#564: Is Protein's Appetite-Suppressing Power Overstated?

(upbeat music) - Hello and welcome to another episode of Sigma Nutrition Radio. This is episode 564 of the podcast. My name is Danny Lennon and with me is Dr. Alan Flanagan. Alan, how are you doing? - I'm very well. I don't know if admiring is the right words, but I'm used, amazed, bizarrely frustrated by the fallout from the keto CT trial. So it's been a car crash in slow motion to watch play out subsequent to publication. - Indeed, and I'm sure many people listening will have seen some of that fallout or at least the initial publication and commentary of that keto CTA study. If not, then actually one of the episodes we recorded a while back with our friend Dr. Gary McGowan in hindsight has become even more relevant to this discussion and many of the problems that have played out were for scene in that particular episode relating their science communication and has been proven to be even more accurate in light of the release of that study. So for people listening, if you haven't listened to that episode yet, that's episode 541. I suggest you go check it out. But yeah, it's certainly been interesting as both a case study of what not to do in terms of actually a study publication, but also in terms of the communication of it afterwards. - I would also echo the referral back to the episode we did with Dr. McGowan because you don't necessarily like being proved right, so to speak, when that's at the expense of the integrity of science and the scientific process and unfortunately that's really the case with this group of authors. But I think there are some points that we make because we've been tracking their work for a couple of years that very much proved to be the case with them. And it just so happens that leopards don't really change their spots or at least the low carb keto leopards certainly don't. And so tragically, the calamity of this study and its publication and the deception within the paper has all been rather predictable for anyone that's followed this group with a degree of objectivity. And that is at the expense of trust in the scientific process and peer review and the publication and stuff like that. So it's a calamity. The whole thing is a pretty sorry affair basically. - And for anyone who hasn't came across this particular issue and doesn't know what we're talking about and does want to maybe look into this a bit more. In addition to that actual study, Dr. Flanagan and a number of other colleagues published a written piece going through some of the problems with that actual study. I will link to that in the description box. I do recommend you go and ever read through that. And that'll give you an idea of some of the things we've just mentioned. And then I'll also give some important context for that podcast episode if you do want to go back and listen to that. It's probably a topic we'll address again in a future episode. But for now, we will let the dust settle on that for a while and maybe revisit it at a future point. Today we wanted to talk about something completely different but also a very interesting topic and something that for a considerable period of time has been as opposed a presumed idea and he and also within evidence-based circles around the impact of protein on satiety, how it impacts on hunger, appetite and we'll distinguish through those terms as we get through this episode. But it's been typically seen as protein is this most filling, most satiating macronutrient. If you consume more protein that's gonna reduce hunger and appetite, it's gonna give you this fullness feeling and therefore you should consume more protein. More protein and meals has this effect. And while as we'll note, there's obviously a basis too much of that. We need to ask where some of the nuance and context comes into that what type of limits to that idea that we see. Do we sometimes over extrapolate some of those ideas or do we maybe misinterpret what some of that research says? And so that is our goal for today. And this comes off the back of actually a listener question, one of our previous subscribers, James Coons, sent in pointing to some of Kevin Hall studies which we'll refer to later in this episode. And basically the question could be summarized something along lines of is protein satiating effect really down to the protein itself or some of these other factors that we might see in a number of different studies where that's eating rate, energy density and so on. And so we will revisit those. But to maybe get us started here, Alan, let's maybe think through the historical context of proteins reputation within nutrition science for its impact on satiety, hunger, cessation. And maybe tied into that a distinction between some of those terms. Where do we see the emergence of this becoming an important nutrient or important characteristic that relates to satiety? It's actually one of the more long-standing observations. We can see the emergence of interest in protein and its relationship with appetite emerge in the 1950s. Melanchop and others published a paper in the Journal of Applied Physiology in 1956, for example, where they had conducted four different experiments looking at the relationship between amino acid concentration and fluctuations in appetite. And this was either through providing of standard food-based protein, high protein breakfast or using intravenous infusions of amino acids. Interestingly, they did that both isolated amino acids and with or without glucose and then oral ingestion of amino acids as a supplement. And they were looking at appetite as a self-reported scale and concluded found based on their experiments that the degree of post-prondial amino acidemia and the circulating levels of amino acids after protein consumption appeared to be associated with the decrease in appetite. And the return to hunger and increases in appetite corresponded and correlated with the decline in circulating amino acids. And that was a relationship that was independent of the changes in blood glucose that were observed when they provided amino acids with or without glucose infusion concomitantly. And so the amino acid infusions alone seemed to cause appetite reduction even when blood glucose levels were low. And so at the time appetite research was very much focused on the relationship with blood glucose, with fluctuations in blood glucose. And Mel and Cough was the first to really propose this amino acidemia theory or what they termed the amino static mechanism of appetite regulation. And this became one of the theories of the potential for protein to have an influence on appetite. And it has found like a lot of these theories of appetite and eating behavior, you can find it a degree of support. But it's not entirely explanatory. And I think that's really a theme of what we're going to explore today is the fact that there can be some degree of explanatory power to certain variables, so to speak, in influencing energy intake and appetite and satiety. But they might not necessarily be entirely explanatory as an isolated variable. And so really you can trace that the amino static theory back to the 1950s and then through the 1990s or through the 80s and into the 90s, you'd much more of an interest in macronutrient composition of diets, not isolated protein. Macronutrient composition and variations and how that influenced appetite. And if it did influence satiety by what mechanism and you had again then emphasis on the thermogenic effect of protein and the fact that we do know that there is a hierarchy of the thermogenic effect of macronutrients with protein typically requiring the greatest contribution of energy to the protein, digestion and utilization compared to say carbohydrate and fat. And the degree of interest then in the relationship between thermogenesis induced by higher protein intake and whether the thermogenic effect of protein might relate to satiety