Is A Ketogenic Diet Bad For Your Gut, Should You Eat Resistant Starch, How Exercise Changes Your Gut Bacteria & Much More With Lucy Mailing.

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My guest on today's show, Lucy Mailing, Ph.D., is a microbiome researcher, educator, and passionate scholar of integrative, evidence-based gut health.

Lucy received her bachelor’s in biology from Kalamazoo College in 2015. In 2019, she received her Ph.D. in nutritional sciences from the University of Illinois, where her graduate research focused on the impact of diet and exercise on the gut microbiota.

She has authored numerous peer-reviewed journal articles, regularly presents at national and international conferences, and was named an Emerging Leader in Nutritional Sciences by the American Society for Nutrition in 2017. Lucy is the founder and sole author of, a website dedicated to integrative, evidence-based articles about the gut microbiome, health, and nutrition science.

During this discussion, you'll discover:

-Why Lucy usually skips breakfast in the morning…6:25

  • More productive on an empty stomach
  • Rhonda Patrick recommends women don't intermittent fast longer than 10-12 hrs (potential for endocan disruption/leptin issues)
  • Intermittent, 24-hour, 7-day water fasting helped Lucy with chronic eczema
  • Physical activity and body type play a role in the need for, and frequency of, fasting

-How Lucy developed her interest in the workings of the gut…9:35

  • The pharmaceutical route proved ineffective in treating her eczema
  • Became familiar with the paleo movement in 2014; saw the connection between gut, diet, and skin health
  • Mark Sisson, Chris Kresser, Sarah Ballantyne were influential
  • Began Ph.D. program at University of Illinois studying the gut microbiome
  • Worked as research assistant for Chris Kresser
  • Book: The Dietary Cure for Acne by Loren Cordain
  • Gut/skin axis plays a role in things like acne
  • The long journey to recovery from eczema provided many opportunities for experimentation and evidence-based research
  • IBS provided further opportunities for expanding her understanding of the gut/diet connection

-The oxygen/gut dysbiosis connection…14:35

  • Article: The oxygen-gut dysbiosis connection: What causes gut health to go downhill, and how to break the cycle of gut inflammation, dysbiosis, and epithelial energy starvation.
  • Butyrate is a short-chain fatty acid, a metabolite produced from the bacterial fermentation of dietary fiber
  • Butyrate provides 70% of the cellular energy for the cells that line the gut and form the gut barrier
  • Fatty acid oxidation: the mitochondrial aerobic process of breaking down a fatty acid into acetyl-CoA units
  • Gut epithelial cells use oxygen from the bloodstream to metabolize butyrate
  • A study at the University of Colorado Denver found that butyrate is required for maintaining the hypoxic state of the colon
  • When butyrate was wiped out with antibiotics, the gut stopped using oxygen and consumed glucose instead
  • The oxygen built up and spilled into the gut lumen (where the microbes hang out)
  • The switch in epithelial cell metabolism from metabolizing butyrate to glucose is a primary cause of gut dysbiosis
  • BGF podcast with Joel Greene (part 1 and part 2)
    • High intake of butyrate via supplementation may actually be feeding cancer cells or resulting in some gut dysbiosis
  • Butyrate supplementation should be done only when not producing enough butyrate or not getting enough ketones
    • Supplementation as an alternative if suffering from gut dysbiosis and can't tolerate fermentable fiber
    • Doses similar to what the gut naturally produces

-How to test butyrate levels and hypoxic status…21:40

  • Fecal butyrate (looking for butyrate-producing microbes)
  • Onegevity (use code BEN20 to save $20)
  • GI Effects (clinical markers)
  • Low butyrate levels indicate a need for increased fiber intake, or supplementing/ketogenic diet if one has SIBO
  • ProButyrate and Butycaps Tributyrin butyrate supplements
  • Inflammation inhibits the growth of butyrate-producing microbes
  • Mucosal hypoxia is the desired effect
    • Butyrate-producing bacteria are all anaerobes

-Other ways to support gut hypoxia and peroxisome proliferator-activated receptor (PPAR) gamma activation…25:55

  • PPAR gene acts as the “control switch” of epithelial metabolism
  • Stimulating the PPAR can reverse gut dysbiosis
  • Butyrate is an activator of PPAR
  • Fasting, ketones, phytochemicals (curcumin, sulforaphane) activate PPAR
  • Insights from Mark Pimentel on the treatment of SIBO

