Fasted Training, Fat Oxidation, and Low-Carb, High-Fat Diets

Steve House:
For years, I’ve classically recommended fasted training. It’s in Training for the New Alpinism and it’s in Training for the Uphill Athlete. Those are half my books and I believed what we wrote at the time. The logic seemed sound. My own personal experiences with trying fasted training myself had been pretty mixed. The fact is, fasted training does raise fat oxidation. But, in most athletes, it simply costs more than it gives back. It pushes people into low energy availability, which tends to show up as bone stress injuries, disrupted hormones, more frequent illness, and training that feels harder while producing smaller gains. Women often feel these effects first and men are certainly not immune to them.

In November 2024, Alyssa Clark sat down with our registered dietitian, Alyssa Leib, to explain where we stand now on fat adaptation, fat fasted training, and low-carb, high-fat diets. We’re re-running that conversation now because it’s one of the most important corrections we have made as a team because plenty of athletes are out there still training fasted on advice that we used to give. Aerobic training is what builds your fat burning engine. Eating enough keeps you healthy for the long run. My name is Steve House and without further ado, here are Alyssa and Alyssa talking about fasted training.

Uphill Athlete:
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Alyssa Clark:
Welcome to the Uphill Athlete Podcast. Our mission is to elevate and inspire all mountain athletes through education and celebration. My name is Alyssa Clark, and I will be your host today. I am joined by our registered dietician, Alyssa Leib, and we are here to discuss a few topics that have sparked many conversations in the endurance world.

Uh, and I also feel like in the Uphill Athlete world, we, um, have some improvements in education and improvements in how we have researched and, and thought about this. So we wanna kind of give an update on our thoughts on fat ox- oxidation, fasted training, and low-carb, high-fat diets. So Alyssa, thank you for being on today to talk about this with me.

Alyssa Leib:
Yeah, of course. I’m stoked to be here. I know this is such an important topic in the mountain sports world, um, and one that comes up a lot. Um, and yeah, I know one that has been, has sort of a storied history with Uphill. Um, so yeah, I’m excited to dive into this today and just provide a little bit of clarity.

Alyssa Clark:
Awesome. Yeah, so just to touch a bit on Uphill Athlete’s previous stance with, in particular, f- uh, fasted training. So in the Uphill Athlete books, um, as well as articles prior to, I think within this year, um, we have been a supporter of fasted training. Um, we’ve had the stance that it can be beneficial to, uh, mountaineering, to, um, trail running, et cetera.

And with current research, with current findings, we have really found that it does not help athletes. It can actually be pretty detrimental and harmful to your training as well as just your overall health, which at the end of the day, we want you to be mountaineers from when you’re in your 20s to when you’re in your 70s, 80s, 90s, and beyond.

So we just wanna make sure we’re very clear that we no longer support fasted training and, um, have found many other ways to improve your aerobic fitness and mountain readiness that does not involve that type of training. So just to, to put our blanket statement out there. Um, but let’s get into our discussions today and start off with fat oxidation.

So L- Alyssa, can you break down what fat oxidation is and why there’s a lot of focus, we’ll call fuss too, on optimizing fat oxidation for ultra endurance?

Alyssa Leib:
Sure. So scientifically, fat oxidation is the process by which the body breaks down fat to use it for energy, in this context during exercise. Um, so what’s important to know is that your body is always burning both fat and carbs to fuel endurance exercise. Um, so fat and carbs in this case are called energy substrates, so the things that are burned for energy.

Um, but as the duration or the intensity of exercise changes, the proportion of energy that is taken from fat and carbs is what changes. So at a lower intensity, w- we actually see a higher proportion of energy coming from fat compared to carbs. And then on the other end of the spectrum, during like really high-intensity, um, exercise, it’s the exact opposite.

We’re predominantly burning carbs as fuel. Um, so when we think about ultra-endurance exercise, you know, we see we’re typically kind of chugging along at this relatively low intensity just for a really long time. So in that situation, our bodies are generally using more fat than carbs for energy. Um, so fat adaptation is really just the ability of the body to be more efficient at using fat as a fuel source.