as well. And so you have the development of various theories that relate to the potential satiety and effect of protein, which relates to the relationship with diet and juice thermogenesis, the relationship with or the amino static regulation, like we just discussed going right the way back. A potential relationship as well with gluconeogenesis as a factor that is not necessarily directly related to satiety, but the metabolic processes associated with gluconeogenesis might have an indirect effect. And then the potential impact of protein in a more direct sense on appetite, regulatory, gut-derived hormones and their bidirectional communication as all of these kind of appetite hormones. For example, like Rallon or GLP1 have receptors in the brain as well, so the reciprocal potential impact of amino acids on those satiety hormone processes. And so they would be the broad areas of interest in terms of explaining whether protein has a uniquely satiating effect at least compared to other macronutrients in the diet. It's maybe worth digging into what you just said and laying out clearly for people. Because when we think about this topic [BLANK_AUDIO] people when they're colloquially talking about proteins impact on hunger or appetite generally, that can mean a lot of different things when we're trying to be very specific in our language show. When we are consuming a meal that's high in protein, for example, are we thinking about when we're going to start feeling full and terminate that meal? Are we talking about the fullness in between meals? Are we talking about that as one acute response versus across the day on average? These are all different things and we have different ways of measuring them and we're using some different terminology. Can you maybe categorize for people some of those distinctions between those things that will maybe set the stage for later on as we're starting to think about some of these impacts that when we generally talk about appetite or hunger and so on? Yeah, I think that there's two, the point of departure here is distinctions in the definitions, because we've mentioned obviously the word satiety but it's important that we don't just interpret that to mean a general sense of fullness is within the appetite literature, there are specific definitions to the word satiety and then satiation and they are overlapping but distinct. So satiation we would describe as the process that leads to the termination of eating and their baby sensory components to that in terms of a feeling of satisfaction or taste pleasantness and stuff like that within the meal. So satiation we could describe as an intra or within meal effect but they are processes that ultimately lead to a decision to stop eating at that point and then satiety would be characterized as the feeling of fullness that persists after eating and that might potentially suppress further energy intake until there is a sufficient return of hunger and increase in appetite and loss of that sensation of fullness such that there is a decision to eat again and Jumblandel's group at leads have done enormous amounts of research over the years in this area and proposed what we would term the satiety cascade. And so for listeners you kind of picture a cascade that begins with the initiation of eating but slightly also is preceded by certain sensory factors that might actually motivate eating. So you know the smell of the roast beef coming out of the oven so to speak these kind of sensory components that go into influencing the sense of appetite and so in the satiety cascade it very much starts in the within meal context even preceding that meal with sensory factors. They overlap with cognitive factors in terms of during a meal for example like a pleasurable response to that meal etc and they influence early responses like we said satiation within that meal effect and then after that meal there are longer term post-ingestive and post-absorbed of factors so we've got the satiation which we described as intra or within meal fullness so to speak and that terminates that eating episode and then we follow that them with early and late satiety and so the early period typically influenced by cognitive and post-ingestive factors. So for those post-injective ingestive factors we would start to look at for example circulating appetite hormones would be a good marker of looking at the relationship between dietary factors and appetite and hunger in that post-prondial period like GLP1 or ghrelin or leptin or otherwise and then the post-absorbed of state is obviously a return to fasting over time and at the decrease in levels of the post-ingestive or post-prondial circulating hormones and eventually a return to hunger and a desire to eat again and so that would be the late satiety impact. So we would have that distinction between satiety and satiation. Satiation itself the within meal effect can be induced by a number of mechanisms such as gastric distension so there's physical aspects to this as well and then hormonal factors like the gut hormone, colocysticin in our CCK and then satiety is influenced both by what are known as episodic which is the immediately following the eating episode that's the early satiety episodic satiety and tonic satiety which is longer term signals and those signals like we said are largely derived of gut hormone release of ghrelin for example and another of other gut hormones in the gastrointestinal tract which then act to induce satiety and then the tonic signals the much more longer term satiety is influenced by hormones that typically provide information about energy regulation such as leptin which is secreted in proportion to levels of adiposity although there are obviously differences in adiposity increases we have evidence of a degree of leptin resistance but if we're talking about the overall processes it's the question we're interested today is how does protein influence satiation in a within meal effect and satiety and in terms of a longer term affect medium and longer term affect and is there an impact then on varying protein levels in the diet in terms of for example return to hunger or subsequent energy intake or indeed on some of these post ingestive gush derived appetite regulatory hormones and all of that has been very much a prominent focus of the protein and appetite research and so just to speak to what you've just said there and I'm sure people are thinking well there's this complex feedback and all these hormones working in concert that influence these processes these this is not necessarily something that's super straightforward there's a variety of different measures of these different markers that we could take there's also subjective markers that can be measured so when it comes to research general in this area that might pertain to some of these questions that we have and maybe some of the studies we'll look at later what are some of those particular measurements are going to be taken what might be the most useful ones most common ones how extensive could this be can you maybe give people an idea of what typically gets looked at here and maybe what directly speaks to some of these things that we've just outlined yeah so really there's two ways we can think about this as far as research goes in terms of measurements or outcome measures that we might look at one would be subjective hunger and appetite and they would typically be assessed using visual analog scales which are 10 centimeter or 100 millimeter long horizontal lines which would have verbal anchors on either the far left or far right so let's say we've got a visual analog scale for hunger on the far left of that might be a verbal cue to the effect of as hungry as I've ever been and to the right of that might be something along the lines of I'm not hungry at all and so those verbal anchors then provide the kind of extremes and the participant will mark a vertical line across where