-Why the paradigm of how we view SIBO has been misguided…31:15

  • A lot of early research was based on culture-based tests which skew the types and abundance of bacteria found
    • A lot of microbes that live in the small intestine don't grow in culture
  • Dr. Purna Kashyap: Symptoms of GI disorders indicate small intestinal dysbiosis, not overgrowth
    • It's not that we have too many microbes in the small intestine, it's that we have the wrong balance of microbes
  • Tendency to overdo antibiotics
  • Small intestine issues can be misattributed to the large intestine, and vice versa
  • Focus on modulating the entire ecosystem to a healthier state
  • We don't want to be in colonic energy starvation
  • Taking butyrate supplements while on antibiotics helps maintain hypoxia in the gut
  • Glutamine, bone broth, and colostrum help heal the lining of the gut
  • Decoction tea recipe (instructions on how to prepare)

-Whether a ketogenic diet is good or bad for the gut…37:35

-How exercise impacts the gut microbiome…45:50

-Resistant starch and gut health…54:45

  • Resistant starch is a starch that is resistant to digestion in its host (the human being); passes thru the GI tract to the colon where it can then be fermented by bacteria into short-chain fatty acids like butyrate
  • 4 primary types of resistant starches:
    • RS1, found in whole or partially intact grains, seeds, legumes
    • RS2, present as raw granules (raw potatoes, green bananas, raw plantains)
    • RS3, retrograded starch – retrograde resistant: starches that are cooked and then cooled
    • RS4, chemically-modified starch (synthetic RS; high maize corn starch)
  • Benefits and cautions regarding resistant starch:
    • Blood sugar regulation
    • Increases production of butyrate in some individuals
    • RS2 can potentially increase gut inflammation and microbial cell damage, increase biomarkers of colon cancer
    • RS3 is more familiar from an ancestral standpoint

-What Lucy is excited about regarding gut health research…1:00:30

  • Virtual Microbiome Summit that Lucy hosted (use code BEN20 to save 20% on the recordings)
  • Understanding industrialization of microbiota
  • Using microbiome to predict medical outcomes

-And much more!

Click here for the full written transcript of this podcast episode.

Resources from this episode:

Lucy Mailing

– Book: The Dietary Cure for Acne by Loren Cordain

– BGF podcasts and articles:

– Lab Tests:

  • Onegevity Health GutBio (use code BEN20 to save $20)
  • GI Effects Comprehensive Profile

– Supplements:

– Other resources:

Episode sponsors:

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JOOVV: After using the Joovv for close to 2 years, it's the only light therapy device I'd ever recommend. Give it a try: you won't be disappointed. Order your Joovv today and receive my brand new book, Boundless as a free gift.

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Do you have questions, thoughts, or feedback for Lucy Mailing or me? Leave your comments below, and one of us will reply!

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15 thoughts on “Is A Ketogenic Diet Bad For Your Gut, Should You Eat Resistant Starch, How Exercise Changes Your Gut Bacteria & Much More With Lucy Mailing.

  1. John says:

    Ben great choice, please bring her back. This was a excellent podcast. It was great hearing from someone who is actually doing the research and not receiving watered down information. Lucy is so knowledgeable and explains things in a way you can understand. Your conversation together was so informative.

  2. Hello where is the list of foods that was supposed to be provided?

  3. Sean McKannay says:

    It is really unfortunate that nobody monitors this site and answers the questions posted in the comments. That is a good way to loose your audience.

  4. Joe says:

    Hey Ben, good podcast. Two questions for Lucy: what are your thoughts on the benefits of dog on microbiome health, and Ingesting coffee grounds (recent Round-up citing study showing benefits).



  5. I respectfully disagree with Dr. Mailing’s conclusions regarding resistant starch. There are 278 published clinical trials on natural sources of resistant starch (excluding RS4). 174 clinical studies have been published with high amylose corn, a natural RS2 resistant starch that is not chemically modified and was bred 30 years ago through traditional breeding techniques. There is also a published meta-analysis based on 14 clinical trials concluding that resistant starch reduces systemic inflammation –….. Another great study was from Dr. Stephen O’Keefe, who used RS2 resistant corn starch in a diet swap study ( He showed reduced inflammation and concluded that resistant starch likely reduced the CRC risk due to its production of butyrate in the gut of healthy adults. A recent Yale animal study was also very interesting which showed that high amylose corn RS2 resistant starch healed a leaky gut –…. The complete reference list for all types of natural resistant starches can be found at There may be differences between different types of resistant starch with raw potato RS2 working differently than high amylose corn RS2 or green banana RS2 but the generalization that type 2 resistant starches need more research because they may exacerbate intestinal dysbiosis and increase gut inflammation contradicts 30 years of published research and more than 200 clinical trials.

