The Truth About Carbs, Fat, Insulin and Fat Loss with Obesity Researcher Dr. Stephan Guyenet

Content By: Ari Whitten

In this episode, I am speaking with obesity researcher, nutrition and neuroscience expert, Dr. Stephan Guyenet about what the science says about carbs, fat, insulin, and fat loss.

In this podcast, Dr. Guyenet will cover:

  • What the science says about carbs, insulin and their relationship with fat gain and fat loss
  • Are carbs bad for you? (what science says about carbs’ influence on health)
  • Why the low-carb philosophy has become so popular in the health and diet sphere
  • Did the low-fat era really make us all fatter?
  • Dr. Guyenet’s advice on how to eat for leanness
  • Does fat make you gain weight?
  • Do calories matter like we all thought? Or is that an outdated view?
  • How the brain controls body fatness
  • What science says about carbs and fats causing a spike in insulin
  • What the science says are the key drivers of fat gain
  • Why counting calories is usually inaccurate
  • Dr. Guyenet’s take on Gary Taubes’ views on carbohydrate and insulin
  • And much more…

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The Truth About Carbs, Fat, Insulin and Fat Loss with Obesity Researcher Dr. Stephan Guyenet – Transcript

Ari Whitten: Hey, everyone. Welcome back to the Energy Blueprint Podcast. I’m your host Ari Whitten, and today I have with me a very special guest, someone whose work I’ve been a fan of for a very long time: Dr. Stephan Guyenet. He has a Bachelor’s in biochemistry, a Ph.D. in neuroscience, and following his Ph.D., he went on to study the neuroscience of obesity and eating behavior as a postdoctoral fellow. He spent a total of 12 years in the neuroscience research world studying neurodegenerative disease and the neuroscience of body fatness.

He’s also the author of the bestselling book, The Hungry Brain, which I highly recommend. That book was named one of the best books of the year in 2017 when it was published, by Publishers Weekly and called “essential” by the New York Times Book Review. He is also a senior fellow at Give Well, and a scientific reviewer for the Research Digest, which I also subscribe to. On a personal note, I will say that back in 2013 or 2014, I first discovered Stephan ‘s work, and I was blown away with it. It completely changed a lot of my thinking around the topic of why people get fat and how to lose fat. I was absolutely blown away by his research, and it profoundly impacted my own work and my own thinking on this topic. Even so much that I extensively quoted him in my book Forever Fat Lossthat I wrote in 2014. This has been a long time in the making. I’m a huge fan of this guy’s work. I cannot recommend his work highly enough, and I’m honored to finally have him on the show. Thank you so much for joining me, Dr. Guyenet.

Dr. Stephan Guyenet: All right, thanks for having me on the show, Ari.


What science says about carbs and insulin as being primary drivers of fat gain

Ari Whitten: Yeah. I want to start off by saying that you were recently on the Joe Rogan podcast, and you had a debate with Gary Taubes about a big part of your work and a big part of something you’ve been doing since at least 2014, maybe earlier.

I came across it in 2013, when you wrote an absolutely epic article called “The Carbohydrate, or A Critical Examination of the Carbohydrate Hypothesis of Obesity,” which is basically the idea that Gary Taubes helped promote but an idea that has been around for a very long time. It claims that the primary driver of fat gain is carbohydrate and that dietary carbohydrate boosts a fat-storing hormone called insulin, and that is the primary mechanism of why people get fat. That was something that I actually believed and taught to my clients for a very long time prior to reading your work and then exploring the research in depth myself. That idea, unfortunately, is still extremely prevalent and very popular, and to question it actually gets you criticized as if you don’t know what you’re talking about. And I mean “you” as in both “you and me and many other people.” With that in mind, I think a good place to start would be, why is that simple hypothesis, which sounds somewhat logical and reasonable, why is it not true? What are the layers of evidence against the basic idea that carbs and insulin are the primary drivers of fat gain?

Dr. Stephan Guyenet: Let me go back to when I first read Gary Taubes’ book, Good Calories, Bad Calories. Obviously, the core ideas that he articulates in there about insulin and obesity predate Taubes.

I read the book back in 2008 when it first came out, and at the time I was quite persuaded by it. That was before I had done my stint in obesity research and before I had really looked deeply into these questions. I was still kind of learning myself, and it was very persuasive to me, and it is very persuasive to a lot of people, but there was one thing that struck me, even as I was reading it, because I had had a longstanding interest in non-industrial cultures and the health and diets of non-industrial culture. I knew a fair bit about that kind of thing.

As I was reading the book, there was one thing that struck me in the back of my mind; even as I was being persuaded, there was a little voice saying, “Well, this is kind of odd because there are  many non-industrial cultures that eat high-carbohydrate diets and are lean and don’t have high levels of chronic disease.” You might say, “Well, that’s unrefined carbohydrate.” A couple of responses to that are, first of all, if you read Good Calories, Bad Calories, which, by the way, is Taubes’ seminal book, it makes somewhat of a distinction between all carbohydrate and refined carbohydrate, but there are a number of places where he claims that all carbohydrate is fattening.

Second of all, even cultures that have a high intake of refined carbohydrate are often very lean. Look at the Japanese, for example. Not only do they eat a diet that is very high in refined carbohydrate and which used to be even higher than it is now, back when they were even leaner than they are now (white rice is, of course, their dietary staple) but the type of rice has a particularly high glycemic index: it’s sticky white, amylopectin rice. That type of rice digests really quickly and yet Japan historically, and even to this day, has the lowest rate of obesity among industrialized nations, at least that I’m aware of. There were just things like that that kept sticking in my mind even as I was reading it and I said, “Maybe it’s not totally correct. Maybe there are some things wrong with it., but overall I’m pretty persuaded.” That was kind of my attitude at the time.