along that horizontal line they feel corresponds best to their subjective sense of hunger and the sense of it wishy washy but they're actually very well validated although they're an important thing to note about some of these appetite measures is they're not particularly for visual analog scales they're not always good predictors of subsequent energy intake but they are validated for actually capturing sense of subjective hunger fullness and composite appetite appetite scores can be produced then from a measure of hunger a measure of fullness and a measure of what's known as propensity to eat or prospective food consumption which is how much you think you could eat in that moment so there would be examples of subjective appetite measures and they could be represented as an area under the curve or they could be represented as just a simple absolute value at different time points and then we can have the actual measures of some of the gut-derived appetite hormones that we mentioned on their actions so a few for example would be grellen so this is a gastric secreted hormone released in the stomach it's effect on appetizers to increase hunger and its mechanism is increasing hunger of hunger via grellen receptors in the brain and it also potentially then has some long-term influences on energy balance one that is a big topic dugeur which is glucose like peptide one or GLP one and that's because of the recent proliferation of GLP one agonist drugs but GLP one is a hormone secreted testinally and increases satiety again via GLP one receptors in the brain and also has a relationship with post-prandial glucose metabolism because GLP one is an incredible hormone and also stimulates insulin production and so GLP one is of interest as it relates to the protein literature because of the potential dickily for way proteins to stimulate GLP one responses and potentially have influences on post-prandial glucose metabolism via slowing gastric emptying so this is an important additional mechanism of GLP one that may relate to their appetite inhibitory effects is the ability to slow gastric emptying and as a result then the kind of rate by which food leaves the stomach induces a longer sensation of fullness and then there's other hormones we probably don't need to necessarily get into as much but I'll just mention for interest like peptide yy which is actually found in terms of its size of production throughout the large intestine the alien the column and the rectum increases satiety again via peptide y2 receptors in the brain also has a role on slow and gastric emptying and intestinal motility. And then there's some other hormones like mentioned, colosistokinin or CCK, secreted in the duodenum and the gigenum. So the small intestine also increased satiation, specifically, so CCK is of interest if we distinguish between satiety and satiation via influencing satiation. And that appears to be a macronutrient that does appear to have some sort of modulation by macronutrients as well. So they would typically often be distinguished as subjective and objective appetite measures of appetite regulatory hormones. But I think in the wider literature it's important to note that actually neither subjective appetite measures like visual analog scales or, and this is the irony, object so-called objective gut derived appetite regulatory hormones are particularly good predictors of subsequent energy intake. And I think that just speaks to the complexity of appetite regulation and energy intake in humans. So then with that, because there's not this one doesn't directly predict the other. We have two questions at this fork and the road then around does protein impacts satiety and satiation directly through some of these measures. And the second would be does more protein then lead to differences in energy intake. And then also that gets us into a whole range of other questions that we can maybe address. But maybe to get into that first part, because we've already mentioned some of it, but for the evidence that we currently have what could we say from a consensus point of view around protein intake and impacting some of these measures that you've just outlined in general. So I think that we could, I mean, certainly up to, and I know we will discuss this study as we go through a 2023 paper from Kevin Hall's group, which was a secondary analysis of two of their very tightly controlled, elegantly conducted metabolic ward studies, one comparing ultra processed to minimally processed food diets, and the other comparing low carbohydrate ketogenic high fat diet to a lower fat carbohydrate plant based diet. And this essentially suggested that other factors rather than protein content of the diet influenced ad libido energy intake, for example, energy density. And this isn't necessarily an unknown energy density has been known to be a factor that could potentially mediate relationships between protein and satiety. And I think there's a few examples that we can discuss. But I think as an overarching synopsis before we get more granular with some of the research, I think the available evidence does support that there is an independent effect of higher protein on overall diet induced satiety. And that sometimes may not necessarily be evident on a meal induced satiety basis. There's a really important distinction there. So in terms of satiation, for example, it may not necessarily be readily or instantly evident in that kind of acute context. But if we're thinking about total diet induced satiety rather than satiation, there does appear to be an effect on, for example, subsequent energy intake in some studies that appears to relate to the degree of thermogenesis. So there's at least some support for a satiating effect that is independent, that is related to satiety that is itself related to diet induced thermogenesis. So in some cases, these are indirect effects. The satiating or the satiety related effect, the diet induced satiety appears to be a indirect process of other metabolic impacts of protein. For example, the thermogenic factor. There's some evidence, although it's probably slightly more inconsistent as it relates to overall proteins as a class, protein-rich foods for the amino-static regulation aspect of this. That appears to be there's more signal in the noise for the amino-static aspect as it relates to specific protein types, way protein in particular. And some research comparing, say, way to casene, whereas casene has more of a slower acting protein, whereas with a way, you get much more rapid aminoacidemia. And again, you can find some support, but I wouldn't describe that as particularly robust in terms of the relationship between circulating amino acids and reduced appetite as it relates to just protein foods generally. It does appear to have a degree of specificity to quay. And then, of course, there are factors that can influence then the signal in the noise, so to speak, for an independent effect of protein. And if we look at studies that have factored in energy density and volume metrics of food, so you're not confending the study by having different energy densities at two different levels of protein intake, or you're not confending the effect of related to food volume, because we know from Barbara Rose's research that food volume has an important influence on how much is eaten in a given meal, accounting for those factors, then with protein on a per meal basis, at levels that are more habitually what people would consume, there does not appear to be any strong signal in the noise for a satiating effect of protein, or even satiety in if studies have looked at, say, subsequent energy intake in some meals, but with a much higher proportion of protein and a kind of meal by meal basis and a much higher total protein intake in the diet, then there can be more of an independent influence at least. Again, relating it appears more to satiety in so far as there is decreased