Podcast Episode 88 Transcript – Low Carb Diets: Effects On Strength Training


Hello, and welcome to The Ideal Nutrition Podcast. This is episode 88, and my name is Aidan Muir, and I am here with my co-host, Leah Higl, and today, we’re going to be talking about low-carb diets and how they affect resistance training and muscle growth. I think it’s a niche area, but it’s something that is particularly relevant for anybody who just prefers low-carb diet, and they also like lifting, and they’re like, “Can I gain muscle? Is this suboptimal? How much does it matter?” That kind of thing. And it’s a tricky space to interpret, because on the one hand, you have a lot of higher carb advocates, particularly in the body building space and stuff like that, trying to, or highlighting that they think that a higher carb approach is pretty much necessary for this, and at the other end of the spectrum, you also have very strong low-carb advocates who think low-carb is the best thing for everything. So, what we are going to try and do is literally just look at what the research says, interpret that kind of stuff, and then let you know our thoughts.


So, let’s start with defining what low-carb actually is. So, there’s pretty loose definitions floating around. In some research, we see that researchers will classify low-carb as anything less than 45% of calories coming from carbs. I’d say that’s still a pretty normal, or even relatively high carb diet. So, that’s not really what we’re talking about in this particular context. What we’re going to be talking about more so refers to less than 20% of calories coming from carbs, as a low-carb diet. We will also be referencing quite a bit of research on keto diets that is going to be very low-carb, but I still think it applies to this kind of general concept, and keto diets specifically are often thought of as less than 50 grams of carbs per day, but that can change depending on things like size, activity, and other factors. So, it could be significantly higher than that number, or it could be significantly lower, but overall, it’s a very low-carb approach, when we’re talking keto.


We’re going to start with why low-carb diets might be suboptimal for muscle growth, because there is obviously arguments that people can make. So, we’re just going to kind of look at those, address them, unpack them, everything like that. So, the first thing is assuming that calories and protein intake are equal, because I think that’s important for this discussion as a starting point, there are still a few things that could theoretically be issues. The first one is that insulin is anabolic, and insulin will be lower on a low-carb diet, all other factors being equal. We clearly see when insulin is being used as a performance enhancing drug, when people are injecting insulin, they gain more muscle, and it makes sense because insulin is a storage hormone and everything like that, and we see when people are using it via injection, it is quite effective. I’m obviously not encouraging that, but it’s just an obvious thing that can’t be overlooked.

But does that mean we can do much from a nutrition perspective, to the point that it actually results in meaningful differences in muscle growth? That’s a much, much different discussion, and I’m personally of the opinion that it doesn’t matter. I have taken a long time to form that quite strong opinion, but when I first got into the nutrition world, I saw a lot of people talking about how you should have carbs post-workout alongside protein to spike insulin, to help increase muscle protein synthesis. But when total protein intake is high enough, post-workout, throughout the day, however you want to look at it, adding further carbs doesn’t increase muscle protein synthesis any higher. It seems like anything we do nutritionally to try and spike insulin or anything like that, the actual change in the nutrition outweighs the benefit you’re necessarily getting from the insulin.

There’s a really good blog post from Nutrition Tactics on this, which I’ve added into the show notes. So, if you do really want a thorough kind of read of this, have a look at that, but the basic premise of it is that we can do stuff with supraphysiological levels of insulin when we’re injecting it does seem to matter, but any change we make through diet just doesn’t seem to matter that much.


The second issue is that low-carb diets will likely involve lower glycogen stores, glycogen being the storage form of carbs in the body, than when we’re looking at optimized kind of higher carb diets. So, lifting weights does burn through less glycogen than, let’s say, endurance type sports, but it still might be relevant. So, the mostly commonly cited study involved six sets of leg extensions to failure, which resulted in a 38% decrease in glycogen of the quads. So, something that is often overlooked in that study is that there is more of a depletion in the Type 2 fibers than there was in the Type 1 fibers. The 38% is just the overall. So, this does mean that there were fibers that were depleted by over 50%, in terms of their glycogen content.

