Dr. Will Bulsiewicz on the Importance of Fiber

There are countless different dietary approaches out there, each with their own purported benefits and lists of foods to eat and not eat. But for Dr. Will Bulsiewicz, gastroenterologist and author of the new book Fiber Fueled, there’s just one rule when it comes to a healthy diet, and that rule depends on just one vital nutrient: fiber. Read on to learn about the importance of fiber for gut health, little-known soluble fiber benefits, different types of dietary fiber, why food diversity matters so much, and more. 

*This is a short clip from our interview with Dr. Will Bulsiewicz. Click here to watch the whole thing!*

You can also listen to an audio version of our interview with Dr. Will Bulsiewicz on The WellBe Podcast. 

Discovering Firsthand the Importance of Fiber

When Dr. Bulsiewicz finished medical school and started his residency, he felt invincible. He was young, had a history of an athlete, and came from a family of slim, healthy people. But then the demands of his residency ramped up.

He was working 16-hour days six days a week, eating fast food regularly, not exercising, and severely sleep-deprived. “Honestly, I could’ve been sponsored by RedBull. I was smashing RedBulls a couple of times a day, plus Starbucks. When I was an intern, I was so sleep-deprived that I could literally drink a RedBull and pass out five minutes later,” Dr. Bulsiewicz recalls. 

At the same time, he noticed that his body was changing. Over the course of a couple years, Dr. Bulsiewicz put on 50 pounds, and became plagued with other ailments like fatigue, anxiety, and high blood pressure. 

“It was getting away from me,” he says. “I really couldn’t find a way out of it.” He tried to exercise his way out of the problem, completing grueling, hours-long workouts each day to try to lose the excess weight, but it didn’t work. He got stronger and faster and gained muscle mass, but, as he puts it, “I couldn’t lose the gut.”

Speaking of the gut: while gut health is a big deal these days, it was not part of the conversation 15 years ago when Dr. Bulsiewicz was struggling. He’d chosen to be a gastroenterologist because he loved the different organ systems, but he didn’t know or understand the importance and complexity of the gut, nor consider that it could be the key to solving his health problems.

Then he met his now-wife, who ate totally differently from him. She ate a completely plant-based diet, while he devoured meat. But what he noticed was that she ate without any sort of restriction, consuming a good amount of food at each meal, and yet she never had any issues controlling her weight.

Seeing this changed Dr. Bulsiewicz’s perspective. “My mind opened up to this possibility that, you know, ‘Maybe, I wasn’t taught everything in medical school,’” he says. With his mind opened to that possibility, he began to do his own research, and ultimately found thousands of studies that sent him on a path to transforming his own health. Those studies all shared one core component: they drove home the importance of fiber.

Once he understood the importance of fiber, Dr. Bulsiewicz changed his diet and was finally able to lose the weight and his other symptoms. “I honestly feel like I reversed aging,” he says. From that point on, he used the importance of fiber as a key part of his practice, and has seen what he describes as “amazing results.” 

So why is fiber so important? Dr. Bulsiewicz explains.

The Different Types of Dietary Fiber and Powerful Soluble Fiber Benefits

Fiber, put simply, is a part of plants. Every plant has fiber in it, and plants are the only thing that contain dietary fiber. As Dr. Bulsiewicz explains it, plants “have a monopoly” on fiber.

But fiber is actually more complicated than that. There are tons of different types of dietary fiber — so many that we actually have no idea of the number. “There’s not even an estimate because it’s so structurally complex,” he says. Given the variety of types of dietary fiber, it’s perhaps not a surprise that the source of the fiber you consume matters a lot. As Dr. Bulsiewicz emphasizes, “fiber is not just fiber. You can’t just count grams.

Although it’s impossible to enumerate the different types of dietary fiber, we can at least break them down into two main categories: soluble fiber and insoluble fiber. Soluble fiber, as Dr. Bulsiewicz describes it, will dissolve if you stir it up in a beverage. Insoluble fiber, on the other hand, will never dissolve no matter how much you stir. This is the type of dietary fiber that we’re referring to when we talk about “roughage,” and it’s also what most of us think of when we talk about fiber. 

While we’ve been taught that, as Dr. Bulsiewicz puts it, “fiber goes in the mouth and it goes through the intestine and it launches out the other end like a torpedo,” this only applies to insoluble fiber, not soluble fiber. Most of us have no idea about the very different soluble fiber properties, as well as the very different soluble fiber benefits. According to Dr. Bulsiewicz this is unfortunate, because soluble fiber is “the secret of nutrition that people are not talking about.”

The power of soluble fiber benefits come from the fact that soluble fiber is prebiotic, which means it feeds and nourishes the microbes in your gut. See, soluble fiber passes through the small intestine untouched, then enters the large intestine, or colon, which is the predominant location of your microbiome — in other words, it’s where most of your microbes live. So once the soluble fiber arrives there, the microbes get into a “feeding frenzy.” They devour the food source, which allows them to grow stronger and multiply. 

This is where the soluble fiber benefits kick in. Now that it has helped produce more anti-inflammatory microbes, the microbes turn around and pay you back for the food by releasing what Dr. Bulsiewicz calls “the biggest secret in all of nutrition:” short-chain fatty acids. These short-chain fatty acids, which are actually postbiotic compounds, are incredibly healing. They’ve been associated with a ton of health benefits, and have been shown to reduce risk of obesity, inflammatory diseases, type 2 diabetes, heart disease, and more.