But the paradoxes kept growing as I continued to investigate the evidence, and one of the things that I figured out pretty quickly when I became an obesity researcher myself is that dietary fat is fattening in a variety of non-human species. You can add fat, whether it’s lard, seed oils, butter, or a variety of different types of added fats, you can add them to the regular low-fat food of rodents, and they will start to gain fat, and that’s kind of hard to explain via a hypothesis that says only carbohydrate is fattening, especially since the amount of carbohydrate in the diet goes down the more fat you add. Essentially, the latest research on this suggests that dietary fat is the primary most fattening macronutrient in rodents. John Speakman did the biggest and most definitive study on this in mice, and, basically, what you see is that mice will get fatter and fatter the more fat you add to their diet up to a point, but once you hit about 60%, that’s where they’re at their maximum  body fatness; if you go to 70 or 80%, then they actually get leaner, and that’s kind of interesting because I think that parallels what we see in humans, as well.

It’s basically like you get leanness on either end of the macronutrient spectrum. If you’re low-fat, you going to be leaner; if you’re low-carb, you’re going to be leaner. That got me thinking that mice have insulin and they have fat cells. The insulin acts on the fat cells the same as in humans, so why would it be any different in humans if this hypothesis is correct? If the primary cause of fat gain is insulin acting on fat cells as a result of eating carbohydrate, why wouldn’t it also be all about carbohydrate in rodents? It should work exactly the same in rodents as in humans if this idea is correct. So, at that point, you have to conclude that something is wrong with the idea. At the very least, it can’t be the only thing going on. The more I looked into it, the more the paradoxes and problems expanded. I think one of the issues that is most profoundly wrong with it, at least as it’s articulated by Taubes, is that it doesn’t acknowledge the importance of calories. Taubes has a very extreme view on this, which is that calories are essentially irrelevant. The number of calories you eat doesn’t really matter. It’s only about how much carbohydrate you eat or, more accurately, the amount and type of carbohydrate you eat via their effects on insulin.

Ari Whitten: If you don’t mind, just to interrupt you for one second, you said that’s an extreme view. I would actually challenge you on that because it has become so popular now for people to think, “Oh, calories! That’s the old model. That’s primitive thinking. That’s already been debunked by science, and really what we know now is that it’s about hormones. It’s about insulin. It’s not about calories.” So, you’re calling it an extreme view, and it would be an extreme view in the scientific community, but I honestly think he’s been very successful in promoting that narrative to the general public under the guise that it is actually the more scientific perspective than a calorie-focused one.

Dr. Stephan Guyenet: Okay. I take your point. It’s kind of ironic if that’s the case because the narrative that he claims is that the research community arrived at this conventional wisdom that calories matter based on faulty evidence or biased interpretation of evidence. I would say that it’s ironic because those conclusions that you just mentioned were arrived at through a faulty interpretation and bias themselves, and now they’ve become conventional wisdom among a subset of people. The very thing that he was pointing out as inaccurate in the research community has come to pass in his own community. In any case. I’m not going to go through the calorie data in detail unless you want to, but basically, there’s just a lot of evidence that calories really do matter. When you vary the amount of carbohydrate and fat in the diet while keeping calories constant, what you see is that the calorie intake is by far the main determinant of whether people gain weight or lose weight or maintain, or I should say gain fat or lose fat or maintain their body fatness.

The amount of carbohydrate or fat in the diet seems to have no or very little effect independent of the amount of calories.  This is one thing I talked about in the debate: you can overfeed people by adding only fat or only carbohydrate to the diet, and what you see is that it makes no difference whether the extra calories are coming from fat or carbohydrate. The amount of fat gain is the same.  all this stuff about carbohydrates and insulin, it’s all fine and good until you actually test it in a scientific experiment and it’s not supported. So, I think there’s this popular, prevalent narrative that has surrounded these issues that’s pretty disconnected from the actual scientific research that has been done on it.

Ari Whitten: Yeah. Let, let me recap a few of these layers that you’re alluding to. I know you’re summarizing a vast body of literature here in just a few paragraphs, but one layer of this story is animal experiments where they’re very clearly showing that fat can be just as fattening as sugar while not boosting insulin at all or to any significant degree. In other words, there’s a mechanism of fat gain that doesn’t have anything to do with insulin.

Dr. Stephan Guyenet: I’m sorry to cut into your summary here, but it’s actually not even as simple as that because another one of the little dirty secrets that they don’t tell you about in that camp is that fat can actually raise insulin. It just depends on the situation. It can potentiate insulin release in a mixed meal, or if you eat enough of it that it causes you to gain fat, it can raise your insulin that way.

Ari Whitten: Right, but it’s generally small, and I don’t want to still operate in the narrative here of insulin is the primary mechanism by which fat is driving the fat gain. I agree with you. I’ve seen research that it can boost it to some degree, but overall, the research very clearly shows that fat can be fattening through almost certainly insulin-independent mechanisms.

Dr. Stephan Guyenet: Correct.


Comparing low-carb and low-fat diets

Ari Whitten: That’s one layer. Then another layer is there are all these tribes all over the world. Modern-day hunter-gatherer tribes still living traditional lives, the [Kidivans???, the Tarahumara Indians, the Tsimane Mani, the Kuna, the Hadza], and all these other tribes throughout South America and the South Pacific and Africa that in many cases eat carb-based diets and have almost no obesity and diabetes and are extremely lean. That’s another layer of the story. You also brought up Japan. And then there’s this other layer of the story, of the evidence I should say, which is directly testing the idea and saying, “Let’s put people on 1,200-calorie diets that are either low-carb, high-fat diets or 1,200-calorie diets that are low-fat, high-carb diets, and we know there’s going to be a difference in their insulin levels. Let’s see if, at the same level of calories, there’s any difference in fat loss. or you can do the opposite experiment and instead of an underfeeding calorie controlled experiment, you can do an overfeeding one where you forcibly overfeed people either pure fat or pure carbs and you see at the same level of calories: do they gain more fat eating a carb-based diet or a low a low-carb diet?

And, basically, what you summarized is that all of those studies consistently show that there’s the same exact amount of fat loss on low-carb versus low-fat, or very slight differences between them that are pretty much insignificant in the grand scheme of things. Would you agree? Do you have anything to my summary there?