So, this raises quite a few questions. So, just going through them, it’s like what if you are doing more than six sets of a muscle group in a session? Are you burning through more of these glycogen stores, and therefore, does it matter more? What if you are coming from a place of suboptimal glycogen stores? So, coming in with suboptimal glycogen stores, are you going to be burning through a larger than percentage of those? We know performance decreases as you get lower in glycogen, but then again, how much does this affect performance in your individual case? And then the important question, specifically talking about muscle growth, although this likely means a low-carb diet will result in slightly worsened performance at the end of sessions, due to early onset fatigue and whatnot from lower glycogen availability, does that actually matter when it comes to muscle growth? Is there a translation to less muscle growth? Especially if you’re taking sets to a similar proximity to failure. It really might not even matter, specifically in the outcome of muscle growth.


Yeah. That last portion is something I’ve thought about a lot, because it really comes into what is the driver of muscle growth? Often, we oversimplified into things like progressive overload and everything like that, and if you thought progressive overload was the be all and end all, even that creates other issues because it’s like, okay, well, let’s have more carbs, more glycogen, and makes it easier to progressively overload. But even in that scenario cycle, if you just didn’t have good glycogen levels all the time, you’d still be progressively overloading in comparison to yourself, and we know it’s even more complex than that. We know that proximity to failure matters, we know that volume matters.

We know that there’s so many things that matter, and even on a lower carb diet, you still have some levels of glycogen, and it’s like we’re questioning whether this even matters that much, and then it probably only matters for the later sets, if you’re training in a certain way where you’re doing a specific amount of volume and et cetera, et cetera. So, there’s a lot of speculative lips, which is why we’re probably just going to look at the research, more so than just being like, “Well, let’s just look at the mechanisms.” But that glycogen thing is a large factor why people encourage higher carb diets, particularly in a calorie surplus for muscle growth.

Another aspect that is relevant is whether ketosis changes anything about this. So, the common argument is that going into ketosis means we start using ketones as fuel, which will improve performance, in comparison to a low-carb diet, where you’re not in ketosis, because it’s like on a low-carb diet without ketosis, you’re getting this glycogen issue, but you’re not getting the benefits of using ketones as fuel to the same degree. We clearly see people drop in performance in the gym, when people transition to ketosis. That’s something that you can just observationally look around, ask people, almost everyone who’s like, “Oh yeah, I switched to keto,” they will say that their performance in the gym dropped initially, and then it usually picks back up after that transition phase. So, I would arguably say that going into ketosis would probably result in slightly better performance than doing a low-carb diet that’s kind of in between the two where it’s like, hey, you don’t have enough carbs to optimize glycogen, but you’re also not in ketosis. But once again, that’s speculation as well, which is why I think we should start looking at the research.


Yeah, and looking at the research, we’re going to break this up into four different parts, because we’ve quite got quite a bit to discuss. Before we get into the individual studies, I just want to address two logistical challenges that we often see in this type of research. The first one is that in research and in the real world, the vast majority of people attempting low-carb diets do end up in a calorie deficit, which is why I guess low-carb is touted as being good for weight loss, because just by the fact of going low-carb, you often, just by chance, end up in a calorie deficit. So, that can be a confounding factor here. We know muscle growth occurs a lot more easily in a surplus. So, the fact that we have these people potentially in a deficit is just going to be a variable that may not be tightly controlled.

The second logistical challenge is that in the real world, in a lot of studies as well, protein intake often increases when people switch to a low-carb diet. So, we don’t see this quite as much as people accidentally creating calorie deficits, but it’s still something that can influence muscle growth outcomes, and is a factor in these types of studies.


So, there are quite a few studies we’ve talked about, or that we want to talk about. There’s not as much research as you would expect on this topic, but I’ve just picked a few that we will talk through. So, I’ve picked about four that we’ll talk through, but there’s not that much more than four to actually talk about.

But the first one I’ve picked is one that just kind of highlights some of those challenges, right? Because I think it’s important to see exactly what we’re talking about. It involved a group of women who did resistance training two times a week for 10 weeks. Obviously, you could criticize that being like, “Well, I train four times a week. Is this relevant for me?” But ignore that for now. Two times a week, resistance training for 10 weeks. One group did a keto diet, and the other group did a quote, unquote, Western diet with higher carbs, et cetera. The keto diet group had 22% of calories coming from protein, and the higher carb diet had 17% of calories coming from protein.