Um, so you can see how it would be attractive to become more efficient at using fat for fuel, especially in this context of ultra-endurance sport, where you’re having to carry everything that you eat with you, and it is this lower intensity, so your body is going to need more fat to fuel that.

Alyssa Clark:
Excellent. Yeah, so, awesome. So within that though, can you explain to us a little bit about metabolic flexibility within kind of like aerobic, anaerobic? ‘Cause it tends to be thought of as very black and white, and it’s not really that cut and dry.

Alyssa Leib:
Yeah, exactly. So, you know, it’s not just, okay, we’re gonna go out for a 100-mile run and burn only fat for it. Um, because if we think about even, you know, whether it’s running or climbing or mountaineering, there’s always going to be something that happens where you need a quick burst of energy. So you are mountaineering and you posthole and you have to pull yourself out of it.

Um, you are on a run and you have to sprint really fast to get through a rockfall section. Um, you’re climbing and you have to pull through a crux. Like these are all things where, you know, you’re sort of breaking from that low intensity and you’re having to switch to this higher intensity movement that does require more carbs.

So the idea of metabolic flexibility is really this idea of how efficiently, how quickly can my body bounce back and forth between using primarily fat and using primarily carbs as that energy substrate. Um, so, uh, pause. I was like talking and then I like– So the, the ability to really switch back and forth between fat and carbs, that’s, that is what is known as metabolic, uh, flexibility or metabolic efficiency.

And ultimately that is really the goal, right? Um, you know, our goal isn’t just be super fat adapted so that we can, you know, run on fat on its own. Our goal is to, yes, be able to utilize fat efficiently as a fuel source, but also to be able to quickly bounce back and forth between carb and fat so that we can account for those moments when we do need that burst of energy.

Alyssa Clark:
Love it. And I mean, that’s so much of what keeps us safe, is the ability if, hey, we’ve gotta move through a rock fall section really quickly. Hey, someone’s injured, and you’ve gotta, A, have like, you know, the adrenaline to help them or get down the mountain quickly. Um, so that is such a huge part of why we train and why some high intensity is very useful, um, for, for mountain sports, is that safety component.

And then for trail running, it can be safety, but it can also be like, hey, you know that sprint at the end of the 100-mile sprint, we’ll say. Or my, my sprint at the end of Moab 240, which was a rousing 15-minute, uh, mile pace, which I, I thought I was, I thought I was sprinting. Um, so that’s super important to safety, you know, to perhaps getting the results that you want.

Um, so what are some of the common mistakes/beliefs around increasing fat oxidation rates that you see from athletes? ‘Cause I think that’s a pretty evergreen pool of content.

Alyssa Leib:
Absolutely. Um, I, there’s a f- there’s several of these and I’m not really sure where to begin. Um, I think one of them is this belief that in order to increase your body’s ability to oxidize fat, you have to eat a low-carb, high-fat diet, um, or fuel your activity on fats alone. Um, so I know we’re gonna touch on both of these things, uh, a little bit later on, but, you know, like I mentioned and like we just talked about, like, we all are going to have those moments where we need to engage that higher intensity energy system, and in order to do that, you need carbs.

Um, so if you are eating a diet that is low carb, high fat, you’re not getting in enough carbs to be able to do that, and that’s, that’s true both on a day-to-day basis during your diet as well as during exercise. Um, if you don’t have enough carbs on a day-to-day basis, your glycogen stores are gonna be really low.

Your muscles aren’t gonna have anything to work off of. Um, kind of similarly, during exercise, if you’re trying to fuel entirely off of fat, then you’re not going to have the carbs available for those higher bursts of energy. Um, and then you are also gonna be at a higher risk of injury. Um, and you’re also probably gonna feel really terrible.

Um, you know, fat is not very easily or efficiently digested and converted into fuel during exercise. Um, this is a little bit of a tangent, but I think it really helps to kind of bring home this point. If you eat a spoonful of sugar on an empty stomach There’s a lot of variation in ind- individual metabolism, but it’s gonna take about 15 minutes for that sugar to make its way into your muscles, where then it can be used for muscle contractions to power you up that hill or get you through that 15-minute mile at the end of Moab 240.

