Fibermaxxing FTW: Meet the Neglected Nutrient That's the Key to Health

Move over protein, it's time to fibermaxx! Fiber has always been the neglected stepchild of nutrients: it's never been seen as essential, and some of its earliest enthusiasts were poop-obsessed oddballs. Take the inventor of cornflakes, John Harvey Kellogg. Inspired by the gorillas he saw at the zoo, he was fiber's first popular champion—but he also went around making people sniff his own perfect poops, and he believed that eating a plant-based, fiber-heavy, and condiment-free diet would help achieve his goal of reducing masturbation. It wasn't until surprisingly recently that scientists realized that fiber doesn't just prevent constipation—eating enough of it cuts your overall risk of death by about a quarter by lowering your cholesterol, helping keep your blood sugar and your weight stable, and preventing cancer. So it's a huge problem that nearly 95 percent of Americans aren’t getting enough of the stuff: on average, we eat about half what we should to hit government recommendations. This episode, we've got the story of fiber's embarrassing origins, the scoop on what all the different kinds of fiber actually are and do, and the emerging science behind its most magical powers—plus, the top tips you need to get more of it, deliciously and without digestive discomfort. Listen in—and, if you don't end up fiber-obsessed, too, we'll eat our whole-grain hats! Learn more about your ad choices. Visit podcastchoices.com/adchoices

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:03Speaker 00

  In Health Watch Now, a new trend called "fiber maxing" is going viral online. The trend encourages people to eat more gut-boosting foods like fruits and beans and legumes to maximize their daily fiber intake.

• 0:17Speaker 01

  All right, well, it's time for your weekend wellness. A new TikTok trend called "fiber maxing" is gaining steam, which involves routinely adding fiber to your diet.

• 0:29Cynthia Graber?

  So you've probably heard of protein maxing. You've seen all the protein-pumped Doritos and Pop-Tarts and ice cream on the shelves in the supermarkets. Well, there's a new trend in town, and it's, you guessed it, fiber maxing.

• 0:40Nicola Twilley?

  Of course, as Gastropod listeners, you all are well ahead of the curve, and you've been writing to us about fiber for a hot minute already.

• 0:48Speaker 04

  Hey, this is Jay, calling from La Cañeda, California, and I had some questions about dietary fibers.

• 0:53Cynthia Graber?

  Jay's son was having, well, some challenges in the bathroom. And so they were trying to figure out whether they were all getting enough fiber in their diet.

• 1:00Nicola Twilley?

  At the time, Jay's son's favorite vegetable was spinach. And before they asked Gastropod, they turned to Alexa.

• 1:06Speaker 04

  And we asked how many grams of recommended dietary fiber intake is per day, but also asked how much spinach we would need to eat per day to get the required recommended amount. And it turned out to be something ridiculous, like four pounds of spinach per day.

• 1:24Cynthia Graber?

  Jay and his son were alarmed. Even a spinach lover doesn't want to eat four pounds of spinach a day.

• 1:29Nicola Twilley?

  This is a common response from folks. Our listener Ranjit started looking into what he would have to eat to hit US recommended daily amounts for fiber. And he quickly concluded that either the fiber RDA was absurdly high or what he thought were high-fiber foods were absurdly low in fiber. He was shocked and a little traumatized once he figured out how many prunes he'd have to eat to hit the target.

• 1:53Cynthia Graber?

  So what's going on? Are we all eating disastrously low amounts of fiber today? Do we really need as much fiber as the recommended daily allowance in the US claims? And if so, what would we eat to get there?

• 2:04Nicola Twilley?

  What is fiber doing for us, anyway? How important is it to get enough?

• 2:08Speaker 04

  There was also questions that we came up with about soluble fiber versus insoluble fiber, like what difference does it make for us?

• 2:15Cynthia Graber?

  We're solving all those mysteries this episode. Plus, we have the wild story of how the father of cornflakes was obsessed with gorilla bowel movements and what that meant for our national diet. A quick note, that cornflake dad, aka John Harvey Kellogg, he had some strange ideas about fiber and sex. And that means we will be mentioning sex this episode. If you listen with kids, you might want to check the episode out yourself first.

• 2:38Nicola Twilley?

  This episode was supported in part by the Bar's Welcome Fund for our coverage of biomedical research, as well as the Alfred P. Sloan Foundation for the Public Understanding of Science, Technology, and Economics. Gastropod is part of the Vox Media Podcast Network.

• 2:56Cynthia Graber?

  Support for this episode comes in part from Ring. Whatever shows up around your home this holiday season, with Ring, you're protected. Ring makes it easy to keep an eye on your home inside and out. Battery Doorbell 2K Plus features Retinal 2K Clarity and up to six times enhanced zoom, so you can take a closer look without losing detail. Indoors, the compact indoor cam plugs in almost anywhere, making it simple to check in on your home and pets while you're away. And Outdoor Cam Plus lets you keep an eye on your yard and other outdoor spaces, no matter where you are. With Ring, you're protected for the holidays. Shop cameras, doorbells, and more at ring.com.

• 3:32Speaker 05

  Support for the show comes from Google Gemini. You know that moment when curiosity strikes? You're exploring a new city with the family, and a sculpture catches your daughter's eye. What is that? Who made it? With Gemini, you can snap a picture and get a fully interactive breakdown of its history, architecture, and even hidden secrets. Gemini is built for moments like that. Whether you're learning something new, solving a problem, or looking for your next favorite thing, Google Gemini is here to help. Download Gemini, Google's AI app, or visit gemini.google.com to learn more.

• 4:12Speaker 06

  We've actually known about fiber for a really long time. It's just not necessarily been in the public awareness as anything that's necessarily worth talking about.

• 4:20Nicola Twilley?

  Emily Leeming is a researcher in nutrition and the gut microbiome at King's College, London. She's also the author of a new book, Fiber Power.

• 4:28Speaker 06

  And I think part of the reason for that is that fiber's been just seen as something that helps you do a nice, smooth poo.

• 4:35Cynthia Graber?

  As far back as the ancient Greeks, people connected their diets to their poops. Greek physicians wrote that whole grains were important to prevent constipation, which they weren't a fan of.

• 4:43Nicola Twilley?

  I mean, really, who is? But no one fully figured out what it was about certain foods that meant that eating them kept things moving in the bowel department until the 1800s.