    1. Lucy Mailing says:

      Rhonda, thanks for your comment and contribution to the discussion. As I did in my original post on resistant starch, I absolutely acknowledge the potential benefits shown in clinical trials for improved satiety, blood sugar regulation, and general metabolic health. The meta-analysis you shared for potential anti-inflammatory effects is also quite interesting and worth considering. It’s plausible that in a population with metabolic syndrome and no gut health concerns, isolated RS could provide an overall benefit. However, with isolated nutritional ingredients, we do have to consider publication bias towards trials demonstrating positive results. Given your extensive background in marketing RS2 as an isolated ingredient, your defense of it is certainly understandable.

      My primary concern is that RS2 supplements (like raw potato starch) are being widely promoted as a solution for gut dysbiosis and addressing chronic GI symptoms when they have not been studied in this context. While some animal studies have found benefits of RS2 for the gut, human studies tracking gut health biomarkers with RS2 supplementation are still lacking. The few studies that have looked at the effects of isolated resistant starch on the human gut microbiota have found that the result is unpredictable and dependent on the baseline microbiota composition. In my work with clients, I have seen many individuals who, like Ben, have adverse symptoms from trying to incorporate isolated sources of resistant starch. Until we understand this better and can predict who might respond best to resistant starch supplementation, I stand by my recommendation to avoid isolated RS2.

  6. patty Klein says:

    Question for Lucy or Ben:

    Wondering about the connection, if any, between gut permeability (given a compromised gut microbiome) and Food Dependant Exercise Induced Anaphylaxis (FDEIA). Any insight is greatly appreciated.

    1. Lucy Mailing says:

      Yes, intense exercise acutely causes gut permeability, and this is thought to be one of the mechanisms implicated in FDEIA. Individuals with FDEIA may also have an altered gut microbiome at baseline that can predispose them to this response during exercise.

  7. Anthony Litrenta says:

    Hey Ben, big fan of your work and very interesting interview! I want to share my personal experience with H2S SIBO that I experienced from Sept 2019 to Jan 2020 in case it can be of any value to anyone. I had all the classic symptoms discussed here, sulfurous smelling gas, diarrhea, painful bloating, and weight loss. I tried various diets on my own to cure it with no luck to the point where I didn’t want to eat anything. Even a purely plant based diet wasn’t working for me although it did seem to be what I could tolerate most. Prebiotic fibers and probiotics made symptoms so much worse. My GP did bloodwork, stool testing, and even referred me for colonoscopy and gastroscopy and everything came back normal. I finally located a functional med doctor and did the SIBO breath test. It was negative for hydrogen and methane producing bacteria so they diagnosed H2S SIBO based on symptoms and ruling everything else out. They told me that no definitive test existed yet for H2S SIBO and that stool tests only gave a good representation of bacteria living in the colon but not the small intestine. I was given a combination of the anti-bacterial hydrasol silver and a biofilm disruptor containing ALA, bismuth subnitrate, and black cumin seed. I took it for 6-8 weeks from what I can recall, however, my symptoms completely vanished after only 2 days on the protocol and have never returned since. To this day I’m still unsure how exactly I got the dysbiosis but my functional med doctors theory is that either the long term antibiotics I went on in 2018 or the consistent use of probiotics I stayed on following the antibiotics were possibly the culprit.


    1. Lucy Mailing says:

      Thanks for sharing your experience, Anthony! Glad you enjoyed the interview. A plant-based diet is only one component of the protocol we might use for hydrogen sulfide overgrowth. Stool testing is often quite useful for identifying this type of dysbiosis, as many hydrogen sulfide producers thrive in the colon, though it may fail to diagnose individuals with primarily small intestinal involvement, as in your case. Glad you were able to find something that worked for you!

    2. Vanessa says:

      Hi Anthony,
      What practitioner did you work with who helped you get to the bottom of your hydrogen sulfide SIBO? I am looking for a practitioner as I have had many failed attempts in the past. Thank you!

      1. Ben Greenfield says:

        Hi Vanessa, have you looked into Dr. Cook?

  8. Stacie Crochet says:

    What are your thoughts about using HBOT to heal the gut and SIBO?

  9. Sean McKannay says:

    Ben, your Onegevity discount code is not working. Also, what is the qualitative difference between Onegevity and Viome? The Viome test is definitely cheaper, so curious why that is not a recommendation other than you potentially get a kickback from Onegevity for the recommendation.

    1. Michael Young says:

      Sean McKannay, Onegevity uses whole genome sequencing, while Viome uses 16S sequencing. That means that Onegevity will analyze all of the DNA in the sample. while Viome will only analyze a small portion of it. The difference is something in the order of 10-100MB for Viome vs multiple gigabytes for Onegevity.

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