Dr. Stephan Guyenet: I think that one caveat is that there have been a lot of studies on the effect of carbohydrate versus fat in calorie-controlled settings, and I should say, first of all, that these tend to be very tightly controlled studies. These are not free-living studies. These are studies where people are in-patients in a research facility, so people have an excellent idea of exactly what they’re eating. There’s not a lot of error, but nevertheless, there is variability in what these studies have reported. Not all of them have reported the exact same thing. However, what you find is that there is a central tendency to what they do report, and when you put those into a meta-analysis, you find that overall there’s very little difference in the effect of carbohydrates versus fat on body fatness.

I don’t want to say that there are no studies that have ever been done that showed that one macronutrient or the other is more fattening. There are studies on both sides that have been done, but when you pull them together, the central tendency of the overall literature is that there is very little difference. The other thing I want to say is that these studies have limitations. They’re not perfect studies, they’re not as long as we wish they were, etc. You could find a million and one ways to poke holes in these studies if you are really looking to do that. However, the fact remains that despite these limitations, there is no world in which these findings are consistent with the carbohydrate-insulin hypothesis as articulated by Taubes. If he was correct, you would see huge differences in favor of carbohydrate being fattening every time, and that’s not what we see, not even close. Despite the limitations, these studies are totally incompatible with the hypothesis.

Ari Whitten: Yeah. Just to emphasize that even more, if you can do studies and 9 out of 10 studies that are calorie controlled don’t show a significant difference between a low-carb versus a low-fat diet, then that cannot exist in a world where Taubes is right and where insulin is the primary thing that’s controlling body fatness. In other words, the studies are very clearly showing that his theory is not correct.

Dr. Stephan Guyenet: Yeah, that’s right. I think that on some level Taubes recognizes that this evidence is a pretty substantial threat to his ideas, so, for one thing, he avoids discussing these things in his writing, but for another thing, when I brought it up to him in the debate, he was kind of groping for reasons to not have to accept the findings.

[He would say that] either meta-analyses in general are not something we should be using, or that the studies were garbage, but he can’t really tell you why they were garbage, or that they weren’t long enough, even though the effects of insulin on fat cells, all of them that I’m aware of, manifest within a matter of hours at most, and most of them manifest within minutes.  I don’t know why it would take months and months for the effect to show up.  [He would bring up] anything he could think of to try to discredit the studies

Ari Whitten: To discredit studies, controlled studies, while also then citing himself obscure anecdotes of random stories of a country or some individual that are totally uncontrolled.  There is a tendency to cite very low-quality evidence and ignore much higher quality evidence.

Dr. Stephan Guyenet: Yeah, that’s exactly right, and I think that’s a very important point. This is something you see over and over again in his writing. If you look in Good Calories, Bad Calories, for example, when he’s trying to make the point that people with obesity actually don’t consume more calories than lean people and that we can become obese while eating low-calorie diets if they’re high in carbohydrate: all the evidence that he cites is from observations that used inaccurate measurement tools to measure calorie intake. In general, it’s really hard to measure calorie intake. It’s hard as an individual, it’s hard as a researcher, and we know that self-reported calorie intake tends to underestimate calorie intake.  So, if you’re looking at observational studies that are based on self-report, what you often see is that people with obesity the same number of calories or report eating the same number of calories as lean people.

And then there are these observations in various countries that Gary likes to cite. It’s really hard to measure people’s calorie intake accurately. When people are out of the house, you can’t measure what they’re eating. So, for hunter-gatherers or for individuals living in an urban area that go to work or whatever, you don’t know exactly what they’re eating all throughout the day, and so those measurements tend to underestimate calorie intake.  There are all these studies throughout the literature using these inaccurate measurements of calorie intake that report that people are eating these implausibly low levels of calories. When you actually look at the studies that use accurate measurements of calorie intake, what you find is that those things drop away, and you get a very different picture. What you see is that calorie intake increases as body mass increases, which really shouldn’t be a shock.

Larger bodies require more energy to maintain, and that’s what we see. If you control for a person’s height and their gender and their physical activity level, you see that people who have more body fat, people who are fatter, have greater calorie intake and greater calorie expenditure pretty consistently. This whole idea that we can become obese with the low-calorie intake and that eating carbohydrate creates a metabolic state that shunts energy away from other tissues and into fat tissue, so it has an energy partitioning effect or, as some people call it, internal starvation. Basically, your lean tissues are being starved by these greedy fat cells that are sucking up more than their share due to insulin, but it just doesn’t fit the facts because calorie intake is increased, lean mass is increased in obesity, metabolic rate per unit lean mass is the same as lean people and obesity.

If you measure circulating fuels like free fatty acids and glucose, they’re the same or higher in obesity, and if you measure circulating fat in general in [chylomicrons and the LDL, it’s] generally higher in obesity. There is no internal starvation happening in obesity; if anything, it’s internal excess.  These ideas are very appealing by virtue of their simplicity, but when you actually sit down and apply the scientific method, which means you ask what do these ideas actually predict and have those predictions been upheld by evidence, you very quickly find that they have not been upheld and that the hypothesis is not supported across the board.


Why adding fat to your meal can be counter-productive when you look at the satiety per calorie equation

Ari Whitten: Yeah. I want to go one level deeper in some of this. We talked about the calorie-controlled experiments and metabolic ward studies. A sort of more nuanced, more sophisticated version of the carbohydrate hypothesis of obesity to try and deal with the facts around the metabolic ward studies is instead of saying that insulin directly results in fat gain, the idea was [to ??? sort of] incorporate calories, but to say that maybe carbs and higher levels of insulin by eating carbohydrates result in less satiety and that eating fat promotes a higher degree of satiety.  There’s a narrative out there that fat is really satiating. I see this all the time, people saying, “Hey if you want to feel fuller from your meals, add more fat to it.” Carbs are not satiating, but fat is really satiating. So, the idea is that carbs are still the driver in a real-world setting, not in a metabolic ward setting, but in a real-world setting. Carbs are still the driver of fat gain by virtue of altering the calories in-calories out equation, increasing people’s appetite and so on. Can you speak to that?