• 4:53Speaker 07

  Well, we have to thank the animal nutritionists for giving us an interest in dietary fiber, because the animal nutrition world knew very much in the 19th century that the amount of fiber in an animal food, particularly those for ruminants, had a major effect on digestion.

• 5:12Cynthia Graber?

  John Cummings is an emeritus professor of gastroenterology at the University of Dundee and a pioneer in fiber research. And he says that agriculture researchers weren't fans of fiber, because they could tell that it came out the other end and wasn't digested, and they wanted to make sure every bite of food was transformed into meat or dairy.

• 5:30Speaker 07

  And they developed, I mean, the first methods for measuring dietary fiber were in around 1810, something like that. And they only measured it because it told them how bad the feed was.

• 5:44Nicola Twilley?

  The 1800s were a difficult time for fiber in general. First, the animal researchers measured it and labeled it as undesirable. And then later that century, John Harvey Kellogg came along.

• 5:56Speaker 06

  I think he might be partly to blame for the fact that fiber has such a bad reputation. So Kellogg had a few strong views. And some of his views was around having, like, really plain, high-fiber, and quite kind of stodgy food as being a way of keeping any kind of sexual desire at bay.

• 6:20Cynthia Graber?

  We're going to get back to that issue of sexual desire. But John Harvey Kellogg was kind of the king of fiber back in the day. To put Kellogg into context, you have to understand just what was going on in the guts of Americans in the mid to late 1800s.

• 6:33Speaker 01

  They had constipation, they had upset stomachs, they had all sorts of things that what Walt Whitman called "the great American stomachache." And, you know, depression and other issues were as common then, if not diagnosed, as they are now. And so everyone blamed everything on their gut.

• 6:52Nicola Twilley?

  Howard Markell is a retired professor and physician, and also the author of The Kelloggs, The Battling Brothers of Battle Creek. He told us that doctors at the time were pretty helpless in the face of this great national blockage. There really wasn't much medical science could do to help Americans with their bowel problems in the 1800s.

• 7:10Speaker 01

  They had, you know, old-fashioned laxatives like Senna, which is, I remember when I was introduced to that as a third year of medical school, the intern said, "This stuff could move a mule. It's really toxic to the gut." So they did stuff like that, Epsom salts, Calamal, which is a mercurial, which will actually turn your stools green. And a lot of people strained its stool and developed fissures and hemorrhoids and tags and stuff like that, too. So it really was a problem.

• 7:42Cynthia Graber?

  John Harvey Kellogg experienced all this firsthand. When he was a kid, he contracted tuberculosis, and he was pretty sickly. But it was really his bowels that knocked him out. He had bloody poop and hemorrhoids.

• 7:53Speaker 01

  He also had a lot of GI issues, including rectal fissures, which are little tears in the, in the rectal area, which hurt like beyond anything you've ever had. They're very painful. And he ate, like many people then, bad food. Fried food, grease from last night's dinner saved to cook the next morning's eggs. Candy, he was a real sugar guy back as a little kid. He loved candy.

• 8:21Nicola Twilley?

  All of this meant John Harvey was kind of the runt of the Kellogg litter. His father wasn't confident he was even going to make it to adulthood.

• 8:27Cynthia Graber?

  His father and his mother both played a major role in shaping Kellogg's future, in part because they both got caught up in the 19th century movement called Adventism. Its leaders and followers believed in the literal second coming, the advent of Jesus, like any day now.

• 8:41Speaker 01

  One of the most biggest proponents was a guy named Miller, who actually made the mistake of saying it'll happen on October 5th, you know, 1820. And he was wrong, and he lost his flock.

• 8:51Nicola Twilley?

  Another Adventist leader called Ellen White was a little smarter. She also proclaimed the imminent return of Jesus Christ, but she didn't put a precise date on it.

• 9:00Speaker 01

  The Kelloggs joined that flock and moved to Michigan, which was, you know, frontier.

• 9:07Cynthia Graber?

  Ellen White was one of the founders of this group that Kellogg's joined. It's now called the Seventh-day Adventists. Seventh-day because they celebrate the Sabbath on the same day as we Jews do, on Saturday, the seventh day of the week, counting from Sunday. But a bunch of Ellen White's beliefs had to do with health, with food.

• 9:22Speaker 01

  Certain foods were considered to be excitatory in the Seventh-day Adventists, you know, health book. Meat, flesh, as they called it, was one of them. Spicy condiments were another, because, you know, you're eating bland food, so you slather with mustard or, you know, relish, whatever. Other spicy foods were considered to be excitatory. And some of these excitatory foods, particularly Ellen White said, but Kellogg would parrot, could make young people a little bit more frisky than they needed to be, put it politely.

• 9:53Nicola Twilley?

  And here we are, back at sex. More on that shortly. But the point is, Ellen White believed that the body was a temple, that people should eat a largely vegetarian diet, and that stimulants like tobacco, coffee, and tea were all bad.

• 10:06Speaker 01

  And John Harvey Kellogg was this precocious, bright little guy who spent all of his evenings at the White family, the Pope S of the church.

• 10:17Cynthia Graber?

  Ellen recruited him to work for their in-house church magazines as a typesetter.

• 10:20Speaker 01

  And then from then, he started writing their health reformer magazine. And from There, they sent him off to medical school and they realized he needed a good education.

• 10:30Nicola Twilley?

  Along with their health-focused magazine, the church also ran a health clinic in Battle Creek, Michigan. It was called the Western Health Reform Institute.

• 10:39Cynthia Graber?

  While Ellen had all sorts of ideas about how to make her flock healthy, it didn't translate into a particularly lovely experience at the institute. The rooms were dark and cold and moldy. The patients there couldn't play board games. Who knows? Maybe that would have been too excitatory. But the worst was the mushy, bland, disgusting food. This all helps explain why the Reform Institute was going bankrupt.

• 11:00Nicola Twilley?

  In an attempt to save this rather sad institution, once Kellogg had graduated medical school, Ellen and the church elders invited him to take it over and run it. Starting in 1876, he utterly transformed it. He spent a large amount of the church's money rebuilding it from scratch into a palace of health. It was the tallest building between Chicago and Detroit. It had state-of-the-art laboratories and luxurious hydrotherapy rooms, a dedicated kitchen garden, and a 1,000-seat auditorium.

