The Peter Attia Drive - November 10, 2025


#372 - AMA #77： Dietary fiber and health outcomes： real benefits, overhyped claims, and practical applications


Episode Stats

Length

24 minutes

Words per Minute

172.96304

Word Count

4,177

Sentence Count

215


Summary


Transcript

00:00:00.000 Hey everyone, welcome to a sneak peek, ask me anything or AMA episode of the drive podcast.
00:00:15.880 I'm your host, Peter Atiyah. At the end of this short episode, I'll explain how you can access
00:00:20.320 the AMA episodes in full, along with a ton of other membership benefits we've created,
00:00:24.960 or you can learn more now by going to peteratiyahmd.com forward slash subscribe.
00:00:30.640 So without further delay, here's today's sneak peek of the ask me anything episode.
00:00:38.900 Welcome to ask me anything AMA episode number 77. In today's AMA, we break down the science of
00:00:46.320 dietary fiber, moving past the sort of generic advice to eat more fiber, to understand what
00:00:53.180 it actually does to the body, where it's truly beneficial to health, how to use it effectively,
00:00:57.760 and where its reputation may exceed the evidence. In this episode, we will discuss how different types
00:01:03.700 of fiber, soluble, insoluble, viscous, and fermentable work in the body and how their
00:01:08.500 distinct properties result in different functional outcomes, whether fiber meaningfully impacts
00:01:14.260 satiety, weight management, and glycemic control, and how those effects compare with more potent tools,
00:01:20.160 how certain fibers influence lipid metabolism and cardiovascular risk, and whether their
00:01:25.320 magnitude of effect justify their reputation, what the mechanistic and epidemiologic evidence
00:01:30.960 really say about fiber's role in colorectal cancer prevention, including the potential influence of
00:01:37.160 short-chain fatty acids like butyrate, who might not tolerate certain fibers well, and how to tailor
00:01:43.240 their intake accordingly, how to move beyond the simplistic goal of getting enough fiber towards
00:01:49.620 a more strategic approach that maximizes its actual benefits. Now, if you're a subscriber and you want
00:01:56.440 to watch the full video of this podcast, you can find it on the show notes page. If you're not a
00:02:01.740 subscriber, you can watch the sneak peek of the video on our YouTube page. So without further delay,
00:02:08.160 I hope you enjoy AMA number 77. Peter, welcome to another AMA. How are you doing?
00:02:21.520 I'm doing well. Thank you once again for having me.
00:02:24.300 Living life, happy, healthy, all is good?
00:02:28.180 Very much so.
00:02:29.460 That's good. People will be sad to learn who listened to our last full AMA. I do not think
00:02:35.620 as of now you are the proud owner of an 87 Caprice Ghostbuster car. Is that correct?
00:02:43.140 Correct. The CFO vetoed the purchase.
00:02:47.200 Yeah. It's a shame. Maybe with enough outcry, one day you can accomplish something where you
00:02:53.680 deserve that car. I don't know if you can, but the hope is one day you can treat yourself to
00:02:58.420 something nice like a 87 Ghostbuster Caprice.
00:03:03.140 Or just an 87 Caprice.
00:03:05.280 You could say it could be a dream of yours. Maybe that's your dream. On that, we're going
00:03:10.400 to talk about fiber, which has nothing to do with anything we just covered. But it's a subject
00:03:16.140 we get asked about a lot and we've never really talked about on the podcast before. And if you
00:03:21.780 think about usually what we cover on AMAs or podcasts, there's some tie in. We've looked
00:03:26.820 at it some way before. But I think fiber is one of those topics we've never really dove
00:03:31.900 deep into. And we get a lot of questions on it. And so we're going to cover what fiber
00:03:37.620 is, where it can, can't be beneficial, how to think about using it. We'll look at all the
00:03:42.840 various claims that people talk about, which is satiety, weight management, glycemic control,
00:03:48.500 cardiovascular health, colon cancer prevention, all things from the outside would be very important
00:03:55.760 and very nice to have. And so we're going to look at all those and then close on some practical
00:04:00.380 takeaways of how people can apply all this as they think about fiber in their life.
00:04:05.720 So with that said, anything on fiber you want to add before we get rolling?
00:04:12.220 Nope.
00:04:12.320 So why do we want to talk about fiber? Why do we think it's worth spending an AMA on?
00:04:18.080 It really comes down to the importance of occasionally checking things that we view
00:04:22.700 as axiomatic or dogma. So the general consensus is that fiber is good. The more you eat, the better.
