The Peter Attia Drive - November 10, 2025


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

Topics
dietary fiber fiber and digestion resistant starch types
Hosted by
Peter Attia
#371 – Women's sexual health： desire, arousal, and orgasms, navigating perimenopause, and enhancing satisfaction ｜ Sally Greenwald, M.D., M.P.H. #373 – Thyroid function and hypothyroidism： why current diagnosis and treatment fall short for many, and how new approaches are transforming care ｜ Antonio Bianco, M.D., Ph.D.

Episode Stats


Length

24 minutes

Words per minute

172.96

Word count

4,177

Sentence count

239


Summary

Summaries generated with gmurro/bart-large-finetuned-filtered-spotify-podcast-summ .

In today's AMA, we break down the science of dietary fiber, moving past the generic advice to eat more of it to understand what it actually does to the body, where it's truly beneficial to health, and where its reputation may exceed the evidence.

Transcript

Transcript generated with Whisper (turbo).
Topics generated with Qwen2.5-3B-Instruct.
Hosts, guests, and mentioned names generated with spaCy (en_core_web_sm), reconciled against Wikidata.
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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