One more layer to this is there’s this narrative that has also been popularized by a lot of the low-carb diet gurus, which is that everybody started eating low-fat diets and it didn’t work. We all just got fatter as a result of eating low-fat diets. Can you speak to that whole landscape?

Dr. Stephan Guyenet: Yeah, I’ll start with the first part. You know what? It slipped my mind, actually: as you were saying the second part, I forgot what the first part was.

Ari Whitten:  That insulin drives appetite. Fat is really okay, but carbs make you hungry.

Dr. Stephan Guyenet: Thanks. I’m going to give that idea credit for being a lot more sophisticated than the basic idea that Taubes promotes because if you can accept that calories are the primary determinant of body fatness, you’ve already made a substantial step forward. If you’ve made the step forward to say, “Okay, calories are important,  let’s try to understand why people eat the number of calories that they do,” I think that is a very important step forward, and that’s a step that underlies my book, The Hungry Brain. That said, the idea is not completely false that fat is satiating. There’s a couple of grains of truth to it. One of them is that if you switch your diet to a low-carbohydrate, higher fat diet, generally calorie intake will spontaneously decline.  In that sense, whether you would call that satiation or not, I don’t know, but people report declines and hunger. I think that could be a fair interpretation of what happens on a lower carbohydrate, higher fat diet. Also, if you add a bunch of fat if for example you’re sitting at your dinner table and you have a piece of bread, a baked potato, a piece of meat and some vegetables, and you slather butter all over everything, you will feel more full at the end of that meal.

However, the question we need to ask is, is the amount of fullness that you’re feeling proportional to the number of extra calories that you’re consuming? That’s really the question that we care about because if you feel 10% fuller but you ate 20% more calories, was that really the effect that you were trying to achieve? I think for most people the response to that would be no, and if we look at studies that actually examine how filling things are per unit calorie, what we find is that added fats are actually one of the least filling things that you can eat.  Yes, they will make you feel more full, but not per unit calorie; per unit calorie, they’re generally not very filling compared to almost anything including carbohydrate.

In studies where they compare diets that are high in fat to diets are high in carbohydrate at like a single meal and then ask people how full they feel, generally, in most cases, people will say that they feel fuller after the higher carbohydrate meal than after the higher fat meal. However, those things in some ways are not perfectly well controlled because fat is more calorie dense and fat can make things palatable, so you have a couple of differences between those two diets. If you actually control for that, if you make the calorie density the same, or in other words, the calories per gram of food the same, and you equate the palatability or how good it tastes, then you see that satiety values are the same.  Basically, I think the body is pretty good at giving you the right level of satiety for the number of calories you’ve consumed, and that it’s pretty good at not caring whether it’s fat or carbohydrate in terms of satiety if the calorie density and palatability are equated.

But it’s really all about the calorie density and palatability, and how full you feel per unit calorie if that makes sense. Even though there is a grain of truth to this idea that fat is more satiating, I don’t think that it is true as a general statement. I feel pretty comfortable saying that it’s not true as a general statement because  for one thing, what I just said, and for another thing, you see similar effects on a low-fat end of the spectrum, so  people who go on a low-fat diet, you see that there’s their calorie intake spontaneously declines just like people who go on a low-carbohydrate diet, except in that case their fat intake is going down and their carbohydrate intake is generally going up. So, you can’t really make general sweeping statements that fat is more satiating than carbohydrate. I don’t think that’s correct, and I would say if anything, it’s the reverse, particularly with added fats, but again, you can get more satiation out of a higher fat diet if it’s a lower carbohydrate diet.  It’s not to say that that’s impossible, it’s just that it happens in a kind of limited subset of possible diets.

Ari Whitten: I would like to just rephrase one thing to make it clear because I think there’s so much scientific nuance in what you said that I’m not sure it will be totally clear to some people. What you said is basically if you add let’s say a bunch of butter to your meal, you’ll feel fuller than if you hadn’t eaten more butter, but it’s basically the same as saying that eating more food will make you fuller. The goal isn’t to eat more food in order to feel fuller; the goal is to be fuller while eating a lower number of calories such that it facilitates leanness. That’s the distinction: can you feel fuller by adding a bunch of butter to your meal? Yes, you can, but you’re likely gaining fat in the process since you’ve just added fat to your meal. The other scenario would be that you’re eating higher fat, very low-carb diet, and in that scenario, the overall net calorie intake could be shifted downwards. Satiety could be shifted upwards, satiety could be shifted downwards such that it helps drive leanness and fat loss.

Dr. Stephan Guyenet: Yeah.

Ari Whitten: The same scenario is true with carbs. If you were to take a standard meal that has a mix of carbs and fats, and you add a bunch of sugar to it, you, if you consume enough sugar, you will also feel more satiated from consuming more overall food than if you hadn’t consumed that food, but at the same time you’re doing it while gaining fat. And the same is true in the case of a whole foods diet that is low in fat and high in carbohydrate—that can also spontaneously lower calorie intake and facilitate fat loss.

Dr. Stephan Guyenet: Yeah, that’s right. The key question is how filling it is per calorie. It’s not necessarily the case that something that’s filling at one meal is going to be a filling dietary pattern in the long run.  There’s a potential distinction there as well that I want to flag.


What Dr. Guyenet thinks is the real issue with Gary Taubes’ Carbohydrate and Insulin hypothesis

Ari Whitten: Got It. With that in mind, what do you think is the real issue with the basic idea that Taubes has promoted, that it’s specifically carbs and insulin that are responsible for fat gain and that everyone needs to go on low-carb diets in order to lose fat? If it is possible, and we know it is, to lose fat on the low-carb diet, and that’s one sort of legitimate path to losing fat, what’s the problem with promoting that basic narrative? Why is it dangerous?

