New Research Shows Simply Adding More Fiber Isn't the Answer to Better Gut Wellness

Sam Westreich, PhD

A new study dives into the benefits — and limitations — of gut supplements
Is this man making pickles for taste, or is he after prebiotics for his gut microbiome?Micah Tindell on Unsplash

Whenever I get asked about what probiotic is best to take, my answer is always the same: “The best probiotic choice you can make is a salad.”

I get asked this question a fair amount; I have a PhD in genetics, focusing on studying the gut microbiome — the collection of trillions of bacteria, a diverse mix of more than a thousand different species, that thrive inside each of our bodies’ lower digestive tract. Each of us has a unique blend of bacteria, like a smellier and messier fingerprint.

Unlike our fingerprints, those bacteria have real life implications for our health. They synthesize a series of molecules called short-chain fatty acids (SCFAs) that provide a bunch of benefits for us, from better gut barrier function to less inflammation to a reduced risk of colon cancer. The microbiome synthesizes many vitamins that we need, and it helps serve as a training ground so our immune system can recognize friend from foe.

Of course, this is provided that we’re giving the microbes the right foods that they need to thrive. When we eat, we aren’t just feeding our own body; we’re also providing food to the microbes who, in turn, help us.

This is where probiotics and prebiotics come in. These are both supposed to help improve our gut microbiome…

…but are they worth it? Or do they just make our poop more expensive?

A new study suggests that it may not be as simple as “probiotics and prebiotics in, better gut microbiome as a result.” Just like our social media feeds, our order at Chipotle, and our fingerprint, the answer may be custom for each person.

Let’s do a brief description of prebiotics and probiotics — what they are, how they differ. After that, let’s talk about what the researchers at Duke University found, and shared in this new study.

Prebiotics vs. probiotics: they’re not the same, people!

Before we talk about their impact on our digestive system, we need to cover the definition of a prebiotic vs. a probiotic. Despite being very similar looking words, they’re not the same thing at all!

Probiotics are cultures of live bacteria. Think about all of the Lactobacillus bacteria that grow in yogurt, happily eating the lactose found in milk. Many probiotics are also sold as pills, designed to help protect the bacteria from the acid in your stomach, so that they can survive and make it to your lower digestive tract.

I’ve talked about probiotics in a number of other articles; for most of us, there’s no reason to eat probiotics. A few quick summarized reasons:

  • Bacteria are evolved to excel in their specific environment. Bacteria that love eating cold yogurt are not going to be well suited to thrive in your hot and largely yogurt-free colon.
  • When there is an open niche (opportunity to survive, like an available food source), bacteria rush to fill it. Unless you’ve just taken antibiotics to kill your normal gut bacteria, there won’t be any open niches for the probiotic bacteria to seize.
  • Most probiotic bacteria are not selected because they exert a needed, often-missing benefit for us; they’re selected because they won’t make us sick, so the manufacturer won’t get sued.

The best probiotic for most of us would be human-derived, which the pills… are usually not.

So let’s put probiotics aside for a minute, and talk prebiotics.

Prebiotics are not live bacteria; rather, they’re the food that bacteria love to eat. Specifically, they’re compounds that we usually classify as dietary fiber.

For years, dietary fiber was considered “roughage”, and only vaguely important. I remember learning as a child that its main purpose was to help “scrub” our insides, and imagined it passing through like the bristles of a big toothbrush.

Now, we know that dietary fiber provides an important food source for our gut microbes. It’s not the only thing they’ll eat, but they’re adaptable; if there’s fiber, they’ll chow down on it. If there’s little to no fiber, they’ll switch to alternate food sources, like the mucus lining that protects the inside of our intestines.

There are different types of dietary fiber — think soluble, which dissolves and forms a jelly, like pectin, and insoluble, which stays intact, like cellulose (yes, the same stuff as in paper! Don’t eat paper for fiber).

Our microbes can feed off both types, although each species of microbe will have a preference that it’s specialized for, its unique niche.

We can buy prebiotic fiber in a powdered form, which are usually extracted soluble fibers from plant materials. We can also get it naturally in our diet by eating more foods with soluble fibers (raw, non-juiced fruits, beans), insoluble fibers (vegetables), and complex starches (beans, lentils, oats, potatoes — not fried in French fries).

