Lettuce Be Honest: Fiber Isn’t Always the Answer

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 the truth is more complex, and more interesting...

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? Sorry, salad. Apologies, artichoke. Bad news, broccoli. Fiber might not be as “essential” as we’ve been led to believe. Now, 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.'

And, if we don’t address the nuances of the matter, we will never advance the discussion. So, in this letter, I’m going to review facts about fiber you should know, citing data on fiber and inflammation, microbiome diversity, short-chain fatty acids, and also explain where fiber elimination can be therapeutic.

This letter is meant to advance a conversation, not end it…

Inflammation

Some people argue that fiber-rich foods are anti-inflammatory. But that’s not entirely true. For example, a 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.

Another interesting discovery in this paper was that fermented foods were more generally anti-inflammatory. So, if your goal is to reduce inflammation, I’d capitalize on kimchi over broccoli.

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, in fact it’s speculation I’ve engaged in myself. 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, it reads, “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.”

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.

Our microbiomes are unique. You are an N = 1. I am an N = 1. Even case studies have relevance.

And, if you want to toss another wrench at the diversity issue, more microbiome 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. Nevertheless, some cardiometabolic benefits were noted, including weight loss and improved inflammatory markers.

So, the takeaways are two:

1. A low fiber, low-carb diet doesn’t necessarily decrease diversity

2. More diversity isn’t necessarily better.