Um, if you eat a spoonful of olive oil, it’s gonna take a couple of hours. Like, it can be converted into an available energy source, but it takes a long time. Um, so that’s something to think about. Um, other kind of common mistakes or beliefs, um, I would say intermittent fasting is sort of a big one, so training during that intermittent fasting window.

Um, so if you’re training fasted and then still waiting a few hours to eat after training, basically you’re missing that really important recovery window there. Um, and then if intermittent fasting decreases your total caloric intake, then you’re also just missing an important opportunity to meet your overall energy needs.

So again, that kind of gets into the fasted training side of things, but it is something that, you know, I definitely hear a lot of athletes trying in order to promote fat adaptation. Um, and then I think the other kind of big one that I see is this idea of trying to go as long as possible during training without fueling.

Um, or kind of similarly feeling like, “Oh, well, I don’t need to fuel because it’s such low-intensity effort.” Um, and both of these strategies can have consequences related to poor recovery, increased injury risk, um, and they just really aren’t needed. Um, we’ll, we’ll talk more about this, I suspect, but, you know, just by simply engaging in aerobic training, you are just hands down, based on that alone, increasing your body’s ability to oxidize fat for energy.

Um, so by, you know, trying to forgo fueling, you’re putting yourself at risk, and the rewards aren’t really as necessary.

Alyssa Clark:
Absolutely. No, those are all really helpful, and I think, I mean, just from a, a training standpoint, we hear this often of, “Oh, well, I wasn’t hungry,” or, “You know what? I co- I could get through it. I could do four hours without fuel.” And we so often respond, “Just because you can doesn’t mean you should.” Because oftentimes with the work that we’re doing, we’re doing back-to-back four or five-hour days.

You know, if you’re doing a big trip, you’re not just doing one acute effort. You’re doing multiple efforts in a row. And so yes, that first four-hour weighted hike might feel fine, but the second and the third, you are gonna be super depleted and feeling really bad. So it’s not just about that one day, it’s about the whole picture.

And that’s one of the things we always say is like, “But your recovery time is increased significantly by not fueling during that effort.”

Alyssa Leib:
Yeah, absolutely. Um, you know, I, I always like to say, as an athlete, everything you’re eating is either fueling training, supporting recovery, or in cases like that where you’re doing back-to-back days, it’s doing both. And so if you are, you know, foregoing fuel during part of that, then not only are you putting that immediate training session at risk, um, whether it is just performance, feeling crappy, po- potential injury, but you are also putting your recovery at risk, which then puts the next day’s training session at risk as well.

So yeah, it really all builds.

Alyssa Clark:
Definitely. And this is the last piece. Oh man, I feel like we could, we could go on about this, but, um, you’re also missing an opportunity to test out fueling that could work for you on the mountain in the race. Um, I know that we’ve talked about this before where there, I mean, sometimes you just have a training session where you just don’t do a good job.

And before I would be like, “Oh, well, whatever.” And now I’m like, darn it, I missed an opportunity to, you know, be tweaking. Like, hey, do I want to eat, you know, five minutes sooner or try this type of fuel? And so when you skip those long sessions, like that is essentially a dress rehearsal for your performance, for your climb, et cetera.

And when you keep skipping those, you are adding a lot of variabilities into the actual day, days of the event where you haven’t tried that fuel, and so you don’t know if it works for you. And that’s, that’s a missed opportunity.

Alyssa Leib:
Yeah, absolutely. I mean, I always– I’m a big fan of tr- like, practicing your fueling strategy on shorter training sessions too. But at the end of the day, you know, you can go out and practice taking some gels for a 90-minute run. It’s not gonna be the same as if you’re out there for five hours. Your body is gonna do totally different things by hour four than it is at hour one.

And so being able to have that opportunity to practice and, you know, not only to practice what your strategy is going into it, but to sort of have that experience of, “Okay, this is how I feel in my body right now. Here’s how I’m gonna respond with food and see if I can change it.” So really being able to dial in those sort of plan B thinking on the spot moments, um, that, yeah, you’re, you’re missing out on that opportunity.

