In this interview, a host and Dave (author of Fast This Way) discuss how fasting influences the gut microbiome, noting that the science is still nascent. They explore which microbial species may be more or less vulnerable during periods without food and raise the question of which nutrients should be reintroduced first after a fast. Dave proposes that Bulletproof coffee, rich in caprylic acid (a component of MCT oil) and polyphenols, may act as an antimicrobial agent that suppresses many microbes while favoring Bacteroidetes over Firmicutes—a shift associated with leanness. Both acknowledge the lack of clinical trials and mechanistic evidence, but suggest that modern consumer‑grade microbiome sequencing and blood‑test platforms enable individuals to run their own experiments, such as comparing different re‑feeding strategies (coffee vs. eggs) while tracking microbial changes. The conversation concludes that companies are already funding such personalized measurement approaches, positioning them as the future of microbiome‑guided fasting research.
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What about fasting? When you fast, there's nothing for the bacteria to eat. What happens? And a lot of people with diabetes, a lot of people with prediabetes are adapting fasting. I'm very transparent plug for my new book on fasting called Fast this Way. What's the role of the gut biome and specifically the species you work with when we're actually fasting?
Well, fasting, of course, it's a super interesting phenomenon. And getting back to almost our caveman days of how our body was intended to function. Not a constant stream of food all day long. I think we don't fully understand the impact of fasting on the microbiome. Obviously there are certain microbes that are colonized in your microbiome so that are less susceptible or more susceptible to not having food there. It is a competition because it's an ecosystem. And so the question is, maybe after you stop fasting, what's the first thing you eat? Who are you feeding first? But this is really in its infancy stage. A lot of interesting things to study there.
I love that you said that. One of the reasons that I hypothesize that bulletproof coffee does what it does is that caprylic acid, which is another fancy name for the specific subset of MCT oil that bulletproof makes it, and even just butter are antimicrobial. So you take those on an empty stomach, you're going to knock everything down. But then some species will grow faster based on polyphenols versus others. So the polyphenols in the coffee feed the bacteriodes versus the firmicutes. And that's a shift that you actually want to do because at least that's the shift that people who are thinner have. So. So what you just said there really ties into one of the six reasons that I think it's doing stuff. And it's fascinating because we have no evidence, no clinical trials, no one knows what to do. We have mechanistic thoughts, and you can sort of try one versus the other. But is that the kind of trial that you think will ever be funded, where we start looking at how to break a fast from a microbiome perspective? Or is that just one of those things that no company's ever going to fund?
You know what's really interesting about the microbiome is the ability to create a product that is natural and can be brought directly to consumers. And the reason that's interesting is because it allows a consumer to now test whatever they want to test. And there are microbiome sequencing companies where you can get information about microbiome changes that are happening, you can get blood tests done, you can basically measure. And of course, it's a phenomenal world we live in now where there's all these new measurement devices that we as consumers have access to in you can measure anything that you want to measure. And so by offering different types of microbes, you're enabling people to really run their own experiments. And so you, Dave, could take this set of probiotics and then you could fast and you could for two weeks break your fast with one particular type of coffee and you could then choose to break your fast with a bunch of scrambled eggs and you could literally see what is the difference between doing those two things. So will a company fund? I think companies are funding it already and enabling people to measure this themselves. But it's really, it is the future. The way personalized medicine is actually going to play out is through these measurement devices.