• 11:32Speaker 01

  Lectures, proper food in the dining hall, exercise, massage. If you needed medical treatment, they had that, and research. It was a remarkable place. And it became the sanitarium, the Battle Creek Sanitarium, and he was the medical director. And he kept making it bigger and better. And Battle Creek was the number one train stop on the Michigan Central because all these invalids, if you will, people with crippled stomachs or constipation or what have you, they all came to Battle Creek to be cured or treated by Dr. John Harvey Kellogg.

• 12:11Cynthia Graber?

  Kellogg was super charismatic and a bit of a showman.

• 12:14Speaker 01

  He was a very entertaining man, about five foot two, very dramatic, knew how to speak, always wore white suits. So if he got a speck of dirt on it, he could quickly change.

• 12:26Nicola Twilley?

  This three-piece white suit, made to measure by Marshall Field of Chicago, it allowed him to immediately see if he'd come into contact with any impurities. Kellogg was all about hygienic living.

• 12:38Cynthia Graber?

  He also had an explosive temper and could be witheringly sarcastic to anyone who crossed him. He was an incredibly domineering boss who made his brother run next to him while he biked around the grounds of the sanitarium to take notes on his thoughts. And his brother had to even follow him into the bathroom so as not to miss a single inspired idea.

• 12:56Speaker 01

  He was very exacting. He wanted things his way. He knew what he wanted. But he also was easily bored and very restless and had a million things on his mind. And he was the kind of guy who would be writing a lecture at the same time he'd be riding his bike or inventing peanut butter or whatever. I mean, he did a million things. John Harvey, I would not want to work for John Harvey Kellogg.

• 13:17Nicola Twilley?

  In terms of the regime at the SAN, as it came to be known, Kellogg followed Ellen White's health prescriptions around exercise and of course, avoiding anything too exciting and eating a plant-based diet. But he also brought his own particular obsession with bowels, which meant serving foods that contributed to frequent and exceedingly smooth poos.

• 13:38Cynthia Graber?

  Kellogg became famous for inventing a cereal that was basically like Grape-Nuts and also with his brother inventing Corn Flakes. The tale of the two brothers and their lifelong feud is fascinating, and you should read Howard's book, The Kelloggs, to get the whole story. But the point is that Kellogg became one of the most famous doctors in the US in large part because of his focus on fiber-rich food and on the nation's bowels. Like we said, that was a huge problem in America.

• 14:02Speaker 01

  So John D. Rockefeller Jr. was a patient who was obsessed with his bowel movements. And he would send his bowel movements from Rockefeller Center on the 56th floor to Battle Creek for analysis. Henry Ford was a patient. Movie stars, he brought them in and gave them free room and board. Eddie Cantor, Johnny Weismuller.

• 14:25Nicola Twilley?

  They were both huge stars in their day. Eddie sang "Making Whoopie", though probably not at the SAN. The flowers and trees and the birds and bees are making whoopie. And Johnny was the original Tarzan.

• 14:42Speaker 01

  They mixed and mingled with the guests while they lived the high life because it was a very beautiful hotel and they ate good foods. He attracted athletes, coaches, and even Booker T. Washington was a patient there at one time. Amelia Earhart was a patient there. Thomas Edison was, you know, so it was really the place to be. If you were sick and rich, or you were the worried well, it was really the place to be.

• 15:11Cynthia Graber?

  Kellogg's books were bestsellers. His magazine, Good Health, was read by millions of people. He was even nominated for a Nobel Prize for his research on digestion and diet.

• 15:20Speaker 01

  But the orthodox physicians, you know, like the people at Johns Hopkins or Harvard, thought he was a nut. Yeah, maybe not a nut, but he was a little wacky. And because he was so flamboyant, you know, he attracted a crowd no matter where he went. And in the state medical profession, he was considered to be a little bit out there.

• 15:40Nicola Twilley?

  To be fair, Kellogg did have some extremely misguided ideas. For one, later in life, he became caught up in eugenics and what he called race betterment, which was unfortunate to say the least.

• 15:52Cynthia Graber?

  And then there were those weird ideas that certain foods would cause, as Howard said, friskiness. Basically, Kellogg was concerned that foods could lead people to masturbate, and that for him was quite possibly one of the worst things ever. His relationship to sex was not the healthiest.

• 16:06Speaker 01

  And even, you know, Dr. Kellogg may or may not have ever had sexual relations with his wife that he was married to for 40 years. No one's quite clear on that. They had four, nearly 40 adopted children, but they lived in separate parts of their house. No one knows.

• 16:22Nicola Twilley?

  Again, to be fair, sex was a pretty risky business in the 1800s. An estimated one in 10 Americans had syphilis.

• 16:29Speaker 01

  You know, 50,000 Americans, just in the US, died of syphilis every year before 1946, when penicillin was developed. And it's not just the death. Syphilis is a long-term debilitating neurologic, cardiologic, bad disease that you don't want under any circumstances. And that doesn't even count for gonorrhea, which is quite common.

• 16:53Cynthia Graber?

  Still, there's obviously nothing wrong with masturbation. And especially now that we have antibiotics, there's nothing wrong with safe sex. That said, Kellogg was right about a lot of things.

• 17:02Speaker 01

  He was talking about the dangers of lead water pipes in Michigan in the 1870s and 1880s.

• 17:09Nicola Twilley?

  Which are still an issue today. Kellogg supported women's right to vote, which was not common at the time. He started a company to make special lights to help people with seasonal affective disorder long before that even had a name.

• 17:21Cynthia Graber?

  He was also ahead of his time when it came to exercise.

• 17:23Speaker 01

  He had a set of records. I have them, 78 RPM records with a brass band where he's telling you how to exercise. It's called the Kellogg Ladder of Exercise. So not only could you do it in the sanitarium, but you could play these records, long before Jane Fonda got to videotape, to do a whole, you know, program of exercises. Rest. Take a deep breath. Raise arms. Turn palms back. Move arms backward and forward at each count. Ready? Begin. One, two, one, two, one, two, one, two.

• 18:03Speaker 01

  But where he was particularly prescient is in terms of eating a whole grain vegetarian diet is probably better for you.

• 18:13Nicola Twilley?

