire. Oh, and dark chocolate is a great source of fiber and polyphenols, which have prebiotic effects in the gut.

Incorporate intermittent fasting. Going without food for a spell gives your gut a break and induces autophagy, which can help with tissue healing.

Get dirty, too, to introduce potentially helpful bacteria. Go out and garden. Go barefoot at the park (do your due/doo diligence, of course) and practice tumbling, or roughhouse with your kids (or friends). Don’t immediately rush to wash your hands all the time (unless you’ve been handling raw meat and/or dog poop).

Whatever you do, don’t stress too much about the antibiotics you had to takeStress is awful for gut health and you’ve already taken the antibiotics—which were probably necessary—so that ship has sailed.

If probiotics with prebiotics aren’t helping (or making things worse), you might want to try going the opposite direction—removing all plant foods and doing a carnivore diet for a few weeks. While I have doubts about the long term viability and safety of eschewing all plant foods, enough people have written to me about their great experiences resolving gut issues with a bout of carnivory that it’s worth trying.

When on a strict keto plan, why do I become so quickly fatigued while attempting a HIT sprint workout?

The first five seconds of a sprint are primarily powered by phosphocreatine (or creatine phosphate), a “quick burst” energy source that burns hot but disappears quickly. This is the stuff used to perform max effort Olympic lifts, short sprints, and other rapid expressions of maximum power. It doesn’t last very long and takes a couple minutes to replenish itself. A keto diet doesn’t affect our creatine phosphate levels. If anything, it should improve them if we’re eating meat.

After five seconds, anaerobic metabolism of muscle glycogen provides the lion’s share of your energy needs. The longer your sprint, the more glycogen you’ll burn. The less glycogen you carry in your muscles, the shorter your sprint. Because once you run out of creatine phosphate and glycogen, you’re left with aerobic metabolism—great for longer distances, not so great for max effort sprints.

Keto dieters tend to walk around with less glycogen in their muscles. If that’s the case, longer sprints will be harder.

If you want to keep sprinting:

Do shorter sprints. Try a 10-second hill sprint rather than a 20-second one. Really go hard. Heck, you can even do 5-second sprints and derive major benefits; just do more of them and make sure to recover in between. There’s no rule saying you have to sprint for 20-30 seconds.

Take longer rest periods. Give your muscles a chance to replenish more creatine phosphate (and take creatine or eat red meat and fish, which are the best sources of dietary creatine).

Eat 20-30 grams of carbs 30 minutes before a sprint session. See if it helps. Alternatively, you can eat the 20-30 grams of carbs after the sprint session to replenish lost glycogen stores (without really impacting your ketone adaptation, by the way).

Most people figure out their sprinting sweet spot while doing keto. They may have to play around with the dosages, durations, and rest periods, but you can usually make it work. Be open to trying new permutations.

If you knew you were going to have a poor nights sleep, what measures would you take to reduce some of the damage?

I would exercise hard that night. Normally, a bad night’s sleep tanks your insulin sensitivity the next day, giving you the insulin resistance and glucose tolerance of a diabetic. A good hard interval session the night before a bad night’s sleep, however, counters the next-day insulin resistance.

I would make the most of it. Don’t dawdle. Don’t beat yourself up because of the impending sleep deprivation. It’s going to happen. You have to accept it, not let it destroy you.

Enjoy it. A little-known acute treatment for depression is sleep deprivation. That’s right: a single night of sleep deprivation has been shown to ameliorate depression in patients with clinical depression. Sometimes the effect lasts up to several weeks. It’s not a long term or sustainable fix for clinical depression, obviously, and you can’t do it every single night—chronic sleep deprivation is a major risk factor for developing depression—but it can improve your mood if you give in to it.

I would set out a jar of cassia cinnamon. I always add cassia cinnamon to my coffee in the morning after bad sleep; cassia cinnamon the day after a bad night’s sleep attenuates the loss of insulin sensitivity and glucose tolerance.

That’s it for today, folks. Thanks for writing in and reading! If you have any input on today’s round of questions, let me know down below.

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References:

De vrese M, Kristen H, Rautenberg P, Laue C, Schrezenmeir J. Probiotic lactobacilli and bifidobacteria in a fermented milk product with added fruit preparation reduce antibiotic associated diarrhea and Helicobacter pylori activity. J Dairy Res. 2011;78(4):396-403.

Bekar O, Yilmaz Y, Gulten M. Kefir improves the efficacy and tolerability of triple therapy in eradicating Helicobacter pylori. J Med Food. 2011;14(4):344-7.