That’s already a red flag, because if the keto diet went on to outperform, we couldn’t tell if it was because of ketosis, low carbs, or the protein difference, because that protein difference really does mean that the higher carb group very much had a suboptimal protein intake for muscle growth.But going into the outcomes, the keto group lost five kilos, and had no change in lean mass. So, we’re talking about muscle growth, and there was no change in lean mass, but they did lose five kilos, no change in lean mass, that’s pretty impressive. And the higher carb group gained 800 grams of total body weight, and 1.6 kilos of lean mass.

A lot of the studies that we won’t be talking about, but a lot of studies have similar findings where it’s like the higher carb group gained muscle, that’s the metric we care about. The keto group, or the lower carb group, had no, or very minimal change in lean mass, but lost a lot of body fat in the process. If we are talking about getting stronger and building muscle, we do care about the outcome of building muscle, but the logistical challenge, in addition to difference in protein, is low-carb groups often end up in a decent size deficit, which kind of makes them a little bit irrelevant, because we want calories to be matched. In a perfect world, we would want studies to have the same calories and protein intake, so we can compare it for the people who have good knowledge of nutrition, know what they’re doing, and everything like that, but just prefer a lower carb diet.


Yeah. Looking at another study, so there was a study in 2021 that included 19 off-season natural bodybuilders. Already slightly more relevant, I suppose, to the group that is usually interested in this topic, but they were randomly assigned to either a keto diet, or a standard Western diet, being slightly higher carb, for two months. So, body fat significantly decreased in the keto group, while lean mass only increased in the Western group, and then max strengths increased similar in both groups. So, both diets were at least designed to be the same calories and similar protein, and bodybuilders are pretty well known for their compliance to nutrition plans, given that that’s their thing, but even in this case, they appeared to end up in a deficit, regardless when they were on the keto diet.

So, the author speculated that this could be because of an increase in energy expenditure, due to a spontaneous increase in activity on the keto diet, but we would speculate it’s probably because of the lower calorie intake, but there is no way to know for sure. Either way, it continues the theme of a lack of muscle growth, which is seen in almost all of the low-carb studies, generally.


Yeah, and backing up your thoughts on, as the authors said, they speculated an increase in energy expenditure, the reason why I also don’t think that that was probably the explanation, is because we don’t see that in other areas.

There’s a lot of low-carb advocates who are looking for a metabolic advantage of a lower carb diet, because they’re, I’m not going to say trying to disprove the calories in, calories out model, but they’re trying to say you could eat the same or more calories on a low-carb diet, and lose more body fat, and if they’re then fitting that into the calories in, calories out model, they have to be like, “Okay, well, calorie expenditure has to be increasing,” and I would speculate that the authors are partly thinking through that lens as well. But because there’s proponents of the calories in, calories out model who have also done that research, just to confirm-

… and just be like on both sides, both groups have done this research to try and… On one side people, are trying to confirm their belief that it increased energy expenditure. On the other side, people are trying to disprove it, and both sides, their research is coming back showing that it just doesn’t matter.

It just doesn’t matter. Yeah. Which is part of why I’m like, I do think these people consumed fewer calories.

Yeah. So, I lied earlier when I said I was going to talk about three studies. Sorry, four studies. We are going to be talking about three. I was just looking at my notes, and we had four parts, but-

So, we’re going to talk about a third study, and this is the most promising one, because I really wanted to present a very balanced view of this topic, and I was like, if I’m going to go through logistical challenges, I also want to talk about something that was about as promising as we can get, right? And the most promising keto study I’m aware of involved 25 college men. I don’t know how familiar people listening to this podcast will be with a guy named Dr. Peter Attia.

