#372 - AMA #77： Dietary fiber and health outcomes： real benefits, overhyped claims, and practical applications
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Summary
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
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Hey everyone, welcome to a sneak peek, ask me anything or AMA episode of the drive podcast.
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I'm your host, Peter Atiyah. At the end of this short episode, I'll explain how you can access
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the AMA episodes in full, along with a ton of other membership benefits we've created,
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or you can learn more now by going to peteratiyahmd.com forward slash subscribe.
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So without further delay, here's today's sneak peek of the ask me anything episode.
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Welcome to ask me anything AMA episode number 77. In today's AMA, we break down the science of
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dietary fiber, moving past the sort of generic advice to eat more fiber, to understand what
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it actually does to the body, where it's truly beneficial to health, how to use it effectively,
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and where its reputation may exceed the evidence. In this episode, we will discuss how different types
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of fiber, soluble, insoluble, viscous, and fermentable work in the body and how their
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distinct properties result in different functional outcomes, whether fiber meaningfully impacts
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satiety, weight management, and glycemic control, and how those effects compare with more potent tools,
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how certain fibers influence lipid metabolism and cardiovascular risk, and whether their
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magnitude of effect justify their reputation, what the mechanistic and epidemiologic evidence
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really say about fiber's role in colorectal cancer prevention, including the potential influence of
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short-chain fatty acids like butyrate, who might not tolerate certain fibers well, and how to tailor
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their intake accordingly, how to move beyond the simplistic goal of getting enough fiber towards
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a more strategic approach that maximizes its actual benefits. Now, if you're a subscriber and you want
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to watch the full video of this podcast, you can find it on the show notes page. If you're not a
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subscriber, you can watch the sneak peek of the video on our YouTube page. So without further delay,
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I hope you enjoy AMA number 77. Peter, welcome to another AMA. How are you doing?
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I'm doing well. Thank you once again for having me.
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That's good. People will be sad to learn who listened to our last full AMA. I do not think
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as of now you are the proud owner of an 87 Caprice Ghostbuster car. Is that correct?
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Yeah. It's a shame. Maybe with enough outcry, one day you can accomplish something where you
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deserve that car. I don't know if you can, but the hope is one day you can treat yourself to
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You could say it could be a dream of yours. Maybe that's your dream. On that, we're going
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to talk about fiber, which has nothing to do with anything we just covered. But it's a subject
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we get asked about a lot and we've never really talked about on the podcast before. And if you
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think about usually what we cover on AMAs or podcasts, there's some tie in. We've looked
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at it some way before. But I think fiber is one of those topics we've never really dove
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deep into. And we get a lot of questions on it. And so we're going to cover what fiber
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is, where it can, can't be beneficial, how to think about using it. We'll look at all the
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various claims that people talk about, which is satiety, weight management, glycemic control,
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cardiovascular health, colon cancer prevention, all things from the outside would be very important
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and very nice to have. And so we're going to look at all those and then close on some practical
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takeaways of how people can apply all this as they think about fiber in their life.
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So with that said, anything on fiber you want to add before we get rolling?
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So why do we want to talk about fiber? Why do we think it's worth spending an AMA on?
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It really comes down to the importance of occasionally checking things that we view
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as axiomatic or dogma. So the general consensus is that fiber is good. The more you eat, the better.
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End of story. Case closed. But if we're going to be critical of our beliefs, we have to acknowledge
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that the recommended daily allowance for fiber is almost entirely based on epidemiologic studies.
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And we know that epidemiologic studies have left us on the wrong track with many of their
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recommendations, not all. And of course, just because epidemiologic studies point to something
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being the way to do it doesn't mean it's wrong either. But basically, we just don't have as much
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clear causal evidence as we would like to make the type of recommendations that we're making. Now,
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I would say historically, it's not that I haven't known this. I've just let it slide
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because my view has been, what is the harm? So what if the epidemiology says you should be eating
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whatever, 50 grams or 30 grams a day? What's the downside in that? But look, there's been some
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polarizing discussions on this topic lately. And you seem to have people accumulating on either
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sides of this debate. You've got the you need zero fiber crowd, obviously more on the carnivore
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side of the diet landscape. And then you've got the if you're not getting 50 grams a day, you're
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probably going to die in the next week or so. Obviously, I'm being facetious, but that's basically
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the way that discussion is sort of shaping out. So I think with that said, we need to take a closer
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look at the data so that an individual who is in the 97% of the population who chooses not to be
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completely dogmatic and extreme can make a well-informed decision. And frankly, that starts with me.