Erginkaya Z, Turhan EU, Tatl? D. Determination of antibiotic resistance of lactic acid bacteria isolated from traditional Turkish fermented dairy products. Iran J Vet Res. 2018;19(1):53-56.

Yang KM, Jiang ZY, Zheng CT, Wang L, Yang XF. Effect of Lactobacillus plantarum on diarrhea and intestinal barrier function of young piglets challenged with enterotoxigenic Escherichia coli K88. J Anim Sci. 2014;92(4):1496-503.

Jitomir J, Willoughby DS. Cassia cinnamon for the attenuation of glucose intolerance and insulin resistance resulting from sleep loss. J Med Food. 2009;12(3):467-72.

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For today’s edition of Dear Mark, I’m answering several questions drawn from the comment board of last week’s post on fasting vs carb restriction. First, how do I square my recommendations with the successful reports of potato dieters losing weight on a high-carb tuber diet? Second, is Leangains optimal for mass gain? Third, how do I use extra virgin olive oil, butter, and ghee? Fourth, could exogenous ketones help a man with dementia, MS, and seizures? Fifth, how should a woman with stalled weight loss integrate fasting?

Let’s go:

Walter Sobchak asked:

If “carbs” are so bad, how do people eat only potatoes and lose large amounts of weight? Andrew Taylor (SpudFit.com) and Penn Jillette (Penn & Teller) are two high-profile people, but there are lots more. Of course, I wouldn’t recommend an unbalanced diet of only one food, but the point is that potatoes are a natural food and are not inherently detrimental.

I agree that potato-only diets are a quick weight loss hack.

Potato-only diets work well because they’re so monotonous. When your only option is a plain potato, it’s extremely hard to overeat. It’s the combination of fat and carbohydrates that’s so easy to overeat, and that causes the most metabolic problems.

Potatoes are surprisingly nutrient-dense. They have complete protein, containing all the necessary amino acids. You won’t be bodybuilding on all-potatoes, but there’s enough protein in there to stave off muscle loss for a week or so.

Cooking and cooling your potatoes converts some of the glucose into resistant starch, which feeds your gut bacteria and cannot be digested by your body. This lowers the effective glucose load.

I could recommend the potato-only diet, ditch the keto/low-carb/Primal talk, and people who listened to me would still lose weight. But they’d miss out on all the other benefits, not least of which is the delicious food. In short, the potato-only diet isn’t the worst thing out there, but I wouldn’t recommend it as a long-term strategy.

Check out what I’ve written about potatoes in the past. You might be surprised.

Mattias Carlsson asked:

I have a question for advice if someone know. According to most sources I find the so called anabolic window persist at least 24 hours after resistance training. How can then an intermittent fasting with 8 hour eating as in lean gains, from what I understand, be optimal on training days. It seems to me that a bit of overeating on carbs and protein during all this time would be most beneficial?

I don’t know that it’s optimal for sheer mass gain. But it does seem to strike a nice balance between “gains” and “staying lean.” You may not bulk up as quickly as you would cramming food in your gullet. You will gain lean mass without gaining so much of the squishy mass that normally accompanies what passes for “gains.”

Michael Levin wondered:

Question: EVOO, Ghee and grass-fed butter–which to use when and for what?

EVOO: salads, marinades, sautéing. It’s actually far more resistant to heat than most people think; the polyphenols protect against oxidative damage.

Ghee: Indian cooking, Thai cooking, high heat searing.

Butter: Cooking eggs and other breakfast items, melted with broccoli/shrimp, finishing steaks and reduction sauces.

Beth Olson asked:

What are your thoughts on exogenous ketones? My dad has MS and dementia and seizures way too often. Should we try adding these?

I can’t give your dad any medical advice. You can talk to his doctors, however, and show them this study where exogenous ketones reduced seizure activity in mice. You can show them that coconut oil and MCT oil—two other routes for generation of ketones—have shown efficacy against cognitive decline in patients with Alzheimer’s or dementia.

I suspect exogenous ketones can help. I also suspect they’d be far more helpful on top of a low-carb, high-fat diet with plenty of healthy lifestyle modifications.