Dr. Stephan Guyenet:Just to acknowledge that it does also have benefits because, as I said in our debate, I think one of the things that Taubes has done that has been beneficial is basically given people permission to try a low-carbohydrate diet, and some people have benefited from that, but what I think is dangerous about the narrative is that it de-emphasizes other things that are actually really important like calorie intake, dietary fat intake, and physical activity.  If you read his writing, and this is another place where I think his positions are quite extreme, it’s basically all or nothing. Carbohydrate is the only thing that matters and none of these other things matter. I think that’s just profoundly at odds with what the scientific literature says. The exercise thing is an interesting case because there’s really good evidence that it impacts insulin sensitivity, which probably, in turn, impacts a variety of chronic diseases.

The evidence on body weight isn’t as strong. The effect on size isn’t as large, but exercise does affect body weight and fatness to some degree, at least in the average person. So, when I see these entire books dedicated to arguing that calorie intake is irrelevant, that physical activity is irrelevant, that dietary fat intake is irrelevant, what I see is someone focusing their attention on one legitimate factor and amplifying that while de-emphasizing and discarding several other equally important variables. I think we’re headed into a time where people are like terrified of sugar but don’t really care about added fats, aren’t thinking about calories, maybe don’t care as much about physical activity; that’s the kind of environment that’s being encouraged by these ideas, and I think that’s profoundly misguided.

Taubes and others have criticized the USDA and other organizations and American culture in general for promoting this low-fat thing and focusing heavily on this low-fat idea that fat is central to body fatness and health. but we’re essentially entering a time where we do the same thing with carbohydrate and sugar that we did with fat, and it’s not going to turn out any better. In fact, I would say it’s probably going to turn out worse, and we can see that in the fact that our carbohydrate intake in the US has been declining since 1999. Our sugar intake has been declining since 1999. Our sugar intake is now somewhere between 15% and 23% lower than it was in 1999. It’s been declining in several other affluent countries like the UK, for example, where it’s been declining for 50 years, and yet obesity and the diabetes rate keeps going up. That doesn’t mean sugar isn’t fattening.

It certainly doesn’t mean it’s slimming, but it’s not this overriding single determinant of all things obesity and health as Taubes argues in his book The Case Against Sugar, and if you think I’m exaggerating or putting words into his mouth, you should read the book. So, that’s what I think is really dangerous—taking the focus off several really important things and putting them all onto one other important thing.


How the brain controls body fatness

Ari Whitten: I want to wrap up one last idea here, and then I want to go on to your central model of how the brain is controlling body fatness. The low-fat era concept, this narrative that has been promoted by many low-carb advocates, that everybody went low-fat in the 1980s, 1990s, and all it did was make us fatter, and the idea that low-fat diets don’t work and have been proven not to work and make us fatter in the process. Can you speak to those basic ideas?

Dr. Stephan Guyenet: Yeah, absolutely. The first thing to note is that we never ate a low-fat diet in the United States. Our absolute intake of fat either increased a little bit or didn’t change depending on what data set you look at. It is true that if you look on it at it on a percentage basis, there was a slight decrease in fat on a percentage basis, but that’s only because our carbohydrate intake went up more than the other macronutrients. If you look at absolute quantities, our fat intake either stayed the same or increased, so this idea that we ate a low-fat diet is just nonsense.

Ari Whitten: There was a low-fat era where the ideas were being promoted but weren’t actually adhered to by the vast majority of people, correct?

Dr. Stephan Guyenet: Correct, and obviously it had some impact. For example, we saw the proliferation of low-fat foods. It was in the media, but I think, human nature is powerful; people like eating fat and if there are tons of foods around that are high in fat and carbohydrate, we’re going to keep eating those. I say “we” in the broad sense to mean “the average person,” but if you look at the types of foods that people were consuming, they weren’t especially low in fat, although maybe they were eating low-fat foods on top of that. I don’t know.

But the other thing to note, if you look at the actual 1980 US Dietary Guidelines that are vilified as this first guideline to promote a low-fat diet that made us eat sugar and made us fat, and if you followed the letter and spirit of those guidelines, you would be eating a diet that was a little bit lower in fat than average; you would be eating a diet that was predominantly unrefined carbohydrate that had less sugar in it, that specifically went out of its way to advise against eating sugar, and you would also incorporate physical activity. You would have a moderate calorie intake. It’s really not at all crazy advice. Now, maybe it wasn’t implemented very well by the food industry or implemented in a tricky way.

The food industry likes to make health claims because those are effective at getting people to buy things and they piggy back on things like this, but the recommendations themselves were not extreme or crazy; they were pretty reasonable. The other piece of it is if low-fat diets cause weight gain? The answer is no. We have tons of randomized controlled trials on this. You could argue that they don’t cause a lot of weight loss, but they do cause weight loss. Almost every randomized controlled trial that has ever been conducted shows that low-fat diets cause weight loss, so why would you think that a low-fat diet would cause us to gain weight when implemented nationally? It just doesn’t make any sense.

Ari Whitten: Right. And one study I think that’s worth speaking to is the really long-term study that was conducted by Chris Gardener. I think it was a 12-month study, low-fat versus low carb, a real-world study where they were actually given pretty good nutritional advice to eat whole foods. It was the best long-term low-carb versus low-fat real-world study with good nutrition advice that I’ve ever seen conducted.

Dr. Stephan Guyenet: Yeah, it was a very good study on a variety of levels. I’m kind of a stickler for study design features these days and it hit all the high notes in terms of detailed preregistration of the study design and analysis plan. Anyway, I’m not going to go through all the technical stuff, but [it was] a very well-designed study, and one of the things that I thought was cool about it was that neither of the diets had a calorie intake target. In a lot of low-carb versus low-fat studies, the low-fat diet will have calorie intake target but the low-carb diet won’t. So, some people have said this is comparing apples to oranges.  If the low-fat diet were eating as much as they wanted, maybe they would be gaining weight.

We already had plenty of evidence to show that wasn’t the case, but some people were arguing that. In this case, I said, “Hey, let’s give both groups a whole food diet, a really healthy version of each of these diets, and not give them any calorie intake targets.” They’re just eating to satiety. Folks like Gary will say that if you let people eat to satiety, that’s where people are going to overeat on a low-fat diet, but what they saw is that over the course of a year, these people lost the same amount of weight in both groups, it was almost identical—I think it was 12 versus 13 pounds.  [That is] very little difference in weight loss and was not statistically significant. Furthermore, they were not able to tell who was going to do better on the low-carb or the low-fat diet based on how much insulin they secreted at baseline.