subsequent adlibitum energy intake observed in some of these studies, so I think that's none of that is exclusive of the role of these other factors in influencing adlibitum energy intake, such as food texture or energy density or eating rate, because these are all part of the equation, I think, and I think when it's come to, I know we discussed this a couple of years ago in relation to set point and settling point theory, when it comes to some of these mechanisms related to appetite regulation, and obviously the implication then being obesity, is there is a tendency for one of these aspects of theories to provide the unifying theory of this relationship, and in fact, I think there are just multiple component parts that are potentially working in concert to influence adlibitum energy intake at least. Yeah, and I think this really speaks to that initial question that I mentioned, the outset that we received, that really, particularly as we go through the analysis of the whole studies, really will come to light of these different aspects you've just mentioned related to food texture, to maybe the degree of processing, to eating rate, and obviously if they are playing a role, as well as potentially this impact of protein, the question essentially centered around well, how much time do we spend trying to disconnect these or account for these, and how much of that role that we might see for protein in one particular place? How many of those studies have accounted for the fact that in general, maybe a class of high protein foods within a diet, have some of these other aspects that let's say a lot of high fat foods might not have, and so now we're starting to really try to try and disentangle these things, and maybe some of those answers might emerge as we go through this. So maybe it will be useful to start working through some of the data we have on a couple of those trials that might start speaking to some of these other aspects and consolidate this together. Yeah, I think maybe a good point of departure is actually a 1998 paper by Barbara Roles and her group published in Physiology and Behavior, and it was a really nicely controlled, most of her researches, experimental within participant, within subject design, where each participant was randomized to five different preloads, and this method of looking at the satiating effect of food is quite common in this literature, which is to provide a preload, quote-unquote, certain dose, so to speak, of the specific food, or whatever the intervention is, and then look then at post-prandial satiety, and then look, then at, let's say, subsequent energy intake to a meal after the preload. So with this, they had five different micronutrient comparisons. Each of the five conditions was high, was designed to be high in a specific micronutrient, either protein, starch, sugar, sucrose, fat, or a mixed composition. They were all the same energy content, 300 calories, and they were all the same volume of food, around 390 grams, and they were looking then at both subjective, hungerfulness, and pleasantness of taste. They were looking at actual then subsequent energy intake, both in absolute energy intake and in the weight of food eaten, and in the micronutrient content of the food eaten. So they wanted to see whether there was a, what they called a micronutrient specific satiety specific micronutrient effect. Like if you ate more of them, one of the macronutrients, if you have the high protein or the high starch meal, did that influence then less protein or less starch or something consumed at the subsequent meal. And what they found was that in relation to subjective hunger and subjective fullness, so both of those subjective ratings, both the high protein and the high starch preloads had a significant suppressing effect on hunger and fullness compared to the sucrose fat and the mixed preload. And then in terms of actual energy intake, weight of food was significantly less after the high protein preload compared to all of the other preloads. And the actual amount of energy intake was significantly less after both the high protein and the high starch preload. And the conclusion was that a degree of sensory specific satiety was evident so that the more someone ate of one micronutrient, the more that they had a decline in that of other foods with that micronutrient. But they actually related the effect of sensory specific satiety that they observed to sensory factors rather than the micronutrient content. And they didn't necessarily find a, for example, to phrase this another way, consuming a food that was high in one micronutrient didn't necessarily selectively decrease the subject of pleasantness or the intake of other foods high in that same micronutrient. But they did see an impact on what they termed general satiety insofar as there is the decrease in subsequent energy content and subsequent weight of food. So this was an effect that was evident for both the high protein and the starch. But they did say that because the foods differed. So for example, the high protein content was achieved with a chicken. The high starch was achieved with faster with pasta and the high fast was achieved with cream cheese. So what they were acknowledging was okay, we've controlled for the calorie content and the food volume between these conditions. But there might be differences in sensory characteristics that could also potentially influence those effects. But nevertheless, in terms of when outcome like subsequent energy intake that was decreased, despite the fact that they didn't necessarily find the evidence for micronutrient specific satiety. So consuming a food high in one micronutrient didn't selectively reduce intake of other foods high in that micronutrient. And then I think there's another study jumping forward a bit more, which was Blatt and colleagues, which was also Barbaroles' research group, although she's senior author by this point published in 2011. And a similar design in terms of a crossover, but what they were looking at was selectively manipulating the protein content of a luncheon dinner from 10% 15, 20, 25 and 30% of energy. And that was in the across these kind of five, five conditions. They manipulated the protein content of those meals while maintaining the energy density and maintaining the actual fat content and the appearance of the food. And they basically found no difference in that study with the amount of energy consumed at all. So there was really no per one percent increase in energy from protein, no significant difference in energy intake and ad-librium energy intake across those conditions. And so the conclusion of that was that actually varying protein content even up to 30% in meals didn't actually reduce daily energy intake or alter subsequent satiety ratings in terms of subjective appetite markers. But if you actually look what I found interesting about that study and this speaks to this difference between thinking about the potential impact of protein as it relates to the total diet versus a meal by meal effect. Because when they were describing the stepwise increases in protein intake, actually in a per meal basis, it wasn't so the 30% protein was a 30% in terms of because this was across both meals was like a total protein content. The actual per meal intake of protein was in the high protein condition, the highest about 17% of the actual meal. So this is potentially one of these discrepancies is actually what's the actual level of intake on a meal by meal basis and what's the relationship between that level of protein intake and total daily protein energy intake because it could be that if you're looking to in acute studies find it more acute post-prandial effect than it may be that the actual proportion of protein in that meal needs to be higher. And it can be somewhat misleading to say a 30% protein diet, but if that is not necessarily what the actual corresponding proportion