And in most training blocks, we really don’t get that many opportunities to practice, you know, being out there for five-plus hours and figure out our fueling.

Alyssa Clark:
Definitely. Well, that I think wraps up fat oxidation. So we wanna shift into fasted training, which I think is like the, the actual like implementation of what’s happening with fat oxidation. Um, so what is fasted training and what are the impacts of fasted training on performance?

Alyssa Leib:
Yeah. So fasted training is what it sounds like. It’s training in a fasted state. Um, so typically we see this as basically training before breakfast in the morning, so you haven’t eaten anything since dinner the night before. Um, so you’ve gone this pretty long window without eating anything. Um, and you know, what this does is when your body is in a fasted state versus the alternative is called a fed state, um, your hormones are doing all kinds of different things in your body just because of what they are having to do in response to not having had any fuel coming in.

Um, and there is some research showing that fasted training does increase fat oxidation rates. Um, so that really is sort of why this whole fasted training thing sort of gained steam, um, was, was that idea of, okay, well, we know that fat oxidation and fat adaptation is beneficial for performance, and now we know that, you know, we can increase that through fasted training, so let’s, let’s lean into that.

Um, so you know, and as, as we just discussed, like, that can be beneficial to performance. Um, so yeah, and I, you know, my, my understanding was sort of how all of this got started was when there isn’t a lot of research available on a topic, then as coaches, as dieticians, like we really are looking at, okay, well, what is that anecdotal evidence that’s coming from our athletes, um, and how can we kind of run with that?

And so that’s kind of my understanding of sort of how, like Uphill Athlete got involved with, with promoting fasted training was that there, there were some instances where an athlete was engaging in fasted training and saw improved performance. Um, and that sort of, that sort of general experience of seeing that has sort of led to more research being conducted, which, um, we’re gonna talk about.

But it isn’t, it isn’t as good as once originally thought.

Alyssa Clark:
Yes. I mean, I can certainly say marked, I don’t know, not safe, but like I, I think both of us probably at the time we were coming up into endurance sports were very much at the forefront of fasted training, um, as like a belief that it should, we should do it. Like, I can definitely think back even just a few years ago where I thought fasted training had benefits.

Um, I, except I could tell you I felt really bad during those sessions. Um, of like, “Wow, I don’t feel good.” Like, “Wow, I feel really lightheaded if I run more than 90 minutes without fuel and I haven’t eaten breakfast.” Shocker. Yeah, imagine that. Uh, so I think that, you know, this isn’t that, uh, far in the past that really kind of this push, I would say like mid-2000, fift- like 15-ish probably around there, is when it was really taking stride.

Yeah. Um, but we’ve learned it’s really not that good for you and especially I think women are, it’s even worse for. Um, but let’s get into what the negative effects of fasted training are on athletes.

Alyssa Leib:
Yeah. So, um, I guess I will start with this concept called low energy availability. So it’s abbreviated LEA. Um, and this is basically a fancy way of saying you’re not eating enough relative to your energy output. Um, and so by skipping your meal before training and any fuel that you would otherwise be taking in actually during that training session, what you’re really doing is you’re digging yourself into a pretty deep energy hole that’s gonna be hard to get out of.

Um, and again, this is especially true if you are training during a fasting window of intermittent fasting, um, so then you’re not eating a recovery meal afterwards. Um, and ultimately, you know, what this does and what a lot of research shows us is that intermittent fasting really does result in lower caloric intake.

So that’s why it’s commonly seen in the weight loss world as this, you know, big thing because if you have fewer hours to eat, then you have fewer opportunities to get in calories. And if your goal is weight loss, maybe that’s a good thing. But in the context of performance and, you know, athletic achievement and training, this, this is really doing the opposite of what we wanna do.

Um, so low energy availability, this is sort of at the center of relative energy deficiency in sport, um, which is abbreviated as RED-S, uh, used to be called the female athlete triad. Um, but RED-S is basically a syndrome, um, that can result in a lot of really detrimental health and performance outcomes. Um, so the best studied of these sort of health outcomes are low bone density, so resulting in bone stress injuries, even osteoporosis happening in like high school and college-aged runners, um, and athletes, which is pretty horrifying.