That’s the thing with dementia: there isn’t a pill that fixes everything, or even a single intervention. In the one study that actually got major results, researchers had Alzheimer’s patients undertake a dramatic diet, exercise, and lifestyle shift. Here’s what each subject did:

  1. Eliminate all simple carbs and follow a low-glycemic, low-grain (especially refined grains) diet meant to reduce hyperinsulinemia.
  2. Observe a 12-hour eating window and 12-hour fast each day, including at least three hours before bed.
  3. Stress reduction (yoga, meditation, whatever works for the individual).
  4. Get 8 hours of sleep a night (with melatonin if required).
  5. Do 30-60 minutes of exercise 4-6 days per week.
  6. Get regular brain stimulation (exercises, games, crosswords).
  7. Supplement to optimize homocysteine, vitamin B12, CRP levels.
  8. Take vitamin D and vitamin K2.
  9. Improve gut health (prebiotics and probiotics).
  10. Eat antioxidant-rich foods and spices (blueberries, turmeric).
  11. Optimize hormone balance (thyroid panel, cortisol, pregnenolone, progesterone, estrogen, testosterone).
  12. Obtain adequate DHA to support synaptic health (fish oil, fish).
  13. Optimize mitochondrial function (CoQ10, zinc, selenium, other nutrients).
  14. Use medium chain triglycerides (coconut oilMCT oil). You could possibly use exogenous ketones here too.

Bring that study to your dad’s doctors and see what they have to say. If they aren’t blown away by the possibilities and open to give it a try, I’d be shocked. Hopefully your dad is game. I’d love to hear how it works.

Lisa Chupity asked:

I went Primal/Paleo back in March of 2012. I lost the 15 pounds I wanted to lose. In 2015, 7 pounds crept on, and for the life of me, I can’t lose ‘em! April of this year, I went Keto. I track my macros, and do my best to keep my carbs to 20 grams per day, tho I don’t beat myself up if I have 24. I haven’t lost an ounce! I’m going to have to do the IF thing, I’m sure. As it is, my breakfast is bone broth (1 1/2 cups) and a mug of Coffee with Brain Octane in it. Lunch is yer basic “Big Ass Salad”. Dinner is good, too, and within Keto guidelines. I try to keep my caloric intake to ~1600 calories/day.

To add to the mess, I have Multiple Sclerosis, so stuff like Cross Fit is outta the picture. I can manage some stationary cycling, and some Pilates, with lighter modifications. Any advice?

If you try IF, do the “early restricted feeding” rather than late. You’re already doing a kind of “fast” in the morning, just drinking broth and coffee with MCTs, and it doesn’t seem to be working.

Eat some fat and protein for breakfast with a few carbs. Eggs and bacon with a side of cantaloupe or berries. An omelet with spinach and onions and cheese. Steak and greens and half a banana. Emphase whole-food fat and protein. Have coffee and broth, too, if you like. This and lunch should be your biggest whack of calories.

Eat your Big Ass Salad for lunch. Drop dinner, or make it really light and no later than 5 or 6 PM.

Terry Wahls has a great Primal-friendly MS protocol. Check out her Ted talk and go from there if it interests you.

Good luck and keep us apprised of your results.

That’s it for today, folks. Take care, be well, thanks for reading and writing!

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inline Fiber.jpegThe tricky thing about fiber is that it’s not a monolith. There are dozens of varieties. Some of them perform similar functions in the body, but others have extremely unique effects. Some rend your colonic lining to stimulate lubrication. Some turn into gelatinous slurries. But we can’t talk about fiber without understanding that the word describes a variety of compounds. As such, anyone making declarative statements about “fiber” without differentiating between the different types and their effects isn’t being accurate (except for me in that exact sentence).

This leads to a lot of confusion. People make blanket statements that might be true for some types of fibers and incorrect for others. 

Today’s post will attempt to illuminate the bulk of the matter. I’ll go through some of the most common misconceptions and myths about fiber from all corners of the dietary world. Whether you’re keto, low-carb, vegan, carnivore, or breatharian, you’ll find something to love and hate in today’s post.

1) “Fiber makes you full.”

This is theoretically sound. Mechanoreceptors in the gut respond to physical fullness by triggering satiety hormones. Big loads of insoluble fiber increase intestinal bulk, while some soluble fibers can gel up and increase the size of the stuff moving through your gut. Both result in added pressure on gut mechanoreceptors.

How does it work in practice?

A review found that while soluble fiber reduced appetite more than insoluble fibers, the overall effect on body weight was quite small, unimpressive, and inconsistent. More recently, a soluble fiber supplement failed to have any effect on satiety hormones, appetite, and subsequent food intake for the first 150 minutes after eating in healthy adults. The plucky researchers aren’t giving in, however, promising “further research… to quantify how soluble fiber influences appetite several hours after consumption.”

Resistant starch, an indigestible type of starch that colonic bacteria ferment, on the other hand does appear to increase satiety in humans, reducing food intake by 15%.