He’s somebody who’s been on like Joe Rogan, Tim Ferriss, in a bunch of places. He does keto himself, but he’s also about as unbiased as you can be, while being an expert in that field. So obviously, he has a bias towards low-carb diets and everything like that, but he also has conversations with smart people on all areas of nutrition. And I don’t know, he was one of the authors on this paper, which I found interesting. He wasn’t the person kind of driving the study, but he was the person who was on this paper, and I think his influence made it a bit of a better designed study, because it addressed a lot of the flaws we’ve kind of been talking about.

So, 25 college aged men, they compared a keto diet to a quote, unquote, Western diet with the same calories and protein for 10 weeks. That’s the key variable that we care about, the same calories and protein, followed by adding carbs in the 11th week to match with some variables. That solves another problem that we haven’t even brought up yet, which I’ll get to. At the end of the 10th week, both groups had increased lean mass by 2.4% and 4.4%, respectively. If you just stopped the study there, you would be like, “Oh, okay, well, high carb diets are better,” right?

As much as we can tell from 25 people, it’s a small sample size, still, but if you just stopped it there, you’d be like, “Okay, that tells us what we want to know.” But if you factor in the 11th week where they added carbs to the keto group, there ended up being a 4.8% increase in lean mass from the baseline in the keto group. Why does this matter? It’s because adding carbs increases lean mass in the forms of water and glycogen. It’s just like this concept of whenever somebody goes keto, they drop weight really quickly due to the losses in water weight and carbs, or glycogen, and there’s a quick loss at the start. Vice versa, when you add the carbs back in, this comes back. Water and carbs, or water and glycogen, sorry, are lean mass. These things matter.

So, once it’s accounted for, it was pretty much the same amount of muscle mass that they had gained. Both groups also dropped a similar amount of fat mass, with the keto group dropping 2.2 kilos of fat, and the Western diet losing 1.5. The reason why I want to again, care about that is because it’s kind of telling us that they were in the same size calorie surplus or deficit, if that makes sense, because if one group had lost way more body fat, then you’d be like, “Oh, they were in a massive deficit in comparison to the other one.” So, they were truly on a similar amount of calories for their needs, a similar amount of protein, and they also got similar outcomes. And if you have been paying really close attention, you’ll notice that no other study we have referenced involved the low-carb diet group actually gaining muscle, but I think that that’s just because they were never in a surplus.

And for that kind of context I was talking about being like, imagine somebody did a low-carb diet where they’re trying to lose weight for ages, and they got to their goal weight or whatever and now they’re like, “Oh, I want to go into a surplus. This has worked for me. I want to continue doing that,” it probably wouldn’t be nearly as hard for that person to go into a surplus on a low-carb diet, as somebody who had just been living life and then randomly switched to a low-carb diet. So, this study I think is relevant for that person. It’s promising, but it’s still a small sample size, and I wouldn’t read too much into it beyond just being like, okay, it’s clearly not the end of the world if you’re trying to build muscle on a low-carb diet, because we’ve seen quite a few examples of people doing it there.


Yeah, and I think in terms of differentiating between lean mass and muscle mass is super important when looking at this research, like you said. We know that they’re not looking at muscle mass, because you look at lean mass when you look at DEXA scans and whatnot, and glycogen depletion is going to play a huge role. So, I think that’s super important as well.

So, kind of take home practical thoughts. So, a low-carb approach seems to be fine for building muscle, assuming you’re able to consume sufficient calories, so that’s a big one. When people do go keto or a low-carb approach, it’s often they find it harder to be in a surplus, or just stay out of a deficit and maintenance. So, you need to be able to consume sufficient calories. Even still, is it optimal to be low-carb when you’re trying to build muscle? We don’t really have enough of a sample size of people doing the research to have a good comparison, but considering even just theory behind kind of performance, and glycogen, and carb availability, I think that it would get to a point where it’s probably not optimal to have a super low-carb intake, but again, the research here is we just don’t have enough to say. So yeah, we’d speculate that a slightly suboptimal amount from the glycogen aspect would affect things, but it wouldn’t be probably too dissimilar. It is likely that logistical challenges of consuming enough calories, for most people, would be the biggest factor at play here that makes the difference.


This has been episode 88 of The Ideal Nutrition Podcast, and as always, if you could please leave a rating and review, that would be greatly appreciated.