00:04:29.080 End of story. Case closed. But if we're going to be critical of our beliefs, we have to acknowledge
00:04:35.060 that the recommended daily allowance for fiber is almost entirely based on epidemiologic studies.
00:04:41.140 And we know that epidemiologic studies have left us on the wrong track with many of their
00:04:46.820 recommendations, not all. And of course, just because epidemiologic studies point to something
00:04:52.080 being the way to do it doesn't mean it's wrong either. But basically, we just don't have as much
00:04:57.900 clear causal evidence as we would like to make the type of recommendations that we're making. Now,
00:05:03.800 I would say historically, it's not that I haven't known this. I've just let it slide
00:05:07.940 because my view has been, what is the harm? So what if the epidemiology says you should be eating
00:05:14.640 whatever, 50 grams or 30 grams a day? What's the downside in that? But look, there's been some
00:05:20.640 polarizing discussions on this topic lately. And you seem to have people accumulating on either
00:05:26.960 sides of this debate. You've got the you need zero fiber crowd, obviously more on the carnivore
00:05:33.380 side of the diet landscape. And then you've got the if you're not getting 50 grams a day, you're
00:05:39.280 probably going to die in the next week or so. Obviously, I'm being facetious, but that's basically
00:05:44.700 the way that discussion is sort of shaping out. So I think with that said, we need to take a closer
00:05:49.900 look at the data so that an individual who is in the 97% of the population who chooses not to be
00:05:57.000 completely dogmatic and extreme can make a well-informed decision. And frankly, that starts with me.
00:06:02.080 Like I actually care deeply about this topic on a personal level because I'm actually not sure how
00:06:07.400 much fiber I should be eating. We've talked about epidemiology in the past, but I think it's worth
00:06:13.060 just kind of double clicking is when you look at epidemiology around fiber nutrition, do you just
00:06:18.600 kind of want to walk through how you think about that and what sometimes the limitations are?
00:06:24.580 Yes. Apologies in advance to the veteran listener who's heard me talk about this at length. I think
00:06:30.080 it's always worth spending a moment on this because I also realize there are people that might be new
00:06:33.980 to this. So epidemiology, in particular nutritional epidemiology, tends to be heavily confounded by
00:06:40.380 healthy user bias. And that means that people who are doing one healthy thing, for example,
00:06:46.100 like eating a high fiber diet, tend to be doing many other healthy things. They might be exercising
00:06:52.400 more, they're much less likely to smoke, they're probably getting more sleep. And while a number of
00:06:57.380 these things can be statistically modeled and corrected for in what is referred to as an adjusted
00:07:03.120 analysis, it is essentially impossible to capture every one of these things and statistically correct
00:07:11.560 for them. In other words, you are very likely to be capturing other healthy habits when you're trying
00:07:20.640 to simply measure one thing. And of course, that's the hallmark of doing experiments. The reason an
00:07:27.200 experiment, particularly a randomized experiment, and ideally a blinded randomized experiment as the gold
00:07:34.520 standard, is so important is because it allows you to isolate one variable at a time. And epidemiology
00:07:41.640 does not allow you to do that. I think the other reason that epidemiology is challenging here,
00:07:48.600 and this is, again, not unique to epidemiology, this is a common phenomenon in all nutrition
00:07:54.480 research, is it is difficult to disentangle the potential impacts of fiber itself from the potential
00:08:01.500 impacts of things that traffic with fiber, the other micronutrients and phytochemicals specifically,
00:08:07.740 because of course fiber is found in plants. And plants are presumably known to contain many things
00:08:14.680 that provide some benefit, and therefore it's difficult to disentangle them. So I would say
00:08:19.500 those are the two main reasons why it has been not easy to come up with, I think, some more concrete
00:08:27.240 points of view on this particular nutrient. Before we get further following that, I think it's always
00:08:33.120 just helpful to even define fiber and how we're talking about it, because I think a lot of times people
00:08:38.160 have heard it. I don't know if everyone knows exactly what we're referring to. So as we look at what
00:08:44.340 we're going to cover, do you just want to define how we're talking about fiber?