of protein in an individual meal is, you might get some kind of discrepancy. So there are two examples of studies suggesting slightly differential effects of protein in terms of subjective appetite and subsequent energy intake. There are, again, studies that we can look out where they have still controlled for energy density, they have still controlled for the actual volume of food and factors like that. And you can see an impact on satiety with protein, with higher protein diets, but that's for example with, and this comes back to the meal factor that I just discussed. So if we look at some of the Marguerite Westertor plantengas research or Clive Westertorps research on the relationship between protein and say diet and use thermogenesis and satiety, there are meals with 25% of energy from protein. It's a really important factor to tease out in this literature like what is a high protein meal and how does that relate to some of these outcomes? And it could be that if a study appears to say well this is a higher protein diet, the actual per meal protein intake might not necessarily be around a level that we might be expecting to see some of these relationships with satiety that relates to other metabolic processes like diet and use thermogenesis. That's really interesting to ponder and it may be at least one of the potential explanations for this discrepancy mentioned earlier between maybe when we measure some markers of satiety, for an example, and then we look at something like actual energy intake, we may see differences between those when we're looking at some of these studies, but if we could have a foresee a situation whereby, as you mentioned, you could have a high protein diet condition let's say, but depending on how that protein is distributed and what the actual meals look like, that may be different to a situation where each meal throughout the day at a certain regular interval is a sufficiently high level of protein to have a very strong acute effect at that moment, and then we're going to repeat that a number of times across the day where we may be able to get more of this reduction in energy intake across the day through that as opposed to what we're having a 30% protein diet, but like you say what those actual meals end up looking like, maybe misleading just to look at this is a 30% protein diet, we're going to call that a high protein condition per se. Exactly, and so there's two questions we could then ask is, okay, we could look at acute studies and we might be able to see a single, a potential effect of a single high protein meal if that protein in the meal is of a certain intake, and then there's the question of, okay, even if we manage to see, so for example, some of the studies we just described at, say, 17% of protein in a given meal in the highest condition, the Black and Colleagues studies, is there, can we see differences in acute studies, which that was acute, can we see differences in acute studies with higher protein intake in a meal, and then if we do see an acute effect, is there slightly more chronic effects over, over several days, and I think there's two studies that I think are worth mentioning in this respect, one is sweets and colleagues, so this is from Westertor Plantango's group, this is in 2008 in the Journal of Nutrition and they were looking specifically at two test meals that were provided as a lunchtime meal, so participants were either randomly assigned to 10% energy from protein, which also concomitantly had 60% carbohydrate and 30% fat, or 25% energy protein, 45% carbohydrate, 30% fat, so intake of fat was the same and other characteristics of the meals, the meals were isocaloric, they provided 35% of daily energy, and they were also matched for the weight, and they were matched for energy density, so the difference obviously is then in the relationship between protein and carbohydrate, but this is an acute single meal provided as a lunch, and what they ended up finding was somehow predictably diet and juice thermogenesis is a percentage of the meal energy was significantly higher in the higher protein condition, fair enough what about satiety, which is what were more interested in, well they were significantly higher in the high protein compared to the 10% protein group, and that was evident both 30 minutes after the meal and two hours after the meal, but interestingly in this study the satiety scores didn't correlate with the measures of Grail and GLP1 or PYY that they had actually conducted, but this was a single high protein meal but 25% energy from protein in that meal, and it did have an acute increase on subjective satiety which was also related to or sorry and they also had increased diet and juice thermogenesis although they didn't specifically look at that relationship, but what they suggested was actually the The effect of the high protein meal may better reflect satiation than long-term satiety, recalling that distinction we drew with the outset. But again, there's limits to the inference we can make, because it was a thing like cute study. But this was, we can compare that to actually study from the same group that did look at a kind of slightly more longer term effect they had participants undergoing three separate thirty-six hour stays in respiration chambers, respiratory chambers, which will sample and measure continuously in direct chlorometry, so to assess energy balance and energy expenditure. And they had, again, distinct diets, high protein and carbohydrate versus a high fat diet, but again, matched for energy, matched for food volume, and matched for the energy density of the meals. So the actual diet composition was 29% protein, 61% carbohydrate, 10% fat, versus 9% protein, 30% carbohydrate, 61% fat. And this was specifically interested in diet-injuice thermogenesis measured over 24 hours, so whole day diet-injuice thermogenesis, and then subject of satiety and the potential relationship between the two. And so because it was a respiratory chamber study, they were able to measure energy balance, intake was controlled, expenditure could be measured accurately, and there was no differences in overall energy balance or activity in juiced energy expenditure. Diet-injuice thermogenesis or thermic-effective food was significantly higher on the high protein. Again, we would expect that related to the oxidation hierarchy of macronutrients, so diet-injuice thermogenesis was around 14.6%, compared to 10.5% on the high fat diet, and the satiety ratings were significantly higher on the high protein diet overall, just overall satiety and also specific measures of fullness, for example. But with the diets, 24-hour thermogenesis, correlated with satiety, there was a moderate correlation in R of 0.6, and then the differences between diets, so there was a within-diet effect of the relationship between 24-hour thermogenesis and satiety, but there was a stronger effect comparing the two diets where 24-hour diet-injuice thermogenesis correlated, and the correlation was 0.8, so a strong correlation when actually comparing the high protein, high carbohydrate diet to the low protein, high fat diet. So, their conclusion was that the high protein, high carbohydrate diet induces greater diet in juiced thermogenesis, which is clearly evident, and that the synchrony between diet-injuice thermogenesis and satiety suggested an indirect of, well, protein is having the direct effect on diet-injuice thermogenesis, but then those metabolic rate components are linked to satiety signals, possibly through increased body temperature or oxygen consumption, and that diet's conversely that are higher and fast, don't elicit those responses, so again, we've got two studies to contrast. This one and the one we've just previously mentioned, where with the previous one, they have a higher actual meal content of protein at 25% of the energy of that meal, and there's some acute effects, but this is actually now