  In terms of both diet and poo, Kellogg aspired to be like the gorillas he'd seen at the zoo.

• 18:18Speaker 01

  He figured that they had five to six bowel movements a day. And since that was the closest primate to humans, he felt that we should have four to five or six bowel movements. He did.

• 18:29Cynthia Graber?

  Which sounds a little excessive, but Kellogg lived until nearly 100. And in his final decade, he was famous for sunbathing and exercising while wearing only a tiny little wisp of cotton that barely covered him up.

• 18:41Speaker 01

  And his grain diet, he was so proud of it that he would show you like a little container of his stool at his older age, when he was like in his 90s. He'd say, "Look, it doesn't smell bad. It's like the perfect stool."

• 18:52Nicola Twilley?

  Kellogg certainly loved fiber with a passion, but as you may have already noticed, he was just the tiniest bit eccentric. To Emily, having Kellogg as the main cheerleader for Team Fiber was a mixed blessing, to say the least.

• 19:05Speaker 06

  So it's almost been related to this taboo topic of, let's not talk about it. It's a bit embarrassing. It's a bit awkward.

• 19:12Cynthia Graber?

  It took constipated sailors, a walk on the beach, and our emerging understanding of our gut microbiome to help fiber claim its true place in the spotlight. Coming up after the break.

• 19:25Cynthia Graber?

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• 20:04Nicola Twilley?

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• 20:20Speaker 01

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• 21:04Speaker 01

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• 21:24Nicola Twilley?

  Despite Kellogg's enthusiastic boosterism around eating a high-fiber diet in the late 1800s and early 1900s, fiber was still largely ignored by the medical establishment all the way up into the 1950s.

• 21:37Speaker 07

  I mean, fiber wasn't even listed in the cumulative index medicus until about the mid 60s. If you looked up fiber, all you could find was fiber optics.

• 21:47Cynthia Graber?

  This is from a TV interview from 1990 with a surgeon and researcher named Dennis Burkitt when he was already in his 80s. Burkitt basically was the fiber man. He's Mr. Fiber.

• 21:56Nicola Twilley?

  As a young man, Burkitt joined the British Empire's Colonial Medical Service. He was posted to Uganda and over the course of his career, he ended up putting together a whole set of clues to tell a brand new story about fiber.

• 22:09Cynthia Graber?

  Like in any good mystery story, there were a bunch of different characters who all gave Burkitt hints along the way. The first was a naval officer named Thomas Latimer Cleve.

• 22:18Speaker 07

  Cleve was an irascible character, a captain in the British Navy who had to manage the bowels of naval ratings on the battleships.

• 22:28Cynthia Graber?

  Cleve found that the sailors were regular if they ate bran, but he primarily blamed sugar in their diets and not the lack of fiber for their constipation.

• 22:36Nicola Twilley?

  Burkitt and Cleve met and talked, so Burkitt knew about Cleve's belief in bran. Then a surgeon called Neil Painter came along and showed that feeding bran to people with a bowel disease called diverticulitis cured that. So that was another score for fiber.

• 22:49Cynthia Graber?

  And then a scientist named Alec Walker broadened the picture. He noticed that some groups of people who ate higher fiber diets had lower levels of cholesterol. And then he confirmed that connection by doing experiments where he fed fiber to people and yes, it lowered their cholesterol.

• 23:03Nicola Twilley?

  And then along came the final piece of the puzzle in the form of Hugh Traul.

• 23:08Speaker 07

  Hugh Traul was a pediatrician and he went out to the same hospital in Uganda that Dennis Burkitt was working at to follow a sort of religious calling that he had to do good in places like Africa.

• 23:23Cynthia Graber?

  Hugh Traul noticed that a bunch of diseases that he called non-infective diseases, or what we call non-communicable diseases or chronic diseases today, conditions like heart disease and type 2 diabetes and high blood pressure. He saw that they were rare in Africa and higher in Europe and North America.

• 23:39Nicola Twilley?

  Burkitt and Traul talked and then Burkitt was walking along the beach on the Kenyan coast and as he liked to tell the story, he had a eureka moment. The connection was fiber. In Africa, where people ate tons of fiber, all these chronic diseases were super rare and the opposite was true in the West.

• 23:58Cynthia Graber?

  Burkitt was a surgeon and for the first step in proving his fiber solves everything theory, he wanted to try to connect fiber to a disease that seemed to be linked logically to poop, and that's bowel cancer. His research involved having people swallow plastic pellets that would come out in their poop and show up in x-rays.

• 24:13Nicola Twilley?

  He collected a lot of feces. He started with his own family storing the shit in the fridge and then he compared these Western poos to those from African people with a very traditional diet.

• 24:23Speaker 07

  I've had to travel hundreds of miles in Africa with a car full of stools because I didn't have an x-ray in the hospital consent.

• 24:30Nicola Twilley?

  Burkitt suffered for his science, but it was worth it in the end. All of this stool collection and x-raying proved his theory. The faster stuff moved through your gut and came out the other end, the less likely you were to get bowel cancer and the difference was fiber. At this point, Burkitt was 100% convinced fiber was the key to health.

• 24:51Speaker 07

  And he wrote a pick in The Lancet in the late 100s saying, related disease, related cause. And the cause is lack of fiber and the related diseases were the ones that we're talking about now, heart disease, diabetes, bowel cancer.

• 25:05Cynthia Graber?

  Burkitt's new fiber hypothesis, that fiber was the solution to basically everything, it was huge news. His book on the topic had a super catchy title, "Don't Forget Fiber in Your Diet to Help Avoid Many of Our Commonest Diseases." It was an international bestseller. He was famous for one particular pithy summary of his theory.

• 25:22Speaker 07

  I think soft stools are a very important factor for preventing a great deal of our Western diseases and fiber is the secret. Possibly if communities pass large stools, they can manage with smaller hospitals. If they pass small stools, they have to have larger hospitals.

• 25:41Nicola Twilley?

  Even today, fiber and gut researchers will quote Burkitt's "large stools mean small hospitals" rule.

• 25:47Speaker 07

  He had an enormous impact. I remember a colleague of mine who heard him give a lecture in somewhere in Yorkshire in the 1970s to have a group of doctors and he phoned me up and said, the next day, all the bran had gone from the shelves in the local supermarket.

• 26:04Cynthia Graber?