00:08:49.160 I think this is actually a very important point, and this is an AMA where I found myself learning a
00:08:54.060 lot in the preparation for it with the research team. So in nutrition, dietary fiber refers to
00:08:59.500 a diverse group of compounds that make their way to the large intestine. So why is that relevant? Well,
00:09:07.440 if you think about how your digestion works, put things in your mouth, digestion actually begins in
00:09:12.000 the mouth. You've got these enzymes in the mouth that are already starting to break things down,
00:09:16.740 but mostly the mouth is for mechanical breakdown. The stomach then undergoes more of a breakdown,
00:09:23.020 right? So now you've got this high acid environment and other enzymes in the stomach that break things
00:09:27.520 down further. The stomach then exits the foodstuff into the proximal part of the small intestine called
00:09:33.620 the duodenum, and then ultimately ileum jejunum. And that's where the bulk of nutritional
00:09:39.820 absorption takes place. But of course, this is what separates fiber from many of the other things
00:09:45.660 we consume, which is the enzymes can't break down fiber. So they actually make their way all the way
00:09:51.160 to the large intestine. Now, these compounds that make their way to the large intestine are virtually
00:09:55.660 always carbohydrate based. And that's why if you look at a nutrition label, you'll usually see fiber
00:10:00.440 as kind of a subset of carbohydrates, but it usually doesn't count towards the caloric content because
00:10:05.440 you're not really using them for energy. The common thread across fiber types is that we can't digest
00:10:11.500 them. But that's really where the similarities end and where the chemical composition of fibers
00:10:16.280 vary widely. This is where you start to get into, at least for me, kind of a nomenclature and a set of
00:10:25.820 explanations that actually is quite complicated and probably worth getting into a bit more.
00:10:29.400 Based on that difference there, and even the last comment on how they can't really be digested,
00:10:36.080 based on that, can we treat them all the same and how they affect the body? Or are there going
00:10:42.120 to be differences there? Yeah, no, there are differences. Not all fibers are created equal.
00:10:46.500 And this is where, unfortunately, we do need to get into a little bit of semantics to be able to really
00:10:51.440 make progress in this topic. So there are lots of things that are classified as fiber, wide range of
00:10:56.320 physical properties that a particular fiber might have, but these different properties produce
00:11:01.260 different effects on the body. So some fibers provide primarily microbiome support because
00:11:07.980 they're actually fermented by gut microbiomes. Some fibers can improve blood sugar, and we'll talk
00:11:14.700 about how that happens, and others simply bulk up stool. And again, the absence of those fibers would
00:11:20.880 lead to altered bowel habits. But importantly, not all fibers do the same things, and they don't
00:11:27.160 all do them equally well. So in the end, the function of fiber comes down to a bunch of properties of
00:11:33.480 that specific fiber. So I think then let's double click on those. What are the properties of fiber
00:11:39.480 that determine the different functions it can do in our body? Okay, so the single most important one,
00:11:45.880 and the one that I think most people will be familiar with, is what's called solubility.
00:11:49.720 Everybody's heard the term. Let's just define it quickly. Solubility means does something dissolve
00:11:54.520 in water, yes or no? So salt dissolves in water. Glucose dissolves in water. Fat does not dissolve
00:12:01.540 in water. So that's soluble, soluble, insoluble. Similarly, we have soluble and insoluble fibers.
00:12:07.760 So let's talk about it. Insoluble fibers obviously do not dissolve in water. So think of an insoluble
00:12:13.600 fiber as sort of roughage, something that remains largely intact as it moves through your digestive
00:12:19.320 system. These are the ones that I talked about earlier. They're essentially there to bulk up stool,
00:12:23.820 mechanically stimulating the gut lining to release water and mucus, to dilute irritants and toxins
00:12:30.180 in the colon, and speed up intestinal transit. So most insoluble fibers are also not fermented by gut
00:12:38.260 bacteria. So we contrast that, of course, with soluble fibers. A viscous fiber is able to absorb
00:12:44.880 water and create a gel, a viscous gel in the gut. And we're going to talk about some of these.