looking over 36 hours and finding that relationship between higher total protein diet and satiety related to the thermogenesis generated from that higher protein diet over, say, 36 hours. And I think this may be important then, because the relationship between protein and some of the post-ingestive gastrointestinal hormones can sometimes be very inconsistent in different intervention trials, may be more of a signal in the lois for GLP1, but for a lot of other markers, it can be a little inconsistent, and it simply could be that, in fact, looking at some of those intermediate markers, if we want to call them that in terms of the post-ingestive circulating hormones may not necessarily be a mechanism through which dietary proteins may impact on satiety directly relative to, say, a kind of process like increased thermogenesis. And that's something we'll certainly maybe come back to as we start playing together, some conclusions on this body of evidence, but at this point, given that we've walked through some of the potential mechanisms by which it could be playing a role, some of the what we can take and garner from that evidence in this area specifically looking at protein itself, that sets us nicely to address maybe those whole studies, which put this focus on some of those other components that you've mentioned throughout this episode so far, whether that relates to eating rate, energy density, and so on. And I think from a practical level, what will also maybe come to is when if someone is trying to eat in a certain way, then it's going to have an impact on satiety across the day as an example, is there going to be more benefit to be putting a focus on the protein level or some of these other components because all of them impact what food decisions we're going to make and by making choosing one thing over another, we can make very different food choices. And so maybe we'll start looking at some of these with relation to some of those publications from Holland, and particularly maybe their analysis of some of the day they got from those trials that did point to this importance of other factors like energy density and so on that you've mentioned rather than protein per se. What for you were the big things that you took from that? Yeah, so I think probably the most interesting one was the 2023 secondary analysis of data from there to previously conducted metabolic or controlled feeding studies. So typical of whole research designs that they used for these studies, participants followed a specific diet for 14 days, then immediately crossed over to a comparative diet. The two trials, I think we mentioned at the outset, one was comparing diets predominantly ultra-processed foods to predominantly minimally processed foods, and the other was comparing a low carbohydrate diet to a low fat diet. And the participants were instructed to eat ad-libertum. So the meals were provided and then participants could eat as much as they want. And these data were based on 35 participants, 30, they're otherwise healthy, 30-year-olds, I think on average, pretty balanced first sex and it was kind of 50 to 55% male-female, and weight-stable participants. But what they were looking at with the secondary analysis was whether specific characteristics of the meals were related to ad-libertum energy intake, how much energy they consumed during each of the respective breakfast lunch and dinner eating occasions, and those meal characteristics they were interested in were energy density, so calories per gram of food, eating rate, so gram of foods per minute, the percentage of protein and the percentage of energy from hyper-political foods, and they were defined hyper-political foods as combinations of fat and sugar, fat and sodium, carbohydrate and sodium, and they were doing, they conducted an analysis of all of the diets combined, which was over 2,500 data from 2,500 meals, and then they looked at it by the diet pattern, so low fat, low carb, ultra-process, minimally processed. And what they found in terms of the hierarchy of what factors, what characteristics had the strongest predictive effect on ad-libertum energy intake? Far and away it was energy density, had the nearly 2,500 times greater impact than the second most influential characteristic which was eating rate, so energy density far and away the most important, at least in this analysis characteristic, predicting energy intake. And protein itself, or the percentage of protein in the meal, had the weakest effect out of all of these, so the actual order was energy density, eating rate, percentage of hyper-political foods, actually sorry, percentage of hyper-political foods was second, eating rate third, and percentage of protein fourth. And then when it was stratified by diet pattern, there were differences, so by low fat versus low carbohydrate diet, the percentage of protein was not significant in either of those diet patterns, whereas in the ultra-process versus on-process diets, the percentage of protein was significantly associated with energy consumed, but it was, again, one of the weaker strengths of predictive value relative to, again, energy density, across all of those stratifications still exerted the greatest influence, and in fact, overall in the mediation analysis, comparing the ultra-process to the on-processed food diets, energy density, mediated 45% of the relationship with overall adlib at a energy intake. And interestingly in that analysis, the percentage of protein was negatively associated with energy intake, i.e. the percentage of protein in a meal was not associated with low lower energy intake at subsequent meals, which is what we would, the hypothesis that we would be predicting based on previous research. And in the low carbohydrate versus low fat comparison, the percentage of protein just had no significant mediation effect. And so some of this was then taken to say, actually, maybe there is no satiating effect of protein, at least not a substantive one, and that the effect of protein as a satiating macronutrient was perhaps explained by the fact that higher protein foods tend to be often lower energy density foods, for example. Like if you think about a non-fat Greek yogurt, it's very protein and rich, but it's not an energy dense food per 100 calories, or a lean chicken breast is not a particular energy dense food per 100 calories compared to a ribeye steak or a fatty meat or something like that, other protein sources. And again, interestingly, the percentage of protein influenced or had an influence, even though it was relatively modest on meal energy intake, only in the ultra-processed versus unprocessed food comparison, but not in the low fat versus low carbohydrate pattern. So again, I think this was the first thing to note, this is a secondary analysis. This is a direct intervention designed to test this. This is looking post-hoc at this at these data from both of these trials, so it's not a direct test of the satiating effect of protein relative to energy density, for example. And many of the trials that we've previously discussed have specifically accounted and matched energy density between their comparative diet conditions. But nevertheless, it was noted by the authors that the kind of association with protein in the comparison between the ultra and unprocessed food diets, which is where they found a significant influence of protein. That influence wasn't on decreasing satiety. The relationship was positive. It increased. It indicated that there was a positive relationship between protein content and energy intake in the degree of processing comparison, which they obviously highlighted as surprising given proteins satiety effects. But I would say again, we have to come back to this relationship between meal-by-meal protein content and total protein intake, because in some of the analyses where we have found or seen more of a signal in the noise for an independent effect of