  At the same time that Burkitt was becoming famous for his fiber pronouncements and research, John Cummings was a young scientist just getting started and he was assigned to do a review on fiber and then he spent the rest of his life on the topic.

• 26:15Nicola Twilley?

  People had assumed that the reason fiber made your poos so much larger is because the poos were full of big fibrous fibers and also the fiber absorbed water, which bulked things up. John and his colleagues looked into this and discovered that actually wasn't the main reason a fiber-rich diet led to larger poos.

• 26:34Cynthia Graber?

  Instead, what John found out was that poop is about 50% bacteria and the more fiber John and his colleagues fed their subjects, the more bacteria they pooped out. It looked like the fiber bulked poop up mostly by feeding microbes, which then multiplied and multiplied and got pooped out.

• 26:48Nicola Twilley?

  John was based at Cambridge University in the UK and his colleagues in the same building were doing all this groundbreaking research into how genes controlled organ development and cell death and much more besides. So John's discovery of the relationship between fiber and the gut microbiome, that kind of got lost.

• 27:07Speaker 07

  They ran away with the Nobel Prizes and we ran away with some what were thought to be rather unimportant discoveries about what the microflora were doing. But historically, starting to identify the microflora as important has proven to be today very significant.

• 27:29Cynthia Graber?

  That's putting it mildly.

• 27:30Speaker 01

  What's been really phenomenal over the past 10 to 15 years is that we, there's been this awakening that this gut microbial community is essential for our health.

• 27:41Nicola Twilley?

  Justin Sonnenberg is a professor of microbiology and immunology at Stanford University and a leader in this wildly exciting and pretty new field of gut microbiome research.

• 27:51Cynthia Graber?

  So what does this all mean? Why is it so important that fiber is feeding microbes in our gut? And fiber and those microbes, how are they all affecting our health? And what does this mean about what we should be eating? That's coming up after the break.

• 28:05Speaker 01

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• 29:35Nicola Twilley?

  So far this episode, we've been talking about fiber like we all know what fiber is. But the truth is, most of us, me included, we kind of do, but we really don't.

• 29:45Speaker 06

  So when we talk about fiber, it's really a kind of catchall term because we have lots of different types of fiber. And when I say different types, I really mean that fiber can have different jobs and different characteristics.

• 29:59Cynthia Graber?

  Emily says one thing we do know is that fiber comes almost entirely from plants, not animals.

• 30:04Speaker 06

  Fiber is a type of carbohydrate. Typically, when we're talking about fiber, we mean the roughage in plants. So fiber tends to be mostly found in the cell wall. And for plants, that cell wall really often gives that kind of plant a bit of structure.

• 30:22Cynthia Graber?

  Animal cells have cell membranes, not cell walls. Ours are more flexible, but plants need something stiffer because they need, say, some extra buffering in the wind and to protect against predators. They can't run and hide.

• 30:33Nicola Twilley?

  But when it comes to our diet and health, fiber also has another definition. Unlike all the other food we eat, which our bodies digest in our stomach and small intestine, fiber arrives in our large intestine, in our colons, still basically untouched.

• 30:49Speaker 06

  And therefore, it kind of has like this, you know, big arrival onto the table of your gut microbes for them to feed on it.

• 30:58Cynthia Graber?

  Erica Sonnenberg, she's a scientist at Stanford. She works with Justin, and she's also married to him. She says saving some of our food for our colon was a useful evolutionary development.

• 31:08Speaker 01

  We just don't have the enzymes encoded within our genome to deal with all the different complex carbohydrates that make up dietary fiber. And in a way, we sort of like farmed that aspect of digesting food off to our microbiomes.

• 31:23Nicola Twilley?

  Just to add a wrinkle, this definition of fiber as the edible food that has to be digested in our colon by our microbes, it includes more than that cell wall roughage stuff.

• 31:34Speaker 06

  You can also find what's called resistant starch. This is a kind of new type of fiber. And these are kind of found as granules within the heart of the cell. And then the resistant starches are simply, again, what it sounds like it says, where they're resistant to being kind of broken down in the small intestine and are therefore, again, useful for feeding your gut microbes.

• 31:58Cynthia Graber?

  So that's the big picture definition of fiber. These are particular parts of plants, and they don't get digested until our colons. But we also categorize fibers by the way they behave. There's insoluble fiber and soluble fiber. Soluble can be dissolved in water.

• 32:12Speaker 06

  This actually is a fiber that most of us probably aren't as familiar with because it can actually be completely invisible to the human eye and might not have that kind of texture that we're used to when we think of something being fibrous. For example, your cup of coffee, which is nice and smooth and just is a nice drink, has some soluble fiber in that, but you just won't know that it's there because you can't necessarily kind of feel that kind of crunchingness to it.

• 32:38Nicola Twilley?

  Other terms you might have heard talking about fiber are prebiotics. Those are the particular types of fiber that scientists know that specific microbes in our guts like to eat. The way bacteria digest them is by using fermentation, so they're also called fermentable fibers.

• 32:53Cynthia Graber?

  There's also viscous fibers. Those typically form a slow-moving gel as we digest them, hence the name viscous.

• 32:59Speaker 06

  All those different fibers are acting in a different way. And actually, what's wonderful is that all the different foods that we eat, when we have whole foods, all contain a variety of different fiber types.

• 33:09Cynthia Graber?

  For example, an apple has both soluble and insoluble fibers. The peel has insoluble fiber and the flesh has soluble. A bean has both insoluble fiber and resistant starch. And oats have a kind of fiber that's both viscous and a prebiotic that feeds microbes.

• 33:24Nicola Twilley?

  And we need them all because fiber is freaking great.

• 33:27Speaker 06

  I would say as a whole, fiber is one of the most evidence-based nutrients that we have. We've got decades and decades of evidence to show how impactful it is, not just for your digestion, but also your heart health and your risk of type 2 diabetes. And now, more recently, your gut microbiome.

• 33:45Cynthia Graber?

  So it seems kind of shocking that a part of our food that we leave to the microbes in our large intestine to digest would lower our risk of type 2 diabetes and heart disease and have all these impacts throughout the rest of our bodies. What is going on?

• 33:57Nicola Twilley?

  Let's start with the heart health benefits because those are the reason the government tells us to eat a certain amount of fiber.