Um, the other kind of main research outcome is menstrual dysfunction. So this was sort of the original female athlete triad. Um, but basically what happened in Hmm. It was in pretty recent years, uh, I wanna say like 2017, was researchers started to realize that men are also susceptible to these negative impacts of low energy availability.

Um, so the female athlete triad got rebranded as RED-S, um, and sort of expanded to include a whole host of things: decreased immunity, so you’re getting sick more often, um, cardiovascular dysfunction, increased injury risk, GI issues, depression and anxiety, and so on. Um, you know, we see decreased testosterone levels, so men are impacted by the hormonal outcomes of this as well.

Um, on the performance side of things, obviously, if you are struggling with injuries or GI issues, it’s gonna be hard to be efficient in your training. Um, we also see research that this low energy availability can result in an increased rate of perceived exertion, so your training feels harder, a decreased training response, so even though you are following all of the recommendations that Alyssa has given you, you’re not getting better.

You’re not improving how you should. Um, and then we also see decreased strength, power, and endurance. Um, so we– these are sort of, you know, things that can result from fasted training in the context of you’re just not eating enough by doing it. Um, fasted training can also increase cortisol levels in the body, so this can impact recovery.

Um, gosh, there’s so many things to talk about here. Um,.

Alyssa Clark:
I know. Yeah, I mean, that was an amazing list. I, I just… It’s like, I think of it as training is so challenging to begin with, especially if you’re, you know, taking time away from family, if you’re, um, you know, really prioritizing that so that you can succeed in whatever your, your sport is. And this is, like, an automatic lowering of what you’re going to be able to accomplish.

It’s like you’re, you’re already decreasing… Like, you’re starting from negative almost, rather than like, “Hey, how well can I perform this?” It’s like how… It just feels like you’re surviving, um, the workouts versus really excelling. First of all, it’s not fun because it’s like, wow, this is why working out isn’t enjoyable.

But second, you’re not gaining all of the benefits that you can from that training session. And so it’s just, I mean, you’re, you’re, you’re kind of putting– You’re digging yourself a hole before you’re even starting the workout, and that’s a bummer if you’ve done so many other pieces to prioritize that session.

Alyssa Leib:
Yeah, and I think that’s so true when it comes to nutrition in general, right? Like how, how many times have you seen an athlete who, you know, they invest in a coach and they, you know, they invest their time and their energy, but nutrition sort of takes a backseat to all of the things. And yeah, it’s like you’re putting all of this energy and this time and these resources into doing this thing, but you’re not achieving what you want to because there are some things that you’re kind of leaving behind.

Um, and I think, yeah, fasted training is sort of like a fast track to that situation.

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Alyssa Clark:
So to, to kind of, I guess, play the devil’s advocate, ’cause I, I’m sure we’re gonna get pushback of people saying like, “Oh, well I did fasted training and it worked for me, and I’m seeing these results. And, um, yeah, that’s great that you have these sciences and the, the, you know, all of this evidence to say it’s not, but anecdotally it works for me.”

What do you say to those athletes that push back on this? ‘Cause it’s still very prevalent.

Alyssa Leib:
Yeah. Yeah. I think, you know, if we’re being honest, there are a lot of people who– Like nutrition in general can get this weird kind of cult following where people identify with whatever, whatever thing they’re doing nutritionally. And, you know, there are definitely folks like that who I’m thinking about like hardcore carnivore diet type people, um, or people who are out there, you know, running 200 milers with no fuel.

Like there are people who, you know, no amount of science is gonna change their mind. Um, but again, from a scientific perspective, you know, your anecdotal experience, it’s not invalid, but we don’t know what the alternative would have been. So maybe you feel great doing fasted training, but maybe if you had been fueling those workouts, maybe you would feel a lot better.

Or maybe you feel great for now, but you know, six months from now you’re gonna come back and say, “Hey, I was feeling awesome, and then I got a bone injury, and, you know, then I had to take time off from training.” So it’s– I think it’s, you know, the anecdote is great and it’s valid, but it’s also a point in time where we don’t actually know what is happening on either side of that, like if there was any alternative.