It didn’t seem to make any difference.

So, you have these diets that differed two to three-fold in the amount of carbohydrate and in the amount of glycemic load, and yet they lost the same amount of weight.  How exactly you can reconcile that with the hypothesis that carbohydrate and insulin are the primary determinants of weight loss, I don’t know.


The key drivers of fat gain

Ari Whitten: Well, I’m going to say that you can’t. Regarding your model of obesity, I know that we could go very in depth and that could be a topic several hours long, but can you give a quick summary of what the key factors are actually driving fat gain, specifically from a nutrition perspective?  What are the key factors driving fat gain if not carbs and insulin?

Dr. Stephan Guyenet: Yeah. Essentially, obesity is caused by two things according to my model.

One is an increase in calorie intake, and the other one is a change in fat regulating brain circuits that cause the body to want to “defend” a higher level of body fatness. These are two key characteristics that we see in obesity, and I would venture to say that it would be difficult or impossible for obesity to occur without both of these features. I think these are two features that have to be explained by any model of obesity, and essentially these are both things that involve the brain because excess calorie intake is related to eating behavior, which is generated by the brain and brain circuits that regulate body fatness are obviously related to the brain as well. The brain is very central to my model. I think most people intuitively recognize that it has to be true that the brain has something to do with it.

If you believe that eating behavior affects your body fatness, then the brain generates eating behaviors. It has to be involved somehow, and we know that it is because genetic studies have shown that the genetic variants that determine differences in body fatness in the general population tend to relate to brain activity.  I really hammered this point home in our debate because I think this is the type of experiment that can tell us whether Gary’s model is correct or where mine is correct.  If Gary’s model were correct, you would see that genes related to fat cells and insulin would be popping up in these genetic studies, but in fact, we see genes related to the brain are the ones that are determining differences in body fatness in the general population. So, essentially, it relates to the brain and it relates to this broader concept that we call evolutionary mismatch.

This is the idea that certain traits that we evolved emerged for a particular environment, and when you put those things in a different environment, they can cause bad things to happen: it can be maladaptive.  Basically, we have these brains that have all these circuits that evolve to do certain things in the environment of our ancestors, but if you put those same brain circuits in the modern environment, they start to misfire and generate negative outcomes because if our hunger circuits were perfectly adapted to 2019, nobody would have obesity. Obviously, they’re miscalibrated in 2019.  Those are just kind of some general principles, but what is it exactly about the way we live now in 2019 that drives all of these things? What drives our brain circuits to misfire that causes us to eat too many calories and causes our fat regulating brain circuits to defend a higher level of body fatness? essentially. There are probably a number of different things that contribute, but I think the main one is the food environment that we live in and the food and the quality of the food that we eat. You see in a variety of species, including humans, that the most fattening diet in the world is basically human junk food, or food that is easy to access, calorie dense,  highly palatable, and that contains both carbohydrate and fat. When you put animals, whether they’re rodents or raccoons or bears or monkeys or humans, in an environment where they have a wide variety of these foods available at all times, they will generally overconsume calories quite substantially and rapidly began to gain fat. In that sense, I don’t think it’s rocket science. Some concrete examples of the types of foods that I’m talking about that are the pinnacle of these properties that I’m describing  include ice cream, pizza, baked goods like brownies and cookies, deep-fried foods like French fries: things that we would intuitively label as junk foods tend to be those types of foods that generate excess calorie intake and excess body fatness.

Ari Whitten: I want to interrupt very quickly here because this is actually one point of confusion where I saw that Joe Rogan got confused and this led to widespread misunderstanding. A lot of people hear those foods that you just listed off and they think of carbs and sugar. In people’s minds, they have come to associate those junk foods as carbs, and the reality is those foods that you just listed off—ice cream and pizza—these are foods that are rich in refined carbohydrates and oftentimes refined fats. Concentrated sources of either flour or sugar and fat together are the foods that drive the most calorie overconsumption.

Dr. Stephan Guyenet: Yeah, that’s absolutely right. We know that it’s not just about the carbohydrate. This is what I was trying to explain to Joe on the podcast. We can put animals or people on diets that are high in refined carbohydrate but don’t have all of these other features of the diet, and it’s not nearly as fattening.  When you put rats or mice in a cage with a bunch of human junk food, they stuff their faces and they get really fat. If you feed them a diet that’s only high in sugar or only high in refined starch, they just don’t get nearly as fat.

In fact, in a lot of experiments, they really don’t gain much weight at all on those types of diets. If you put them on a diet that’s just high in fat, they’ll gain some amount of fat, but again, they won’t gain nearly as much as they do when you put them in the presence of these foods that have this combination of properties.  It’s not just about the carbohydrates, and it’s not just about the sugar and the fact that people’s minds go there is a reflection of where we are culturally right now in 2019. That’s where the conversation is, but the fact that the conversation is there has nothing to do with the science. It just has to do with the current dietary fads. That’s an important point. I want to talk about some of the brain systems that underlie this. I mentioned the fact that we have these ancient brain circuits that evolved to support the survival and reproduction of our ancestors, and that those basically aren’t playing well in 2019 with our current food and food environment. I want to talk a little bit about what some of those circuits are.

The first thing I want to talk about is the brain circuits that determine the seductiveness of food. Basically, they determine how much we want food and how much we like food. And these circuits are kind of centered around the basal ganglia in the brain and particularly the parts of the brain that are influenced by the neurotransmitter dopamine.  Dopamine is a key driver of motivational states, including motivational states for eating. Different foods caused different amounts of dopamine release, and what we know is that those brain regions that are stimulated by dopamine are most activated by combinations of carbohydrate and fat; they’re somewhat activated by carbohydrate, and they’re somewhat activated by fat. When you put that and carbohydrates together, you see more activation and you see a greater motivational drive to eat the foods that contain those things.