Remember, probiotics are specifically introducing new bacteria. Prebiotics are not bringing in any bacteria, but provide a food source for bacteria that are currently present.

Almost all of us have a problem getting enough fiber. The average American only eats about half the total fiber per day that is recommended. We should all be taking prebiotics; more fiber means more microbes happily making short-chain fatty acids (SCFAs) to help reduce inflammation and give us a stronger gut.


New research suggests that our microbiome might have a “fiber cap”

Enter Zachary Holmes, Lawrence David, and the other researchers at Duke University, who decided to investigate this notion of “more fiber is always better”.

They ran two studies, looking at how people responded to fiber supplements that acted as a prebiotic. Their theory: adding more fiber to someone’s diet in the form of a supplement would lead to a boost in SCFAs, no matter how much fiber that person already got in their typical diet from fruits, vegetables, beans, and other fiber-rich foods.

In the first study, they gave prebiotics to an “artificial gut microbiome”, one growing in a laboratory setting and not inside a person. They found that the microbiome adapted to make use of this new food source — but not immediately. It took several hours for the microbiome’s individual bacteria to begin changing their consumption, adapting to eat this newly introduced prebiotic food source. Once the microbiome had adapted, however, it “remembered” how to eat the prebiotic for multiple days afterward.

Takeaway: if you introduce a new prebiotic or fiber source, it won’t immediately pay off. You’ll have to keep eating it for at least a couple days to start seeing benefits, but you’ll keep seeing benefits as long as you eat it semi-regularly afterward.

In the second study, they got a bunch of human volunteers to consume prebiotic fiber supplements. The researchers found that the type of prebiotic didn’t matter much. Instead, the biggest predictor of how much increased SCFA production? The amount already being produced — which was directly linked to the amount of fiber being consumed in the normal diet, excluding the supplement.

Takeaway: more fiber, regardless of the type, is better — up to a limit. Our microbiome can only eat so much fiber, and create a corresponding amount of beneficial SCFAs. Adding more and more fiber has diminishing returns.

Our microbiome will take advantage of whatever fiber you provide to it. If your diet is lacking in fiber, adding some in will increase the amount of digestion, although it will have a lag time of a couple meals before the microbiome shifts to the new food source.

But if your diet is already full of lots of fiber, no matter the type or source? Adding a prebiotic into your routine won’t provide much benefit, since your microbiome is already running SCFA production at full blast.

Additionally, this means that there’s no real need to “personalize” the type of fiber/prebiotic for individuals. You might be happy consuming one brand or source of fiber, while I’m perfectly happy with another. If we were to switch diets, it would take a day or two for our microbiomes to adjust, but then they’d do just fine with the new, different fiber source.

In summary: what does this mean for me?

First, and probably the most actionable takeaway from this: you’re likely not getting enough fiber. Fiber intake recommendations vary, but most groups settle on a range of 20 to 40 grams of fiber per day. To get that, you’d need to eat EITHER:

  • 9 raw apples (4.4 grams of fiber, each)
  • 12 bananas (3.1 grams of fiber, each banana)
  • 14 cups of broccoli (2.4 grams/cup)
  • 2 cups of oats (16 grams/cup of raw oats)
“No dessert until you finish all the bananas in this picture, for your fiber content.”Oren Elbaz on Unsplash

It makes sense why we might look at prebiotic fiber supplements.

Adding more fiber to your diet is good, but there’s a cap on the benefits. If you already have a high-fiber diet, adding even more fiber will not spur your microbes to make even more beneficial butyrate and short-chain fatty acids. They’ll keep making these up to the cap that they can manage, and any excess fiber will simply pass, undigested.

Eating more fiber, whether in whole foods or as a supplement, will be most beneficial to those people who aren’t getting enough in their diet. And the type of fiber doesn’t seem to matter; the microbiome will adapt to whatever you give it.

We’ve learned a lot from studying the microbiome; fiber is not just “roughage” or there to bulk up your feces. It’s an important food source for your gut bacteria, and they repay you by producing molecules to reduce inflammation and strengthen your gut barrier against intruders.

It doesn’t matter how you go about it, just eat more fiber!

Do you have a preferred fiber-rich food?


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A microbiome scientist working at a tech startup in Silicon Valley, Sam Westreich provides insights into science and technology, exploring the strangest areas of biology, science, and biotechnology.

Mountain View, CA

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