protein, on markers of satiety, either subjective or in the case of diet and juice thermogenesis, that kind of indirect relationship, the correlation with satiety, is these are a thresholds that are higher than the whole and colleagues' experimental diets in both of the diets and both of these studies in the comparison's protein was I think 15% of energy in the diet. So as much as this study is interesting for the relationship between energy density and eating rate, I don't think that we should overly read into the correlations observed with protein in this study because I simply don't think that the protein content of the diets is of a sufficient magnitude to actually have a more robust test of that association. And that point actually speaks to where different contexts of different populations could see differentially impact because one of the areas where protein is often most discussed is in maybe a fitness-related or an athlete-specific type population, even more so if we're talking about a body-building sphere and we're looking at populations of people who are undergoing energy-restricted diets, but tend to be very high in protein much higher than we would see even in high protein conditions for typical diets. And even in most of the studies where we're looking at a generally normal intake, let's say even in the studies we look at so far, if we have higher or lower levels of protein, they tend to don't reach the amounts we might see in some of these very specific conditions. Even more so on a meal-to-meal basis, we could think of specific meals that or snacks that people may have, that maybe the majority of calories in that meal could be coming from protein, which would be very atypical in some of these other studies. And so we're looking at a slightly different exposure and people may have that question of maybe in those situations, do we see a situation where protein is more of an important player here, and this lack of effect could be down to just the types of thresholds we're seeing. But then as a flip side, we could go to the other extreme and there are people who may presume that, well, just going to more and more protein per meal continues to have a beneficial impact on, let's say, a feeling of fullness afterwards. And so for that, there's two ends. One is, do we need to pass that certain threshold that maybe you've suggested for on a meal-to-meal basis for protein to start having some of those effects? And at what point does more protein, maybe not necessarily have effect? Probably very difficult to give specific answers, but to speak to any of those points, is there anything that you've come across that may be relevant to those types of questions people could have? When it comes to that question, I tend to try and look at, because a lot of what we've looked at so far and the studies that we've discussed so far are very controlled metabolic warge studies where, you know, an arbitrary level of protein is essentially selected. So we're not necessarily looking at some studies that have looked at dose responses or a range of protein intakes, but that's often then, like I said, the range of protein intake related to the total protein content, the diet, not necessarily the range of meal-by-meal protein. I think generally when it comes to this, like, is there a level at which there may be an effect that is warranted in terms of an evidence-based justification for real-world outcome? And I think this is where we kind of are better turning to studies that have actually looked at some kind of real-world outcomes in terms of weight loss and appetite. And I think one that I've always really liked is this weagle and colleague study in 2005. And the reason this is a really informative trial is because it had three distinct phases to the study. It had a two-week initial baseline weight maintenance phase, which was a generic macronutrient composition that you would see in Western industrialized countries, 15% protein, 35% fat, 50% carbohydrate. And then they followed a two-week isocaloric, so the same energy intake as the previous two-week baseline, but they switched to a high protein, 30% total daily protein intake, 20% fat, 50% carbohydrate. So the carbohydrate remained the same. And again, calorie intake remained the same as it had when they were consuming 15% protein. And then following that, so there's a four-week total run-in phase where they've gone from 15% protein in two weeks to 30% protein, then they entered a 12-week ad-libitum. They were able to eat as they desired, but maintaining this high protein, 30% energy, diet. And then they were studied during both of these respective periods when they were doing the kind of isocaloric run-in, and then under the ad-libitum conditions, an appetite was assessed using visual analog scales. And then during the inpatient visits, they also had measures of plasma, ghrelum, and leptin, and insulin. And what they were really interested in then was changes in appetite and satiety and the ad-libitum energy intake and how that changed over time. And so what we ended up seeing in this study, during the period where they were consuming a weight maintenance isocaloric diet, but recalled that for two weeks, there were 15% protein before switching up to 30%. They doubled their protein intake in the count, and that was at the expense of fact coming down to 20% of energy. They showed significantly higher satiety and decreased hunger scores that were assessed as subjective appetite markers using visual analog scales. And then when they then switched, so you're seeing an effect on subjective appetite that was independent of energy intake, because energy was designed to maintain their body weight. And then when they switched into the free living ad-libitum phase, but maintaining their higher protein diet, they saw a spontaneous decrease in energy intake of around 500 calories per day. And that was observed within days of switching to an ad-libitum diet. And it was maintained at a decrease of about 440 calories at week 12 compared to baseline, 440 calories per day reduction on average. And this was largely independent of changes in resting metabolic rates. So I think the study is a really good example of the impact of that higher thresholds, relative to most diets, and certainly the habitual general population, tend to sit at around this 15% energy intake. So it is a substantial increase. It's a doubling of increase. Did this clearly had a very real world, so to speak, free living impact on ad-libitum free living energy intake, such that there was a reduction, a spontaneous reduction in energy intake. And that corresponded then to about a 5% body weight loss over the 12-week period of the intervention. So I think I tend to then, in terms of trying to reconcile, is there some sort of justification for what constitutes higher, look at outcome data. And I think this is typically where we can find this. And I can think of a meta analysis then from Munsoor and colleagues which looked at differences in weight loss and low carbohydrate versus low fat diets, but the interesting thing about that study was the massive discrepancy observed in total protein intake. On average, the low fat diets had about 17% of energy from protein and the low carb diets had around 30 to 35%. And in the overall analysis, this was one of these studies that people would point to and say, you see low carb diets lead to significantly greater weight loss. And low fat diets, but actually, we can't attribute that necessarily to the low carbohydrate component given wider research as much as we can to the high protein content. And the reason that the Wagle and colleagues study is a very good test of that is the fact that they maintained carbohydrate content because ordinarily, you typically see a replacement of protein with carbohydrate, at least in free living context, as people proportionately increase protein intake at the expense of