00:12:50.440 And if anybody's ever played with these as supplements, it's actually kind of cool to watch
00:12:53.700 how quickly something that is dry as a fiber, when mixed with water, will form a viscous gel
00:12:59.140 that your kids would want to play with. So this can lead to slower gastric emptying. It can blunt
00:13:04.440 blood sugar spikes, contributes to lower cholesterol, though I would argue not so meaningfully that we
00:13:09.720 should rely on this for lipid management. So some examples of these would be pectin, which you'd find
00:13:15.080 in apples and other fruits, beta-glucan found in oats, and the psyllium husk, which I think is probably
00:13:23.280 the most common one that we would look at. So that's actually a plant that you can most commonly
00:13:29.540 consume as a supplement. These are all viscous, but there's some variability in how viscous they are.
00:13:34.820 We'll get to that later. Okay, so the other category of soluble fibers are the fermentable fibers,
00:13:39.540 which are broken down by gut bacteria to produce short-chain fatty acids such as butyrate. These
00:13:45.040 are also known as prebiotic fibers, which the most common examples of this would be inulin and
00:13:50.100 pectin. While most prebiotic fibers are soluble, there are some exceptions. So for example, resistant
00:13:57.020 starches found in cooked and cooled starchy foods, oats, potatoes, and rice, or in beans. We're going to
00:14:03.180 talk more about these later, but I just kind of want to start high level. Important to be aware that
00:14:06.980 these two properties, viscosity and fermentability, are not mutually exclusive. You'll notice that
00:14:11.940 because I just use pectin as an example in both. So some soluble fibers are fermentable, but don't
00:14:17.740 form gels. Others are gel-forming, but poorly fermentable. And some can do both, and some may
00:14:23.480 do neither. All of this is to say this is a bit of a mess and sometimes can be a little hard to wrap
00:14:30.640 your head around. For the sake of my lack of brain damage and anybody else's listening, we have
00:14:37.000 included a table in the show notes of common fibers along with all their properties and the food
00:14:43.080 sources you can find them in. So if at the end of this podcast, you're thinking to yourself, all right,
00:14:47.560 I kind of want to beef up my intake of this fiber and this fiber, and I frankly care a lot less about
00:14:52.560 this type of fiber, we want to be able to help you do that in terms of understanding what sorts of
00:14:56.340 foods you'd find that in, and of course, what supplements. Not to cause you to incur more brain
00:15:01.980 damage and not to go through the full table, but I think sometimes it is helpful just to highlight
00:15:07.640 one or two examples of what you just said so people can kind of put an understanding to what you just
00:15:13.200 talked about and where they might find it in their day-to-day diet. So can you just go through
00:15:17.820 just a few examples to kind of paint the picture for people as they're listening or watching?
00:15:21.940 Yeah. So when you take fiber from a whole food source, which I think is what we mostly want to
00:15:28.180 be able to do, you're typically going to get a mix of different fiber types and therefore they're
00:15:32.600 going to have different properties. So cellulose and lignin, which are insoluble fiber, are structural
00:15:38.140 components of plant cell walls. So that means that any whole plant-based food you consume will have
00:15:43.580 some amount of insoluble fiber, but the exact quantity is going to vary across plants. Though vegetables
00:15:49.440 will typically contain more and within fruits, peels and skins would also contain more than the meat of
00:15:55.960 the fruit, so to speak. When it comes to soluble fibers, some foods are especially high in certain
00:16:01.560 types. So for example, oats are packed with beta-glucan, which is a soluble gel-forming fiber
00:16:07.780 that's also fermentable by the gut biome. So when you eat oats, you're getting insoluble fiber
00:16:13.440 from cellulose and then the beta-glucan and you're getting both gel formation and the fermentation
00:16:19.680 driven prebiotic effects that make those short-chain fatty acids, which also have a positive and
00:16:26.540 favorable effect on blood glucose. Now beans, on the other hand, don't contain any single dominant fiber.
00:16:32.580 They're high in fiber overall because they provide a diverse mix, including resistant starches,
00:16:37.720 what I want to come back and talk about, insoluble fiber and soluble fibers with varying degrees of
00:16:43.920 fermentability. So when you get fiber from whole food sources, just remember you're getting a mixture