So, I think this is a major explanation for why people see success on low-carbohydrate and low-fat diets, because essentially you’re taking out one of the primary factors that augment our eating drive, and if you’re on a low-carb diet, you’re cutting out carbohydrates, you’re getting less dopamine, and even if you replace that carbohydrate with fat, you’re still cutting out the combination of carbohydrate and fat together. You’re cutting down on dopamine, you’re cutting down on your motivational drive to eat, and that just makes it easier to eat less so. You see the same thing on the other end of the spectrum with low-fat diets. I think that’s a key piece of the puzzle. another brain region that’s kind of mismatch in the modern environment is our satiety circuits or the brain circuits that generate that feeling of fullness, that tells us to terminate the meal.

The amount of satiety that you experience per unit calorie really depends a lot on the food properties that you’re eating. Foods that are higher in calorie density, lower in protein, lower in fiber and higher in palatability are less filling per unit calorie, and that, of course, describes junk foods.  If you want to have a meal of those types of foods, you might have to eat twice as many calories to feel full and feel satisfied with that meal than you would have if you had eaten simple, unrefined food. So, that’s another brain region, and I won’t keep going through them, but those are two examples that I think are particularly important. The brain regions that determine our motivational state for food and the brain regions that determine satiety.

Ari Whitten: One thing that I noticed in the podcast with Joe Rogan that I wish was explained more in-depth and that I think got lost in the overall grand scheme of things is the evolutionary mismatch that you were just referring to. I think context matters a lot. If you have no reference for history and you just this as the modern environment, this is the normal human diet, the one that we’re all eating today is what’s normal, then what explains the difference between why some humans eating in this modern food supply, modern food environment, are lean and some are overweight? The explanation immediately focuses on biochemical differences. Is it hormonal differences, is it genetic differences, is it neurological differences? We can explore all those differences and focus all of our attention there. And how do we modify the activity of these brain regions or how do we modify the amount of insulin or this gene or that gene?  But I think when we see it in a different context, we then realize that in other humans who don’t eat this modern food supply, they’re almost universally lean and have no obesity epidemics.

So that then shifts the focus away from genetic or biochemical differences towards the food supply itself and how that is causing a sort of dysfunction or is mismatched with our biology and neurology. And that’s what you were getting at here. With that in mind, it’s specifically this modern food supply, junk food, foods rich in refined carbs and fats in particular and combinations of those that are the most mismatched with our brain circuits that regulate our food intake. Then when we’re in a food supply that has lots of those foods, we tend to get fat, and there are lots of animal experiments supporting this.

For example, if you take rats and put them on standard rat chow, they stay lean, they eat the amount of food that’s appropriate, and then if you give them access to a human junk food diet, they will all of a sudden start overeating and make themselves obese. Then if you put him back on standard rat chow, they’ll get lean again. There is lots of evidence suggesting that the same thing occurs in humans as well. With that in mind, from this understanding of the food reward theory of obesity that you just spoke to, what are the top two or three sorts of practical strategies that come out of that paradigm for people to lose fat?

Dr. Stephan Guyenet: Yeah, I think the number one thing, or at least one of the top things, is to control your food environment. Essentially, the problem is that it’s very easy for us to eat foods that don’t support our goals, and it’s harder for us in our everyday lives to eat foods that do support our goals, so  if you engineer a food environment for yourself in your home and at your work that makes it easy for you to eat foods that do support your goals and makes it harder for you to eat foods that don’t support your goals, then you don’t have to exert much willpower in the moment anymore to guide your behavior; you can just kind of go with the flow. I think most people do a lot better that way, not having to exert willpower frequently to avoid eating unhealthy foods, but just going with the flow because the environment has already been set up to favor healthy foods.

When we talk about these regions of the brain that regulate your eating drive and your motivational state toward food, the main thing that they respond to is food cues in your personal environment such as the smell of food and the sight of food are the two biggest ones, as well as a little taste of food; those are things that get your dopamine flowing and trigger a motivational state. It’s just like a smoker being in a place where somebody else is smoking or the bar where he normally smokes or walking past the convenience store where he normally buys cigarettes. It’s exactly the same process.

Those cues spike your dopamine and create a motivational state that can be hard to control at that point, so controlling your personal food environment and making it so that there’s not a lot of visible food around, and the food that is visible isn’t that tempting and is that will support your goals, like lower calorie density, healthier items such as fresh fruit. I think it’s particularly useful if there are little effort barriers in place for eating as well, like have oranges on your counter and that’s really the only snack that’s available.  if you want a snack, you can have one, but you’re going to have to go through the effort of peeling that orange, so you’re probably not going to do that unless you’re genuinely hungry. In other words, your body genuinely needs those calories.

Ari Whitten: I always think of fully shelled walnuts where you’ve got to put them in the cracker to break them open and then fiddle with them to get all the pieces out. If you have to put in a little effort to get the food, It dramatically influences how much food you eat compared to if you just had access to peeled walnuts that were just sitting there.

Dr. Stephan Guyenet: Yeah, that’s right. It’s not that hard to peel an orange, and it’s not that hard to crack some walnuts, but I think that emphasizes the fact that a lot of the time when we eat, our motivation to eat isn’t really that strong. We’re not starving. Maybe we’re just a little bored or a little bit hungry or we just walked past the food and it caught her eye. It doesn’t really take much of an effort barrier to shut down some of that excess calorie intake that isn’t even really that strongly motivated, particularly in-between meal snacking.

Little effort barriers like that can be important. The second thing I want to talk about is the composition of food and how it affects your satiety level.  As I mentioned before, foods that are higher in calorie density, in other words, more calories per gram (which usually tracks with the amount of water and fiber in the food), foods that are higher in calorie density, lower in fiber that are lower in protein and highly palatable, those are the least filling types of food, and conversely, lower calorie density, higher fiber, higher protein and moderately palatable are a lot more filling. That’s what we see when we feed people common foods and measure their level of satiety, and that can make a big difference.