• 34:03Speaker 06

  Most of the evidence, and particularly the evidence that informs the fiber guidelines, particularly for the US and for the UK, around its benefit for heart health.

• 34:12Cynthia Graber?

  There's a fascinating thing happening in our bodies when we eat fiber that helps our heart. It has to do with that viscous fiber, the kind that forms a thick gel, and its relationship to bile. That's a digestive juice that our liver makes and that usually is kind of recycled in our bodies.

• 34:26Nicola Twilley?

  But the viscous fiber gel, it actually soaks up some of that bile.

• 34:30Speaker 06

  And then carries out of your body to be excreted when you go to the loo. So then your liver then has to make new bile to be able to use by your gut. And it then grabs cholesterol to be able to make that new bile out of your bloodstream.

• 34:45Nicola Twilley?

  Which means you have less cholesterol circulating in your blood, which is a very good thing because we all know that the higher your cholesterol, the higher chance you have of heart problems.

• 34:55Cynthia Graber?

  There's a bunch of other great things that fiber does for feeling full and helping you maintain a healthy weight and for reducing your risk of bowel cancer and type 2 diabetes. These all work in two ways. First, there's the physical way, and then there's what the microbes are doing.

• 35:07Nicola Twilley?

  In terms of feeling full, the physical way fiber works is easy to picture. If you eat bulky, fiber-rich food, your stomach walls get gently stretched, like a little workout, and that tells your brain you've had enough to eat. And then, as our good friend Mr. Fiber, aka Dennis Burkett, discovered, fiber moves things through your gut faster, and that includes any chemicals you've eaten that are potentially cancer-causing. So that physically reduces your risk of cancer.

• 35:34Speaker 06

  That fiber is helping that compound not having too long in contact with the gut wall to then cause a problem.

• 35:41Cynthia Graber?

  For diabetes, Emily says that fiber acts a bit like a net to slow down how quickly sugars in your food enter your bloodstream. So those are the main physical ways fiber is keeping us healthy. But what's also happening is that fiber helps protect against all of these conditions through the actions of our good friends, the microbes.

• 35:58Nicola Twilley?

  Yes, you can drink, although pace yourselves because there are two big ways microbes and fiber are interacting in our guts to affect our health: mucus and waste chemicals. Let's deal with the mucus first.

• 36:10Speaker 01

  Our colon lining is lined with mucus, and that's an important barrier that helps keep the microbes like in the lumen or center of the tube of the intestine and away from our cells. Like, that's an important barrier that needs to be maintained. But that barrier is also made up of carbohydrates.

• 36:29Cynthia Graber?

  And that makes it a backup food source for hungry microbes. If they're not getting enough fiber, they'd prefer fiber. That's their favorite food. But if they're starving, they'll turn to that mucus lining in our bodies instead for sustenance.

• 36:40Speaker 01

  If you're not eating dietary fiber, essentially, your gut microbiome starts eating you. That is, it's backup strategy if you're not eating dietary fiber.

• 36:48Nicola Twilley?

  This is as bad for us as it sounds because once the fiber-starved microbes eat this mucus buffer, that means they're right up against our body.

• 36:57Speaker 01

  And we think that that may be resulting in inflammation in the colon because now all of a sudden, you know, our human cells are like, wait a minute, these bacteria are getting really close. This is not a good situation. We need to incite inflammation and make sure that those bacteria stay, you know, within the tube of the intestine.

• 37:15Cynthia Graber?

  Inflammation isn't necessarily a bad thing. In general, your body reacting to something in the moment with inflammation can be really helpful. Like, if you get cut, your inflammatory system sends all kinds of molecules to the cut to help you heal.

• 37:26Speaker 06

  What we don't want is what we call low-grade inflammation. And this is effectively when your body is just constantly on this kind of like low-grade alert.

• 37:35Nicola Twilley?

  And that low-grade, constant inflammation, that's linked to not just heart disease and diabetes and cancer, but also all sorts of autoimmune disease and mental health issues, too.

• 37:46Cynthia Graber?

  And this mucus lining, or the lack thereof, is another one of the links between fiber and bowel cancer. If there isn't fiber and microbes eat the mucus lining, then that bad stuff in our waste can more easily get in contact with our bowel tissues.

• 37:58Nicola Twilley?

  So this mucus lining issue is the first major way that fiber and our gut bacteria interact to affect our health. But there's something else important because when our gut bacteria eat lots of tasty fiber, they ferment it to digest it so they can extract the energy to use. But that fermentation process produces a bunch of waste chemicals. And it turns out that those waste chemicals are also doing a lot of things in our bodies.

• 38:23Speaker 01

  And butyrate is the big one that everybody talks about in the colon. It's the major short-chain fatty acid produced there. And the bacteria don't have any use for it at that point. But our own cells can take that up and use that to also fuel our own cells' growth. But it also seems to do a bunch of things.

• 38:45Cynthia Graber?

  It feeds the cells in your lining that make mucus, so it helps keep that mucus production going. But that's not all.

• 38:50Speaker 01

  It induces aspects of the immune system that decrease inflammation. And it also induces GLP-1. So most of the GLP-1 in our body is produced in our gut.

• 39:02Cynthia Graber?

  We talked about GLP-1 in our episode "You've Lost That Hungry Feeling." It's an important chemical our bodies make, or that we can take as medicine, that in part helps regulate how hungry or full we feel.

• 39:12Nicola Twilley?

  Like Erica said, butyrate is the poster child for these short-chain fatty acids that are produced when our gut microbes eat fiber. But there are a few of these chemicals that we know are important.

• 39:22Speaker 06

  These are all slightly acting in a different way. And again, this is something that we have quite a new understanding. Propionate, for example, is seen to perhaps be able to signal to your brain to kind of lower cravings and to feel full and satisfied.

• 39:37Cynthia Graber?

  And some of these short-chain fatty acids, they actually serve as food for cells throughout our body, kind of like sugar is a food for cells, but the way they provide energy is different.

• 39:46Speaker 07

  The important thing about it is that it doesn't cause insulin stimulation. So you can have energy obtained from your large bowel without any insulin production. So that's an absolute winner for a diabetic.

• 39:59Nicola Twilley?

  Increasingly, scientists are thinking that quite a few of the benefits of a high-fiber diet are connected to the waste products that our gut bacteria are excreting.