carbohydrates. To maintain carbohydrate constant, you can see that independent effect of protein when it comes at the expense of decreasing dietary fat. So that 30% threshold seems to be at least in the terms of total daily protein intake, or some of these more consistent findings in terms of relevance, energy intake and weight loss outcome data tend to be observed, it seems. And so with all this said, if we return to the original question that we set out with the star, that people may be wondering about of i, is the, let's say, appetite suppressing effect of protein, or is the fullness promoting effect of protein overstated or not? Is this something that has an impact? Where should we fall down and are trying to give a relatively straightforward conclusion as much as we can to the question of how protein impacts, satiety, how it impacts association, and then how we put that in the context of some of these other things we've said, what is the best takeaway point people maybe could have? I think with these warmer isolating independent characteristics or components of diet, the question is the degree of independent effect, I think, and then interrelated effects. I think when we look at some of the body of controlled feeding studies, particularly the respiratory chamber studies, I think we can say that there is a degree of independent effect of protein, on satiety, possibly mediated by diet and juice thermogenesis, and at least there might be slightly stronger evidence for that than maybe the amino static regulation or that, like I said, when it comes to specific protein types, like way, you can perhaps make more of an argument for the amino static component. I think we could say that there is an independent effect of protein, but it's not entirely itself solely explanatory, that there are other factors in terms of sensory characteristics of food and in terms of eating characteristics, do influence eating rate, do influence, add a little bit of energy intake. I think that if we distill even further down to studies that have factored in and made sure that they have accounted for energy density that they've accounted for isochloric comparisons, if they're manipulating macronutrients and the volume component, the food volume component, then that independent effect is evident in some of these tighter controlled studies, but it's not always entirely independent of these other factors that are important, and we have an increasing body of literature in relation to factors like food texture and energy density itself, so all of these are likely converging to influence energy intake in a free living context. The lack of, or the somewhat sometimes more inconsistent correlation between protein and some of the post-ingestive gut-derived appetite hormones is not necessarily unique to protein, we see that across multiple diet comparisons and it may simply be the fact that the levels of some of these that are induced by diet, it just pale in comparison to the kind of levels of increases that you would see with pharmacological intervention, and you see that very clearly with GLP1, agonist drugs, and so it simply might be that no single isolated dietary variable kind of has that much of an isolated impact on gut-derived appetite hormones to exert the kind of effects on appetite suppression that you might see with pharmacological intervention, that's another factor, and of course there are multiple components because of the relationship between intramural satiation and post-meal satiety, early and late satiety, or as we described it earlier, the acute and tonic satiety, that those factors are themselves multifactorial processes that are influenced by multiple factors, so I think to a degree, there's always an exercise in futility in trying to isolate, does this single component influence all of these parameters in a way that congruently explains an outcome? I think with protein, what we have more consistently is the relationship with the outcome, with these higher protein interventions, they, in terms of real-world outcomes, do appear to have an influence on spontaneous energy intake, that is typically in the direction of reduced energy intake, that's certainly supported by studies, not just acute studies, but studies over several days in a controlled eating context where decreased subsequent energy intake is observed with higher protein contents of preceding meals, and weight loss outcomes tend to be favored by those higher threshold protein diets, at least compared to a lower threshold protein diet, so I think those outcomes are certainly consistent, do we necessarily need to see impacts on every single potential mediation factor, I don't think so when we have that kind of outcome data, there I think there are several plausible mechanisms through which dietary protein might act, like we said the relationship between diet and juice thermogenesis, the potential, and I do think this is relevant, the relationship between protein rich or high protein foods and the fact that they may be lower energy density foods or slightly less, they're not hyper-palatable foods by any degree, so perhaps the specific characteristics of some higher protein foods might again lend itself to, if you're feeding someone a plain chicken breast, it might be easier to stop eating that than it would be to stop eating a Snickers bar, so all of these factors I think are relevant, but I think the convergence of the outcome data certainly does appear to, at least if we were in the pursuit of kind of weight loss interventions, the needle I think evidentially certainly points towards more effectiveness to higher compared to lower protein intakes, and I do think that some of the, we can draw at least a degree of biological plausibility to some of the mechanisms that we've seen in the controlled interventions that relate to the relationship between diet and juice thermogenesis, impacts on some of the intermediate post-ingestive hormones like GLP1, but those might be weaker just simply because the magnitude of the increase that you get from diet is nowhere compared to what you'd get with pharmacological intervention. Right? Exactly. Perfect summary, I think, and with that we will wrap it up, I think, and hopefully for people listening, this has been somewhat useful to give you some answers to some of these questions and to dig into some of the interesting studies in this particular area of which will be linked in the description box where you're currently listening. We'll also reference some of those previous podcast episodes, so not only what we reference at the start of the episode, but also Alan and I, along with Dr. Neve Aspel, did a full episode around the topics of appetite and hunger, going much beyond protein and a range of different nutrients that would certainly be worth checking out. If you want to look at some of the stuff specifically related to eating right and food texture, there's an episode with Dr. Kiron Ford, episode 499, and indeed in reference to Professor Barbara Oles, interesting was on the podcast over 10 years ago now, unbelievable. So that is one of the early episodes, if you want to check that out, I think episode 54 of this podcast, if some of the stuff around energy density, volumetrics, and so on. So they will be linked up worth checking those out, if this is an area of interest for you. Dr. Alan Flanning will be back with another episode very soon, so please do rejoin us then. But in the meantime, thank you for listening in. Hopefully you've enjoyed this episode and we will talk to you soon. Thanks so much for listening in to today's episode. Before you go, I just wanted to remind you about Signetration Premium, our subscription for those of you podcast listeners who want to significantly deepen your understanding of nutrition science and become truly confident in your knowledge. So what's the idea of this subscription? 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