How full you feel after a meal of the same number of calories can differ dramatically depending on the types of foods that you eat.  If you’re eating simple unrefined foods, whether those are things like potatoes or whole grains, beans, meats, eggs, a fresh fruit, those are the foods that are going to create a high level of satiety per calorie and allow you to terminate your meal while having eaten a number of calories that is supporting your goal of leanness. Essentially, you can eat fewer calories and lose fat without even feeling deprived.  I would say those are my top two right there.

Ari Whitten: Excellent. I have one last thing that I want to mention here. I know you’ve got to run, and we’re a little bit over here, but your new site that I’m very excited about. Is it called Red Pen or Red Pen Book Reviews?

Dr. Stephan Guyenet: Red Pen reviews.

Ari Whitten: Okay. Can you just give a brief overview of what you’re doing there?

Dr. Stephan Guyenet: Yeah, absolutely. Red Pen Reviews provides the most informative, consistent and unbiased health and nutrition book reviews available anywhere. We use a method that I developed along with some really thoughtful and nutrition scientists that actually gives numerical scores to books in three areas which are scientific accuracy, reference accuracy, and healthfulness. We have a structured scoring method that has a rubric that allows us to score books quantitatively, so when you land on the review page, you see actual percentage bars in those three categories that I mentioned as well as an overall score of those three average together. Basically, you can land on a review page, whether it’s Grain Brain, The Good Gutor The Longevity Diet, and within one or two seconds you can get a pretty good idea of whether that book is factually accurate and worth reading. Then below that, there’s a summary.

If you want to take a minute, you can read a summary of what we thought of the book, and then below that is the full review where we explain how we came to the scores that we came to.  Really, it’s a totally novel way of reviewing books that is a lot more powerful because it’s very consistent, we take many steps to reduce the amount of bias in our reviews, and because of that consistency, it allows those reviews to be compared between books.  You can see which book got the best score for a specific topic, and you can directly compare those scores to one another whereas, with typical book reviews, that’s really hard to do.

Ari Whitten: Yeah. I’m personally really excited about this project of yours. I hope it blows up big time. I hope you get tons of funding. In fact, I may be interested in helping to fund that myself, and I hope that this can be a massive book review site because I think it’s absolutely needed. And I’ll just mention a couple of reasons why.  I think there are only maybe 10 book reviews on there now, but, for example, Steven Gundry’s book, The Plant Paradox, which has popularized this whole low lectin movement, has a scientific accuracy rating of 26%. Then you have another popular book in the health space for the last several years, Grain Brainby Doctor Perlmutter, that has popularized this whole sort of anti-grain movement along with the idea that grains, carbs and insulin are the primary drivers of neurological disease: it has a scientific accuracy rating of 20%.

These are abysmal scientific accuracy ratings, and the reason that I’m saying this is such an important project, if you consider what I just said is that,  and as I’m speaking to everybody listening is that there are people, there are diet gurus, health gurus out there right now writing books that are absolutely just totally out of line with the existing scientific evidence and making millions of dollars off of them and creating millions of people out there who are believing this narrative that is wildly inconsistent with scientific evidence.  There’s this massive mismatch between what the general public believes and what the actual scientific evidence says, and I think we really need to reign this in and regulate a lot of these authors and health gurus that are promoting nonsense and pseudoscience and making riches off it.

I applaud your efforts there, and I really encourage everybody listening to go and read some of those book reviews for yourselves and see why some of these books score terribly. Finally, what’s your plan for developing your site? I really want to see that explode in popularity and become a massive resource, another objective review for all the stuff that’s on where people will go to check those reviews to see what the actual evidence says.

Dr. Stephan Guyenet: My main plan is just to keep adding reviews. I think that’s where most of the value is going to be for us, and there are other opportunities for growth that I would like to consider in the future, as well. I would like to expand the scope of the types of books that we review to include things like physical activity related books and medical books, and for that we would have to recruit reviewers who have expertise in those areas. I think it would be cool to do books in other languages, and again, that would require recruiting reviewers who are fluent in other languages.  There are a lot of ways that we can expand. I think partnering with different organizations is one way we can do it. Right now, those things are all in the future, and the main thing is to put one foot in front of the other and just put out new reviews.

Ari Whitten: Excellent. I love it. I hope that it explodes in popularity. Again, to everybody listening, I want to highly recommend that you go pick up Stephan’s book on called The Hungry Brain, and Stephan, or Doctor Guyenet, do you want people to follow you? Is there a place to direct them? Should they go to your website? Is there a good place for them to sign up to follow your work?

Dr. Stephan Guyenet: Yeah, first of all, Red Pen Reviews has an e-mail list now, and we alert people to all new reviews. You can sign up for that on our homepage. I’m most active on Twitter. My Twitter handle is @whsource and my website is, or if you don’t want to spell my name, you can go to and that’ll take you to the same place.

Ari Whitten: Wonderful. Well, Dr. Guyenet, this has been a pleasure. I’ve looked forward to this for many years, and I’m so glad that we finally made it happen. Thank you for your time, and I hope everybody listening will go buy your book and follow your work. Thank you very much. I appreciate it.

Dr. Stephan Guyenet: Thank you, Ari.

The Truth About Carbs, Fat, Insulin and Fat Loss with Obesity Researcher Dr. Stephan Guyenet – Show Notes

What science says about carbs and insulin as being primary drivers of fat gain (1:56)
Comparing low-carb and low-fat diets (14:33)
Why adding fat to your meal can be counter-productive when you look at the satiety per calorie equation (25:04)
What Dr. Guyenet thinks is the real issue with Gary Taubes’ Carbohydrate and Insulin hypothesis (34:38)
How the brain controls body fatness (39:24)
The key drivers of fat gain (46:26)


Check out the Joe Rogen Podcast mentioned in this episode.

Get Dr. Guyenet’s book “The Hungry Brain” from Amazon 

Go check out the nutrition book review page called

Follow Dr. Guyenet on Twitter

Check out Dr. Guyenet’s website

The science on fat loss - Dr. Stephen Guyenet
Listen to the podcast with Dr. Spencer Nadolsky on the science of fat loss

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