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Like I actually care deeply about this topic on a personal level because I'm actually not sure how
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much fiber I should be eating. We've talked about epidemiology in the past, but I think it's worth
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just kind of double clicking is when you look at epidemiology around fiber nutrition, do you just
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kind of want to walk through how you think about that and what sometimes the limitations are?
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Yes. Apologies in advance to the veteran listener who's heard me talk about this at length. I think
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it's always worth spending a moment on this because I also realize there are people that might be new
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to this. So epidemiology, in particular nutritional epidemiology, tends to be heavily confounded by
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healthy user bias. And that means that people who are doing one healthy thing, for example,
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like eating a high fiber diet, tend to be doing many other healthy things. They might be exercising
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more, they're much less likely to smoke, they're probably getting more sleep. And while a number of
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these things can be statistically modeled and corrected for in what is referred to as an adjusted
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analysis, it is essentially impossible to capture every one of these things and statistically correct
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for them. In other words, you are very likely to be capturing other healthy habits when you're trying
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to simply measure one thing. And of course, that's the hallmark of doing experiments. The reason an
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experiment, particularly a randomized experiment, and ideally a blinded randomized experiment as the gold
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standard, is so important is because it allows you to isolate one variable at a time. And epidemiology
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does not allow you to do that. I think the other reason that epidemiology is challenging here,
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and this is, again, not unique to epidemiology, this is a common phenomenon in all nutrition
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research, is it is difficult to disentangle the potential impacts of fiber itself from the potential
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impacts of things that traffic with fiber, the other micronutrients and phytochemicals specifically,
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because of course fiber is found in plants. And plants are presumably known to contain many things
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that provide some benefit, and therefore it's difficult to disentangle them. So I would say
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those are the two main reasons why it has been not easy to come up with, I think, some more concrete
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points of view on this particular nutrient. Before we get further following that, I think it's always
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just helpful to even define fiber and how we're talking about it, because I think a lot of times people
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have heard it. I don't know if everyone knows exactly what we're referring to. So as we look at what
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we're going to cover, do you just want to define how we're talking about fiber?
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I think this is actually a very important point, and this is an AMA where I found myself learning a
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lot in the preparation for it with the research team. So in nutrition, dietary fiber refers to
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a diverse group of compounds that make their way to the large intestine. So why is that relevant? Well,
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if you think about how your digestion works, put things in your mouth, digestion actually begins in
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the mouth. You've got these enzymes in the mouth that are already starting to break things down,
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but mostly the mouth is for mechanical breakdown. The stomach then undergoes more of a breakdown,
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right? So now you've got this high acid environment and other enzymes in the stomach that break things
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down further. The stomach then exits the foodstuff into the proximal part of the small intestine called
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the duodenum, and then ultimately ileum jejunum. And that's where the bulk of nutritional
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absorption takes place. But of course, this is what separates fiber from many of the other things
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we consume, which is the enzymes can't break down fiber. So they actually make their way all the way
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to the large intestine. Now, these compounds that make their way to the large intestine are virtually
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always carbohydrate based. And that's why if you look at a nutrition label, you'll usually see fiber
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as kind of a subset of carbohydrates, but it usually doesn't count towards the caloric content because
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you're not really using them for energy. The common thread across fiber types is that we can't digest
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them. But that's really where the similarities end and where the chemical composition of fibers
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vary widely. This is where you start to get into, at least for me, kind of a nomenclature and a set of
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explanations that actually is quite complicated and probably worth getting into a bit more.
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Based on that difference there, and even the last comment on how they can't really be digested,
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based on that, can we treat them all the same and how they affect the body? Or are there going
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to be differences there? Yeah, no, there are differences. Not all fibers are created equal.
