'Lettuce' Be Honest: Fiber Isn’t Always the Answer🥬🤥(Refs linked in...
Nick Norwitz MD PhD@nicknorwitz
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Apr 23, 2025
~3 min read
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'Lettuce' Be Honest: Fiber Isn’t Always the Answer🥬🤥(Refs linked in letter at the end) 🧵
1/6) We’ve been told for years that fiber is a must-have for gut health—that without it, your microbiome will crumble faster than a stale bran muffin.
But what if I told you… you don’t actually "need" fiber?
But before you throw your kale smoothie at the screen, let’s break down the science—because the truth about fiber is way more complex than a simple 'good' or 'bad.'
#fiber #microbiome #guthealth #educational #SCFA #ketodiet #carnivore #plantbased #metabolism #metabolichealth
1/6) We’ve been told for years that fiber is a must-have for gut health—that without it, your microbiome will crumble faster than a stale bran muffin.
But what if I told you… you don’t actually "need" fiber?
But before you throw your kale smoothie at the screen, let’s break down the science—because the truth about fiber is way more complex than a simple 'good' or 'bad.'
#fiber #microbiome #guthealth #educational #SCFA #ketodiet #carnivore #plantbased #metabolism #metabolichealth
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2/6) Fiber & Inflammation🔥🥬
Some people argue that fiber-rich foods are anti-inflammatory. But that’s not entirely true.
For example, a landmark randomized controlled trial published in Cell found that some people were inflammatory responders to dietary fiber. Reading from the paper, “Taken together, these data suggest divergent immune system responses to the high-fiber intervention, with high-inflammation participants exhibiting broad increases in steady-state immune activation.”
To be clear, this was NOT the majority of participants.
And it's also worth noting that those with lower microbiome diversity tended to be the inflammatory responders, raising the 🤔hypothesis🤔 that there might be protocols by which one could train-up a microbiome such that it responds with a healthier anti-inflammatory response to fiber...
It’s possible.
But the fact remains that some people have a pro-inflammatory response to fiber that could have negative health consequences and contribute to or exacerbate chronic disease.
Some people argue that fiber-rich foods are anti-inflammatory. But that’s not entirely true.
For example, a landmark randomized controlled trial published in Cell found that some people were inflammatory responders to dietary fiber. Reading from the paper, “Taken together, these data suggest divergent immune system responses to the high-fiber intervention, with high-inflammation participants exhibiting broad increases in steady-state immune activation.”
To be clear, this was NOT the majority of participants.
And it's also worth noting that those with lower microbiome diversity tended to be the inflammatory responders, raising the 🤔hypothesis🤔 that there might be protocols by which one could train-up a microbiome such that it responds with a healthier anti-inflammatory response to fiber...
It’s possible.
But the fact remains that some people have a pro-inflammatory response to fiber that could have negative health consequences and contribute to or exacerbate chronic disease.
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3/6) Fiber & Microbiome Diversity🥬🤔
One point that is often raised is that fiber depletion or elimination will decrease microbiome diversity, a presumed marker of good health. This is fair speculation at a population level. However, there are deeper nuances.
First, microbes in the gut can feed off more than just fiber and eating a low-fiber diet doesn’t necessarily lead to decreased diversity.
For example, in one impressively comprehensive case study, a man who had been on a carnivore diet for 4 years had his microbiome compared to that of omnivores and, reading from the study, “[T]he comparison showed surprising results. The carnivore’s gut microbiome did not stand out regarding α- and β-diversity, indicating that it did not lack richness or diversity when compared to its omnivore counterparts.”
And “Our study indicates that adherence to a carnivorous diet does not cause detrimental changes in the gut microbiome. Instead, it suggests that the effects on the gut microbiome are due to the combined influences of dietary regime and lifestyle, rather than meat consumption alone. Further research is needed to identify which components of the carnivore diet could act as prebiotics in the absence of plant-derived prebiotics and maintain gut health over time.”
Granted, this is a case study. But even an N = 1 it’s sufficient to make the point that even complete fiber elimination for 4 years doesn’t starve off the microbiome.
One point that is often raised is that fiber depletion or elimination will decrease microbiome diversity, a presumed marker of good health. This is fair speculation at a population level. However, there are deeper nuances.
First, microbes in the gut can feed off more than just fiber and eating a low-fiber diet doesn’t necessarily lead to decreased diversity.
For example, in one impressively comprehensive case study, a man who had been on a carnivore diet for 4 years had his microbiome compared to that of omnivores and, reading from the study, “[T]he comparison showed surprising results. The carnivore’s gut microbiome did not stand out regarding α- and β-diversity, indicating that it did not lack richness or diversity when compared to its omnivore counterparts.”
And “Our study indicates that adherence to a carnivorous diet does not cause detrimental changes in the gut microbiome. Instead, it suggests that the effects on the gut microbiome are due to the combined influences of dietary regime and lifestyle, rather than meat consumption alone. Further research is needed to identify which components of the carnivore diet could act as prebiotics in the absence of plant-derived prebiotics and maintain gut health over time.”
Granted, this is a case study. But even an N = 1 it’s sufficient to make the point that even complete fiber elimination for 4 years doesn’t starve off the microbiome.
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4/6) Fiber & Microbiome Diversity cont’
And, if you want to toss another wrench at the diversity issue, more diversity isn’t even necessarily better. In fact, in a recent 2025 study published in Cell, researchers tried to restore the human gut microbiome of those in industrial settings with a diet mimicking non-industrialized dietary patterns.
Ironically, the diet was primarily plant-based, featuring beans, sweet potato, rice, cucumber, cabbage, artichokes, peas, and onions, but it decreased microbiome diversity. I’ll repeat that, the plant-based, carbohydrate-rich diet decreased microbiome diversity.
Nevertheless, some cardiometabolic benefits were noted, including weight loss and improved inflammatory markers.
And, if you want to toss another wrench at the diversity issue, more diversity isn’t even necessarily better. In fact, in a recent 2025 study published in Cell, researchers tried to restore the human gut microbiome of those in industrial settings with a diet mimicking non-industrialized dietary patterns.
Ironically, the diet was primarily plant-based, featuring beans, sweet potato, rice, cucumber, cabbage, artichokes, peas, and onions, but it decreased microbiome diversity. I’ll repeat that, the plant-based, carbohydrate-rich diet decreased microbiome diversity.
Nevertheless, some cardiometabolic benefits were noted, including weight loss and improved inflammatory markers.
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5/6) So, the takeaways are two:
(1) A low fiber, low-carb diet doesn’t necessarily decrease diversity.
(2) More diversity isn’t necessarily better.
(1) A low fiber, low-carb diet doesn’t necessarily decrease diversity.
(2) More diversity isn’t necessarily better.
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6/6) For more on Fiber Facts, Myths and Misperceptions, including coverage of short-chain fatty acids (SCFAs) and therapeutic fiber elimination, see today’s Stay Curious Metabolism Newsletter:
open.substack.com/pub/staycuriou…
open.substack.com/pub/staycuriou…