Alyssa Clark:
I also think, I mean, just from a performance standpoint, especially I can just speak coming from the ultra world, that we are seeing records drop like flies. I mean, we also just saw a marathon world record drop a couple, or literally yesterday, two days ago. And it– I believe much of that is because of our improved knowledge on how important carbohydrate intake is, and we’re seeing athletes put up really high, incred- like, I’m honestly just floored, impressed by the amount of carbohydrate intake that is fueling these performances.

So, and that’s not to say that, you know, we like, we always… Well, I mean, no. I mean, to be honest, like if we look at kind of the sharp end of, per se, what sports are doing, that’s going to benefit not just an elite, but also someone who is, is improving their performance. I, I hate middle-of-the-pack terms, but like, you know, w- no matter where you are in that race field, like, we have, I think, more, um, investment maybe in terms of like lab time, um, you know, changing up the nutrition for an elite athlete, but that resonates and that, that sharp, sharp, pointy end comes and infiltrates like all of the other athletes.

So I would say, “Hey, look at what the pros are doing.” They are not doing low carb. They are not doing fasted training. They are really, like, embracing high carbohydrate input, and that is paying off by so many records falling.

Alyssa Leib:
Yeah. Yeah. I saw somebody put together on Instagram just from, like, looking at social media posts and articles and just whatever they could find, they put together, I think it was after Western States, this list of, like, the top 10 male and female finishers and the grams of carbs that they were eating per hour during the race.

And I mean, the top 10, you know, for, for men it was all, like, 120 grams per hour. Like, it was really just pushing the physiological limits of what bodies can tolerate with training. Um, women it was, you know, a little bit lower, but I was still seeing, you know, some women breaking that 100 gram per hour mark, which is freaking awesome.

Like, and I, I don’t think, you know, just, like, as a mid-packer, like, I think most mid-packers probably don’t need to push that hard, um, and that high. But I agree. Like, I think this is definitely just a good indication that, like, fueling is working, and that is why course records are getting broken. Um, Leadville 100 this year, the men’s course record, which we thought was pretty much, like, the g- gonna be the record forever, like, that went down.

David Roche shared, I think he was taking in 130 grams of carbs per hour. Uh,.

Alyssa Clark:
It’s insane.

Alyssa Leib:
Yeah, so much fuel happening. Same with, uh, the women’s record at Leadville too, um, was almost, almost broken. Um, and yeah, Mary was, you know, aiming for, like, 110 grams per hour. Like, th- there’s something to this and, you know, like, there’s a reason that the people who are breaking these records and who are winning these major races are eating as much as they are, and that’s because it works.

Alyssa Clark:
Absolutely. Yeah, it’s exciting. I mean, I feel like this is just the beginning of what we’re going to see for the future, and, and yeah, like what, what, what are these numbers we’re gonna hit of grams of carbs per hour? See like 200, 300, like oh my

God god.

Alyssa Leib:
Someone might, come up with something. Yeah, I mean, my… I think based on our current research, I think this 120, 130 kinda seems to be a ceiling. But I mean, yeah, who knows? Somebody could, you know, discover some new transporter in the gut that can help with that, and it’s, it is really exciting just to see fuel being such a priority.

Alyssa Clark:
Absolutely. And it helps recovery. You don’t feel nearly as bad on the other end either.

Alyssa Leib:
Yeah, which I think is something that a lot of folks don’t realize. Um, you know, I think we all have in, in our minds this idea of like protein equals recovery. But oftentimes when I’m working with an athlete and they mention excessive soreness, we just add some extra carbs afterwards or during and, you know, often they’ll come back to me and be like, “Cool, like I added a banana to my recovery smoothie and like I feel great.”

And it’s like, yes, the power of carbs. I love this.

Alyssa Clark:
That’s awesome, and an easy fix, and carbs taste good. Yay, carbs. I think the, uh, title of this should just be, “Yay Carbs!”

Alyssa Leib:
I mean, that should be the title of everything. I support that.