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And this is where, unfortunately, we do need to get into a little bit of semantics to be able to really
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make progress in this topic. So there are lots of things that are classified as fiber, wide range of
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physical properties that a particular fiber might have, but these different properties produce
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different effects on the body. So some fibers provide primarily microbiome support because
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they're actually fermented by gut microbiomes. Some fibers can improve blood sugar, and we'll talk
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about how that happens, and others simply bulk up stool. And again, the absence of those fibers would
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lead to altered bowel habits. But importantly, not all fibers do the same things, and they don't
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all do them equally well. So in the end, the function of fiber comes down to a bunch of properties of
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that specific fiber. So I think then let's double click on those. What are the properties of fiber
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that determine the different functions it can do in our body? Okay, so the single most important one,
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and the one that I think most people will be familiar with, is what's called solubility.
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Everybody's heard the term. Let's just define it quickly. Solubility means does something dissolve
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in water, yes or no? So salt dissolves in water. Glucose dissolves in water. Fat does not dissolve
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in water. So that's soluble, soluble, insoluble. Similarly, we have soluble and insoluble fibers.
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So let's talk about it. Insoluble fibers obviously do not dissolve in water. So think of an insoluble
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fiber as sort of roughage, something that remains largely intact as it moves through your digestive
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system. These are the ones that I talked about earlier. They're essentially there to bulk up stool,
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mechanically stimulating the gut lining to release water and mucus, to dilute irritants and toxins
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in the colon, and speed up intestinal transit. So most insoluble fibers are also not fermented by gut
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bacteria. So we contrast that, of course, with soluble fibers. A viscous fiber is able to absorb
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water and create a gel, a viscous gel in the gut. And we're going to talk about some of these.
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And if anybody's ever played with these as supplements, it's actually kind of cool to watch
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how quickly something that is dry as a fiber, when mixed with water, will form a viscous gel
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that your kids would want to play with. So this can lead to slower gastric emptying. It can blunt
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blood sugar spikes, contributes to lower cholesterol, though I would argue not so meaningfully that we
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should rely on this for lipid management. So some examples of these would be pectin, which you'd find
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in apples and other fruits, beta-glucan found in oats, and the psyllium husk, which I think is probably
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the most common one that we would look at. So that's actually a plant that you can most commonly
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consume as a supplement. These are all viscous, but there's some variability in how viscous they are.
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We'll get to that later. Okay, so the other category of soluble fibers are the fermentable fibers,
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which are broken down by gut bacteria to produce short-chain fatty acids such as butyrate. These
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are also known as prebiotic fibers, which the most common examples of this would be inulin and
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pectin. While most prebiotic fibers are soluble, there are some exceptions. So for example, resistant
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starches found in cooked and cooled starchy foods, oats, potatoes, and rice, or in beans. We're going to
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talk more about these later, but I just kind of want to start high level. Important to be aware that
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these two properties, viscosity and fermentability, are not mutually exclusive. You'll notice that
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because I just use pectin as an example in both. So some soluble fibers are fermentable, but don't
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form gels. Others are gel-forming, but poorly fermentable. And some can do both, and some may
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do neither. All of this is to say this is a bit of a mess and sometimes can be a little hard to wrap
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your head around. For the sake of my lack of brain damage and anybody else's listening, we have
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included a table in the show notes of common fibers along with all their properties and the food
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sources you can find them in. So if at the end of this podcast, you're thinking to yourself, all right,
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I kind of want to beef up my intake of this fiber and this fiber, and I frankly care a lot less about
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this type of fiber, we want to be able to help you do that in terms of understanding what sorts of
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foods you'd find that in, and of course, what supplements. Not to cause you to incur more brain
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damage and not to go through the full table, but I think sometimes it is helpful just to highlight
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one or two examples of what you just said so people can kind of put an understanding to what you just
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talked about and where they might find it in their day-to-day diet. So can you just go through
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just a few examples to kind of paint the picture for people as they're listening or watching?
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Yeah. So when you take fiber from a whole food source, which I think is what we mostly want to
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be able to do, you're typically going to get a mix of different fiber types and therefore they're
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going to have different properties. So cellulose and lignin, which are insoluble fiber, are structural
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components of plant cell walls. So that means that any whole plant-based food you consume will have
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some amount of insoluble fiber, but the exact quantity is going to vary across plants. Though vegetables
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will typically contain more and within fruits, peels and skins would also contain more than the meat of
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the fruit, so to speak. When it comes to soluble fibers, some foods are especially high in certain
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types. So for example, oats are packed with beta-glucan, which is a soluble gel-forming fiber
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that's also fermentable by the gut biome. So when you eat oats, you're getting insoluble fiber
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from cellulose and then the beta-glucan and you're getting both gel formation and the fermentation
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driven prebiotic effects that make those short-chain fatty acids, which also have a positive and
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favorable effect on blood glucose. Now beans, on the other hand, don't contain any single dominant fiber.