• 40:08Speaker 01

  They're making tons of stuff, like just so many different compounds, most of which we have no idea what they are. We can find these things in our bloodstream and this. This is really where this idea that, you know, what happens in the gut doesn't stay in the gut because those chemicals are circulating throughout our bodies and can influence, you know, our central nervous system, our metabolic health, their basically everything.

• 40:34Cynthia Graber?

  We do have to give one caveat, and it's that a lot of studies that are trying to tease out how these gut bacteria waste chemicals work in our bodies have been done on mice. At the same time, though, as Emily says, fiber is the nutrient that we have the most and the clearest evidence of benefit in human studies, too, that a high-fiber diet is related to a lower incidence of a bunch of diseases.

• 40:53Nicola Twilley?

  Which is why it's so shocking and worrying to find out how little fiber most of us are eating.

• 41:00Speaker 01

  One alarming study led by scientists at the American Heart Association found that nearly 95% of American adults and children don't get enough fiber on a daily basis. Recommendations vary by age and gender, but the average is around 28 grams per day. The AHA says most Americans get just half that.

• 41:19Cynthia Graber?

  We've now spent quite a long time talking about what fiber is good for and what happens when you don't have enough, and Justin says you can see how not having enough has played out in general in the West.

• 41:28Speaker 01

  You can imagine in a society that eliminated dietary fiber and reduced butyrate production, suddenly inflammation would go up and metabolic health would go down. And you would actually predict that the two major classes of drugs that you would need in that society would be drugs that would, you know, decrease inflammation and induce GLP-1.

• 41:51Nicola Twilley?

  Which, hello, America.

• 41:53Cynthia Graber?

  Eating enough fiber is really one of the most significant things you can do for your health. Eating a high-fiber diet reduces your risk of death by around a quarter compared to eating a low-fiber one. It's kind of mind-blowing, but we still don't do it.

• 42:05Speaker 07

  We've been recommending higher fiber intakes for 30, 40 years and totally failed to really make much difference to people's intake. You know, talking about 25, 30 grams a day, and most countries are down in the 15 and 12 grams a day. It's very difficult to get people to do it.

• 42:24Cynthia Graber?

  So I'm guessing that basically all of you are joining me now and asking, what can we do? How can we get more fiber in our diets?

• 42:31Speaker 06

  I think when it comes to what we can do with our modern-day plate, it's really just prioritizing those whole foods, and particularly beans, whole grains, and nuts and seeds. So yes, fruits and vegetables are wonderful, but beans, whole grains, and nuts and seeds do tend to contain about three to four times more fiber than fruits and vegetables.

• 42:50Nicola Twilley?

  If you remember, our listeners Ranjit and Jay were deeply discouraged by how many prunes and how much spinach they'd have to eat to reach their fiber targets, and that is because a lot of fruit and vegetables are not as high in fiber as you'd think.

• 43:04Speaker 06

  We perceived things like salad leaves and spinach as being high-fiber foods when really they're not. Salad leaves, for example, contain about 2 grams of fiber per 100 grams, which is kind of a generous bowl. Still great, I still would advocate absolutely for eating salad. It's a fantastic source of different vitamins and bioactive compounds.

• 43:28Cynthia Graber?

  But it's not going to be your main source of fiber, and there's a logical reason for why grains like whole wheat have more fiber in their cells than salad greens do.

• 43:36Speaker 07

  The wheat, of course, grows over many months and needs a very strong cellular structure to grow up and everything, whereas spinach grows quickly, lies almost on the ground, and is very quick to cook, et cetera. So they're just botanically very different.

• 43:52Nicola Twilley?

  In short, if you're looking to fiber max, beans, nuts, and grains are really your friends.

• 43:57Speaker 07

  They must be whole-grain cereals. Don't mess around. Whole-grain bread, porridge oats, et cetera, and just get used to them. No sob stories about, I don't like that sort of bread.

• 44:08Nicola Twilley?

  You tell them, John. But there are some surprising sources lurking out there. Avocados, raspberries, and dark chocolate are not only foods I personally love with a passion, they're also pretty good sources of fiber.

• 44:20Cynthia Graber?

  Another important tip is to not peel most of your fruit and veggies. Don't peel potatoes, don't peel apples, and also don't peel carrots, just wash them. The carrot peel seems like it's the same as the inside of the carrot, but it actually has quite a bit more fiber.

• 44:32Speaker 06

  If we're talking about juicing, what happens when we juice something? We tend to squeeze that juice out, and then there's the pulp that then gets chucked away. That pulp is where a lot of that fiber is. So if you are someone who loves juice, I would say, could you maybe switch to having a smoothie instead?

• 44:50Nicola Twilley?

  Emily told us about another cool trick, which is a way that fiber gives you yet another reason to embrace your leftovers. If you cook cool and even reheat pasta, rice, potatoes, bingo, now they have something like a third more fiber than they did when they were first cooked.

• 45:06Speaker 06

  These starches, the kind of the way that they're attached and they're shaped in terms of the structure of them, they kind of break apart, and then they kind of come back together in a new way that makes them then resistant starches.

• 45:21Cynthia Graber?

  And as we said earlier, resistant starches are fiber in the sense that they're not digested until they get to your colon. So another reason to love my favorite meal, which is the one I cooked yesterday.

• 45:30Nicola Twilley?

  But I hear you all saying, what about the farts?

• 45:34Speaker 06

  We all should be farting. Like farting is normal, farting is okay. The reason I say that is that when your microbes feed on fiber and they're happy and they're making those molecules that we want for your health, they also make gas as a side effect. So that is something that, you know, if you've, that's why with fiber, that's what's happening, it's your microbes being very happy.

• 45:54Nicola Twilley?

  Drink again, and toot.

• 45:56Cynthia Graber?

  That said, Emily and Justin acknowledge that excessive farting and even gastric discomfort can be the result if you ramp up fiber too quickly.

• 46:03Speaker 01

  You know, if you try to introduce an extra 20 or 30 grams of dietary fiber from one day to the next, it's going to be pretty uncomfortable.

• 46:11Nicola Twilley?

  Increase your fiber intake slowly, drink more water, obviously, consult your doctor if you have digestive issues already, and also know that scientists have found that your farting goes right back to your previous levels just as soon as your body adjusts to your new, higher-fiber lifestyle, which only takes three to six weeks.