Why Food Diversity Is So Important for Your Health 

Besides the importance of fiber, one of the points that Dr. Bulsiewicz emphasizes in his practice is the importance of food diversity in your diet. Based on his research and experience, he believes that eating food in abundance, and eating many different types of food, is essential. “But that is actually quite divergent from what we’ve been told for the last 15 to 20 years with most of the popular diets the people have been doing,” he says, nothing that these diets have all focused on restrictive rules and what not to eat. 

What he’s proposing is quite different, and quite straight-forward: “There’s one rule. It’s simple. You don’t need to count your calories. You don’t need to weigh your food. You don’t need to worry about macros. You just need the diversity of plants.”

So why does food diversity and diversity of plants matter so much? It all comes back to those soluble fiber benefits. Remember how soluble fiber feeds the microbes in your gut? Well, all of those microbes eat different types of dietary fiber, and because every type of plant contains its own mix of fiber, you need to eat many different types of plants to keep all your healthy microbes satisfied. In other words, food diversity means fiber diversity means microbe diversity, which, in turn, means a healthy gut and a healthy you (download our free guide to gut health here!). 

While this argument for food diversity makes sense on an intuitive level, there’s also hard research to back it up. Dr. Bulsiewicz cites the largest study to date — including more than 11,000 people — which correlated diet and lifestyle with the diversity (and thus the health) of the gut microbiome. The results were unequivocal: “They found one factor that was the clear-cut number one thing associated with a healthy gut microbiome, and that was the diversity of plants in your diet.”

So what happens when you turn away from food diversity and start eliminating categories of food from your diet? Dr. Bulsiewicz says that it’s “not the end of the world” if you turn away from one food or a small category of foods. If you choose to eliminate nightshades, for example — a family of plants that includes peppers, tomatoes, and eggplants — as long as you focus on plant-based food diversity in the rest of your diet, you’ll be okay.

Where things get troublesome is when it comes to categorical elimination: for instance, the elimination of legumes or whole grains, two food categories that are commonly cut out on various diets. “The studies are very consistent in their findings, whether it is looking at a gluten-free diet, whether it is looking at a low-FODMAP diet, whether it’s looking at the paleo diet, where people completely eliminate whole grains and legumes,” says Dr. Bulsiewicz. “What we consistently find is that it actually damages the gut.” 

This is because there are specific microbes in your gut that thrive when you consume legumes, so if you get rid of legumes entirely, those microbes die. Similarly, there are healthful microbes that survive on the consumption of whole grains, as well as harmful microbes that thrive in their absence. All of this underscores the importance of fiber and food diversity in your diet. Makes you want to go to the farmer’s market, right??

Takeaways: How to Use the Importance of Fiber for Your Health

As Dr. Bulsiewicz knows, fiber is hugely important, and hugely misunderstood by people. But as his research and personal experience has shown him, the importance of fiber cannot be underestimated. It’s a complex topic, but here are the key things to remember about fiber and your health:

• Fiber comes from plants. Every plant contains fiber, and you can’t find dietary fiber anywhere but plants. 

• There are tons of different types of dietary fiber. So many that we don’t even know the number. However, we can break them down into two broad categories: soluble fiber and insoluble fiber.

• Insoluble fiber is the fiber most of us think of when we think of fiber. It passes untouched through our digestive system and provides “roughage.” Soluble fiber, on the other hand, is broken down in our large intestine, and the benefits of soluble fiber are powerful and not widely known. 

• Soluble fiber is so good for our health because it is prebiotic. This means that it feeds the microbes in our gut. This promotes gut health and allows the microbes to produce short-chain fatty acids, which reduce risk of disease.

• Each microbe eats a different type of dietary fiber, and each plant contains a different mix of fiber. This means that plant-based food diversity in your diet is vital for a healthy gut.

• Categorical elimination of certain types of food starves certain good microbes in your gut while simultaneously promoting the growth of harmful ones.

• For optimal gut health and overall health. Dr. Bulsiewicz recommends a mostly plant-based diet (you don’t need to go fully vegan or vegetarian — 80% is enough!) with as much food diversity as possible (aka, don’t eat the same veggies every day!).

Watch our full interview with Dr. Bulsiewicz to learn:

•  The harmful substance your microbiome produces when you cut out whole grains and eat red meat

• Why the elimination of processed foods is so essential to gut health (and how many food additives are approved for human consumption in the U.S.)

• His concerns about the paleo diet

• What he thinks about processed foods that contain fiber

• Why he takes a prebiotic supplement but not a probiotic

• His opinion on raw vs. cooked vegetables

• How many edible plants there are on the planet, and much more.

You can also listen to an audio version of our interview with Dr. Will Bulsiewicz on The WellBe Podcast. 

The information contained in this article comes from our interview with Dr. Will Bulsiewicz, MD, MSCI. His qualifications and training include graduating from Georgetown School of Medicine, a Master of Science in Clinical Investigation from Northwestern University, and a certificate in Nutrition from Cornell University. He trained in medicine at Northwestern Memorial Hospital and gastroenterology at the University of North Carolina Hospitals. He is double board-certified in internal medicine and gastroenterology. You can learn more about him here.

How do you make sure you have plant-based food diversity in your diet? Tell us in the comments below!