2) “Every diet needs the same amount of fiber.”

As it turns out, fiber becomes more critical the more carbohydrates you eat.

Soluble fiber slows down digestion, reducing the rate at which energy is absorbed. This can be helpful for people with glucose intolerance or type 2 diabetes by slowing the release of glucose into the blood. 

The byproducts of fiber fermentation in the colon by gut bacteria often have beneficial effects on carbohydrate metabolism. Eating resistant starch, for example, lowers the postprandial blood glucose spike. This reduction may also extend to subsequent meals, indicating it’s honing your ability to handle glucose. Everyone can benefit from better glucose management, but it’s far more critical for people eating significant amounts of glucose.

3) “All the healthiest people studied eat fiber!”

Observational studies are fun and all, but they’re not a good way to prove the healthfulness of fiber. Looking at fiber intake is just about the best way to capture the “healthy user”—that person who does everything right, like walk daily, exercise regularly, abstain from tobacco, avoid binge drinking, and eat whole foods rather than refined ones. It doesn’t say anything definitive about the health effects of the specific dietary variable they’re observing.

That said, the fact that most healthy populations eat whole foods containing fiber indicates that fiber probably isn’t actively harmful.

4) “Fiber is just roughage for big impressive poops. No functional use.”

That’s mostly true of insoluble fiber, which is pure waste material that shreds your intestinal lining and increases stool volume.

There’s considerable evidence that people with type 2 diabetes can really benefit from prebiotic fiber supplementation:

  • Chicory-derived inulin, a potent prebiotic fiber, reduces liver enzymes and HbA1c, improves blood pressure and fasting glucose, and increases calcium homeostasis.
  • Inulin improves immune markers and glycemic control.
  • Resistant starch lowers insulin resistance and inflammation.

A review of studies found that while the prebiotic inulin reduces LDL-C (an imperfect biomarker of dubious utility) in all populations, only in type 2 diabetics does inulin improve HDL and blood glucose control. 

Prebiotic fiber may also help certain patients with non-alcoholic fatty liver disease (NAFLD). The usual therapy for NAFLD patients is weight loss. You lose enough body fat elsewhere and the fat you’ve accumulated in the liver starts to disappear, too.

What about lean NAFLD patients without any real weight to lose?

In lean patients with NAFLD, a synbiotic—blend of prebiotic fiber with probiotic bacteria—reduces liver fat and fibrosis by improving inflammatory markers. Pre-emptive consumption of prebiotics may even protect against the development of NAFLD.

Another function of fiber that occurs in everyone is the production of short chain fatty acids by gut bacteria. When gut bacteria ferment prebiotic fiber, they produce short chain fatty acids, many of which have beneficial metabolic effects.

Butyrate is the most important short chain fatty acid. It fuels colon cells and may prevent colon cancer. Its relationship with existing colon cancer cells is more controversial. Read more about that here.

One interesting line of research is studying the interaction between the ketone body beta-hydroxybutyrate and the short chain fatty acid butyrate. Initial indications suggest that the two may have synergistic effects on cognition, inflammation, and overall health. That alone may be a reason to make sure you get prebiotic fiber on your ketogenic diet, just to hedge your bets.

Now, might a low-carb or ketogenic diet work better for people with type 2 diabetes than adding fiber to their normal diet? Sure. Could such a diet reduce the need for fiber? Yeah, I could see it. The same goes for NAFLD—low carb diets are also excellent in this population. And perhaps people who aren’t eating so many carbs don’t need the short chain fatty acids to improve their metabolic function and insulin sensitivity. But the evidence for fiber in type 2 diabetes and NAFLD stands, and I suspect short chain fatty acid production matters even in low carb or keto dieters.

5) “Fiber cures constipation.”

It depends.

In one 2012 study, patients with idiopathic constipation—constipation without apparent physiological or physical causes—had to remove fiber entirely to get pooping again. Those who kept eating a bit or a lot of it continued to have trouble evacuating. The more fiber they ate, the worse their constipation (and bloating) remained.

A 2012 review found that while fiber may increase stool frequency, it doesn’t improve stool quality, treatment success, or painful defecation. Similarly, glucomannan, a soluble fiber, moderately improves defecation frequency in constipated kids, but has no effect on stool quality or overall treatment success.

However, galactooligosaccharides, a class of prebiotic fiber, do appear to improve idiopathic constipation. And inulin, another prebiotic fiber, improves bowel function and stool consistency in patients with constipation.

6) “Fiber aggravates gut issues.”