• 46:30Cynthia Graber?

  But also, as Emily says, farting is normal, farting is great, and she has a suggestion for how to embrace it. Take a fart walk.

• 46:36Speaker 06

  Which is like after a meal, going for a walk outside, just letting it all go, and then feeling confidence come back.

• 46:43Nicola Twilley?

  Fart walks for the win. But maybe you're still struggling, and you're tempted to just boost your fiber intake with a supplement. Is that going to have all the same magical health effects?

• 46:54Speaker 06

  That fiber type that is within a supplement or you're stirring into a drink is usually just one type of fiber. Sometimes they're now blends of, you know, a couple of different types of fibers. They are still going to be having a kind of net positive effect. It just isn't going to be to the same wide-ranging impact as having a variety of different fiber types. So think of it not as, you know, yes, okay, that's as good as, I wouldn't put it in that bracket, but it's still, you know, a positive in terms of helping you to close that gap if you're really struggling to do that from whole foods alone.

• 47:31Cynthia Graber?

  As we already said, very few people in the Western world are getting enough fiber in their diets. Do we need a major overhaul and to get everyone up to 25 to 30 grams a day?

• 47:41Speaker 06

  What we're aiming for, yes, it's meeting that recommended amount, but even something as small as adding in an extra seven grams of fiber, that's the equivalent of, say, half a can of chickpeas, is related to a significantly lower risk of type 2 diabetes, heart disease, and bowel cancer. So it's even just making a small increase to your everyday that's likely going to have a great impact.

• 48:08Nicola Twilley?

  Every little counts, but that said, all our guests agree, even though we don't hit the recommendations, we also don't know if the recommendations are high enough. We don't actually know if there's an upper limit for fiber, a point at which you stop seeing more benefits.

• 48:23Speaker 07

  At WHO, we drew graphs relating fiber intake to the incidence of different diseases. And at the higher end of the graph, we didn't have enough numbers to suggest how high you could go. So we were up to sort of 28 grams a day, but when we tried to see, look at 30, we didn't have enough information. So that's where we are.

• 48:47Speaker 01

  We could all benefit just from trying to reach those targets, but I don't think that those numbers were really based on any sort of evidence of upper limits. If you look at like these Hadza hunter-gatherers, for example, like they eat up to like 100, 150 grams of dietary fiber per day.

• 49:07Cynthia Graber?

  The Hadza are nomadic hunter-gatherers who live in Tanzania, and the Sonnenbergs have studied their diets and digestion. Scientists have also been able to dig into ancient poop to figure out that our ancestors also probably ate around 100, 150 grams of fiber a day. That's about 10 times more fiber than most Americans get in their diet.

• 49:25Speaker 01

  So I think, you know, in our Western society, it'd be really tough to get that high, but so I don't, I don't think we have to worry about eating too much. Yeah, and I think there really is no evidence that eating too much fiber is unhealthy.

• 49:40Nicola Twilley?

  Another interesting thing about the Hadza and about ancient poo is that scientists have found that there are a bunch of bacteria that we in the West don't have compared to people who eat or who ate a really high-fiber diet. And Justin and Erica have actually found that if you feed mice low-fiber diets, some species of gut microbes just go extinct. They're not found in the next generations.

• 50:02Cynthia Graber?

  Scientists have seen this in immigrants, too. People who ate more fiber and then moved here, they lose some species in their guts. We don't know exactly what that means for our health, but we do know that in general, diversity is a pretty good thing. Justin and Erica wanted to see if mice that had lost species regained them when they went on a higher-fiber diet again, but they didn't.

• 50:20Nicola Twilley?

  But before you start gnashing your teeth and wailing in despair, they have seen evidence that you can get back some of that lost microbial diversity by adding fermented foods to your diet as well as upping your fiber intake.

• 50:33Speaker 01

  If you eat fermented food, you get more species into your gut, and then if you give them fiber, that provides the sustenance they need to thrive and, you know, continue to survive in your gut, and so.

• 50:47Nicola Twilley?

  We think consuming both of those things is probably the ideal. We haven't tested that directly yet, but that seems to be, I mean, all arrows are pointing to that being the ideal pattern.

• 51:00Cynthia Graber?

  And the great news is that if you do make these changes, they come with all sorts of extra benefits on top of everything we've been talking about this episode. Fiber-rich foods have all sorts of other good-for-us things in them.

• 51:10Nicola Twilley?

  And it can displace a ton of bad things in your diet. So if you're eating a low-fiber diet, what are you eating instead? Probably, you know, if you look at the American diet pattern, a lot more saturated fat, a lot more simple carbohydrates. And so fiber is this amazing thing that can keep you full. There doesn't seem to be any negative to eating a bunch of it and can keep your consumption of things that we know are bad for us low as well.

• 51:39Cynthia Graber?

  Honestly, it's kind of funny. If you look at what everyone is worrying about getting enough of these days, it's protein, which basically everyone except weightlifters and the elderly already gets enough of.

• 51:50Speaker 06

  Fiber in comparison, we do have this big, significant gap. So I would say it sounds a bit depressing, but actually I find it a really positive message because it kind of means for most people that upping your fiber is really where this lever is for your health, that you're filling this gap, you can feel confident about it. It's not just fiber, the fact that fiber-rich foods contain lots of other different micronutrients, lots of different bioactive compounds, then you're likely meeting many other nutritional needs simply by doing the action for your health. So it's a kind of simple, you know, easy hack to just like game your health.

• 52:29Cynthia Graber?

  Emily says it's simple, but honestly, she knows it's hard for a lot of people in the West to get enough fiber. That's why she wrote her book Fiber Power. It is full of meals and recipes that can help you get more fiber in your diet deliciously.

• 52:43Nicola Twilley?

  Thanks this episode to Emily Leeming, Howard Markell, John Cummings, and Justin and Erica Sonnenberg. We have links to Emily and Howard's books and John and Justin and Erica's research on our website, gastropod.com, plus a newsletter filled with extras for our special supporters to say thank you for helping us make the show.

• 53:01Cynthia Graber?

  Thanks as always to our fantastic producer Claudia Guyb. We'll be back in a couple of weeks with a brand new episode. Till then.

• 53:10Speaker 01

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