00:16:50.460 of fiber types as opposed to just one. You mentioned their resistance starch. I think it's worth just
00:16:56.180 doubling down on that right now. Can you just walk through what a resistant starch is and how cooking
00:17:03.020 relates to that, cooking or cooling? I think you mentioned earlier.
00:17:06.260 Yeah. Yeah. This is actually something that I learned in the prep for this podcast. I didn't
00:17:10.400 appreciate the temperature component of this. So a resistant starch is a type of insoluble fiber
00:17:15.620 where the word resistant is referring specifically to the fact that they resist digestion. Now they're
00:17:21.840 subdivided into five types, RS1 to RS5, which actually I think would really be a better car name,
00:17:28.300 the Audi RS1 or the Audi RS5, but we'll put that aside for now. But the most common types we encounter
00:17:34.560 are RS1, RS2 and RS3. So RS1 are the starches that are physically inaccessible to digestive enzymes in the GI
00:17:42.760 tract. They're typically found in whole grains, seeds, legumes, and other minimally processed foods
00:17:49.720 that are high in fiber. RS2 are naturally resistant starch granules. So they're present in raw potato
00:17:57.700 starch, unripe green bananas. Anybody's up for that? And some high amylose maize starches. That's just
00:18:05.620 a type of corn. This is the type you're most likely to find in supplement form. If you're looking to
00:18:12.580 double down on RS2, you're going to be buying an RS2 supplement. RS3s are known as retrograde starches
00:18:20.440 because these are starches that have been cooked, which disrupts the starch structure, but then
00:18:26.180 cooled, which causes the starch to retrograde into a crystalline form that resists digestion.
00:18:34.760 In practice, and by the way, again, this is not something I knew until a few weeks ago. In practice,
00:18:40.320 this looks like cooking your starchy food, like your potatoes or rice, and cooling them in the
00:18:45.500 refrigerator overnight. Now it can be warmed up, but if it gets too hot, the resistant starch will
00:18:50.480 break down. So what's really interesting here is I'm the only person in my family, Nick, that loves
00:18:55.820 eating cold, starchy food. So if we have leftover mashed potatoes, if we have leftover potatoes of
00:19:03.180 any kind, if we have leftover rice, I am simply too lazy to heat that stuff up. So I'm always eating
00:19:10.580 it cold, much to my wife's chagrin. And I finally realized I'm doing one thing right here, which is
00:19:16.520 I am actually getting the maximum amount of RS3 resistant starch. So anyway, we'll note this in the
00:19:22.740 table. And again, remember RS2, you're going to buy in supplement form, whereas RS1, you're going to
00:19:28.900 get in a whole oat. By the way, you're not going to get that in an instant oatmeal. Worth making that
00:19:33.740 point here. If you process the heck out of oats, you sort of lose this capacity. Anyway, hopefully that
00:19:38.980 helps with odor resistant starches. With that background laid out, I think now we can kind of
00:19:45.760 move to what people want to know next, which is how fiber relates to their health. So do you want
00:19:52.440 to just kind of go through quickly, which claims that we see most often around fiber and then how
00:20:00.520 you want to talk about them, how you want to cover them for the rest of the show? Yes. You mentioned
00:20:05.040 them at the outset, but I'll restate them. So satiety and weight management, glycemic control,
00:20:10.220 cardiovascular health, and colorectal cancer prevention. So these are the big ones. And
00:20:16.560 this being the drive, we do everything in structure and therefore we have a framework.
00:20:22.680 And so the way we want to do this is we want to, for each of these claims, ask the question,
00:20:26.540 do we know the mechanism or mechanisms that would account for it? Do we have a sense of the effect
00:20:31.140 size? Do we have a better tool to accomplish this? In other words, should we consider fiber
00:20:36.860 as an adjunct versus is this sort of the primary tool and the one that we should rely on mostly as
00:20:44.020 our arrow in the quiver? So after we go through each of these, we'll try to wrap it up with, hey,
00:20:48.620 what's the recommendation on how much fiber and what type of fiber you should prioritize?
00:20:52.940 And effectively, it's just going to come down to how much data do we have to understand
00:20:56.560 this and go through this framework. Perfect. Let's start with weight loss. What do we know
00:21:02.680 about the mechanism of which fiber can help with weight loss?
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