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They're high in fiber overall because they provide a diverse mix, including resistant starches,
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what I want to come back and talk about, insoluble fiber and soluble fibers with varying degrees of
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fermentability. So when you get fiber from whole food sources, just remember you're getting a mixture
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of fiber types as opposed to just one. You mentioned their resistance starch. I think it's worth just
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doubling down on that right now. Can you just walk through what a resistant starch is and how cooking
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relates to that, cooking or cooling? I think you mentioned earlier.
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Yeah. Yeah. This is actually something that I learned in the prep for this podcast. I didn't
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appreciate the temperature component of this. So a resistant starch is a type of insoluble fiber
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where the word resistant is referring specifically to the fact that they resist digestion. Now they're
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subdivided into five types, RS1 to RS5, which actually I think would really be a better car name,
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the Audi RS1 or the Audi RS5, but we'll put that aside for now. But the most common types we encounter
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are RS1, RS2 and RS3. So RS1 are the starches that are physically inaccessible to digestive enzymes in the GI
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tract. They're typically found in whole grains, seeds, legumes, and other minimally processed foods
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that are high in fiber. RS2 are naturally resistant starch granules. So they're present in raw potato
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starch, unripe green bananas. Anybody's up for that? And some high amylose maize starches. That's just
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a type of corn. This is the type you're most likely to find in supplement form. If you're looking to
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double down on RS2, you're going to be buying an RS2 supplement. RS3s are known as retrograde starches
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because these are starches that have been cooked, which disrupts the starch structure, but then
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cooled, which causes the starch to retrograde into a crystalline form that resists digestion.
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In practice, and by the way, again, this is not something I knew until a few weeks ago. In practice,
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this looks like cooking your starchy food, like your potatoes or rice, and cooling them in the
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refrigerator overnight. Now it can be warmed up, but if it gets too hot, the resistant starch will
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break down. So what's really interesting here is I'm the only person in my family, Nick, that loves
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eating cold, starchy food. So if we have leftover mashed potatoes, if we have leftover potatoes of
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any kind, if we have leftover rice, I am simply too lazy to heat that stuff up. So I'm always eating
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it cold, much to my wife's chagrin. And I finally realized I'm doing one thing right here, which is
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I am actually getting the maximum amount of RS3 resistant starch. So anyway, we'll note this in the
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table. And again, remember RS2, you're going to buy in supplement form, whereas RS1, you're going to
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get in a whole oat. By the way, you're not going to get that in an instant oatmeal. Worth making that
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point here. If you process the heck out of oats, you sort of lose this capacity. Anyway, hopefully that
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helps with odor resistant starches. With that background laid out, I think now we can kind of
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move to what people want to know next, which is how fiber relates to their health. So do you want
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to just kind of go through quickly, which claims that we see most often around fiber and then how
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you want to talk about them, how you want to cover them for the rest of the show? Yes. You mentioned
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them at the outset, but I'll restate them. So satiety and weight management, glycemic control,
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cardiovascular health, and colorectal cancer prevention. So these are the big ones. And
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this being the drive, we do everything in structure and therefore we have a framework.
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And so the way we want to do this is we want to, for each of these claims, ask the question,
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do we know the mechanism or mechanisms that would account for it? Do we have a sense of the effect
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size? Do we have a better tool to accomplish this? In other words, should we consider fiber
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as an adjunct versus is this sort of the primary tool and the one that we should rely on mostly as
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our arrow in the quiver? So after we go through each of these, we'll try to wrap it up with, hey,
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what's the recommendation on how much fiber and what type of fiber you should prioritize?
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And effectively, it's just going to come down to how much data do we have to understand
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this and go through this framework. Perfect. Let's start with weight loss. What do we know
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about the mechanism of which fiber can help with weight loss?
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