Some say fiber cures gut issues like IBS and IBD. Others say fiber aggravates them. Who’s right? Maybe both.

Both IBS-D (irritable bowel syndrome with diarrhea) and IBS-C (irritable bowel syndrome with constipation) patients can benefit from soluble fiber (psyllium) while insoluble fiber (bran) is far less effective.

Wheat bran works okay for IBS, if the patients can tolerate it. They tend to tolerate something like hydrolyzed guar gum much better.

For IBD, the evidence is mixed. One survey of Crohn’s patients found that those eating more fiber (23 grams/day) had fewer flareups than those eating less (10 grams/day), while colitis patients reported no difference in symptoms based on fiber intake.

On the other hand, studies indicate that a low-FODMAP diet, which eliminates most sources of fiber, especially fermentable prebiotic fiber, is an effective treatment for IBS and IBD. Low-FODMAP diets have been shown to reduce bloating, abdominal pain, quality of life, and overall symptoms in intestinal disorders.

These contrary results may not even be contradictory. If your gut’s messed up, one solution could be to add back in the fibers you’re missing. Another could be to take all the fiber out and start from scratch.

7) “Fiber reduces nutrient absorption.”

For a long time, the consensus was that fiber tends to bind with minerals in the gut and thus reduce their absorption. These days, researchers understand that many of these fiber-bound minerals become available after fermentation in the colon.

Another wrinkle is that dietary fiber often comes with phytic acid, which binds minerals and prevents their absorption. Take wheat bran. Often deemed “wheat fiber” and lambasted for its tendency to bind minerals, wheat bran isn’t just fiber. It’s also a significant source of mineral-binding phytic acid.

Prebiotics increase absorption of magnesium, heme iron, and calcium. This makes sense. Even if the prebiotics are binding minerals, they release them once they reach the colon for fermentation by gut bacteria.

Fiber may reduce absorption of plant polyphenols, however.

8) “No one needs fiber.”

On the surface, this appears to be a sound conclusion. The human host digestive system cannot digest it. The majority of the fiber we eat gets pooped out as literal waste material. Certain classes of fiber may improve our gut health, but no one is keeling over from a lack of fiber in their diet.

Some have argued that a sterile gut is ideal if you have the right diet, that employing vast hordes of gut bacteria is just an adaptive measure taken to deal with a substandard diet full of roughage. The problem is that most people throughout history and prehistory have eaten that roughage, employed those gut bacteria, utilized the metabolites those bacteria produce. I suspect thinking long and hard before you consider it immaterial to human health.

If that were true, why would breast milk—the only food specifically designed for human consumption—contain loads of indigestible oligosaccharides that feed the growing gut biome? Even if it turns out that feeding the gut biome is only vital during infancy, that’s still a population of humans who truly need fiber.

Here’s where I come down: Fiber is an intrinsic part of many whole plant foods (and even whole animal foods, if it turns out that our gut bacteria can utilize “animal fibers” like other top carnivores). The Primal-friendly plants, the ones our ancestors grew up eating approximations of, like fruits, vegetables, roots, and tubers, are mostly higher in soluble fiber and lower in insoluble fiber. The only way to get huge doses of insoluble fiber these days is with supplementation or by eating grains. I don’t suggest eating grains or supplementing with insoluble fiber. I do suggest eating fruits, vegetables, roots, and tubers (while managing your carbs).

As for the carnivore issue, I’m open to the possibility that a properly-constructed carnivorous diet (which may, remember, include gristly animal fiber) obviates the need for plant fiber, prebiotic or otherwise. I’m not confident enough to try it myself, though.

Do I think everyone should be supplementing with prebiotic fiber? No. I add inulin to my Primal Fuel protein powder, mostly to improve mouth-feel but also to feed beneficial microbes and increase butyrate production. I add prebiotic cassava fiber to my collagen bars, again to improve texture and feed gut bacteria. And I’ll sometimes use raw potato starch for its considerable resistant starch content, often just mixing it into sparkling water and drinking it straight. But for the most part, the fiber I eat is incidental to the foods I consume. Berries, non-starchy vegetables, jicama, garlic, onions, green bananas, nuts—these are all foods rich in fiber, particularly prebiotic fiber, and I eat a fair amount of them while remaining low-carb and often keto.

As you can see, the fiber story isn’t simple. At all. There are many variables to consider. If you’re confused and unsure of how to think about fiber, you’re on the right track.

What do you think, folks? How has fiber helped or harmed you? I’d love to hear from everyone.

Take care and be well.

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