Alyssa Clark:
True. I think it should be our new merch. Um, so the last, the last bit that we’re gonna talk about, which, I mean, this is kind of, we’ve danced on this a lot, but the low carb, high fat diet.

So can you explain what the low carb, high fat diet is and, uh, why someone might choose to follow this dietary approach, and kind of what the research says about it?

Alyssa Leib:
Yeah. So low carb, high fat diet, like it sounds, is one that is low in carbs and high in fat. Um, there isn’t just one definition of what this looks like, um, and I think a lot of people will kind of throw the term around without really, like, thinking a whole lot about it. Um, but a pretty common example is the keto diet.

So this one is actually, like, fairly well defined. Um, you’re basically eating fewer than 20 to 50 grams of carbs per day, um, which to give you some concept of what that is, a banana has about 30 grams of carbs. So that’s the entire carb intake for a day for someone following a keto diet. Um, and so then you have, you know, 75, 80% of your…

Alyssa just held up a bunch of bananas that are just sitting on the shelf.

Alyssa Clark:
Yeah, this, the, the two bananas I have in my hand would be, uh, too much, which is w- yeah, sad. I, I would be very sad eating that many.

Alyssa Leib:
Agreed. Agreed. Um, but yeah, I think the, the important piece of this low carb, high fat diet is it’s not just cutting carbs, right? So you’ll see diets out there that, you know, are low carb, but they actually really prioritize protein versus in this case, it really is emphasizing this really high fat intake.

Um, so with the keto diet, the goal is to enter a state of ketosis, which is basically where, um, your body produces ketones from those fatty acids, and then ketones become that main energy source for your body because you don’t have any carbs coming in. Um, so as far as what research says, a low– There’s– This is– There’s a lot here.

Um, I’ll try to kind of summarize it as best as I can, but a low carb, high fat diet can increase fat oxidation during both exercise and rest. So drawing back to what we talked about at the beginning, this means that more fat is going to be used as fuel, and this makes sense if you consider that that is the only energy substrate that is coming in.

Um, if you’re really just not taking in very much carb, then the body has to do something to make fuel. Um, but what we see is that the research doesn’t necessarily support that this increase in fat oxidation actually results in improved per-performance. Um, so we also see that a low carb, high fat diet can decrease the ability to use carbs as a fuel source.

So what that means is that in those moments where you have to recruit that burst of energy, like stepping out of the post hole, pulling through a crux, doing your sprint finish, um, it’s gonna be really hard if your body is so used to using fat as fuel that even if you do give it carbs, it’s gonna be that much harder to actually use.

Um, one really interesting study found that there might be a performance benefit, but it was a really small study, and there was actually– So they found– They, they measured performance based on improvements in VO2 max. Um, and basically what they found was that this improvement in VO2 max might have actually resulted from weight loss and not from, like, the diet itself.

Um, and like we just talked about, you know, weight loss in the context of training and performance is not necessarily beneficial. Um, another finding that they found in that same study too was that the low carb, high fat diet may have worsened hypoxemia. So that has obvious effects, especially thinking about anybody who is recreating at altitude, um, mountaineering or, you know, like mountain running.

Um, so potential benefits that, that we don’t really see played out. Um, and then, uh, kind of the other thing that I wanna mention with this is that eating this low-carb diet can put you in a state of low carbohydrate availability. So we talked about low energy availability earlier. Um, low carb availability is this sort of new thing that is starting to be researched where we’re actually finding that even if you are eating enough total calories, if you’re not getting enough carbs to support your activity level, then you can face a lot of the same outcomes that we see with RED-S.

So like bone injuries have been one of the big ones that have been studied from this. Um, and what’s really important about this is the effects of low carb availability, they’re not a long-term thing. So there was a study that came out in, I think somewhere in Scandinavia, um, pretty recently, and they found that, um, having a low carb availability just for six days can, um, result in some negative impacts on bone health and immunity.

So it’s not, it’s not necessarily a long-term thing. Um, it’s just a few days of not getting enough carbs can have negative impacts. Um, and I think one other piece I want– I have a lot to say about this. Um,.

Alyssa Clark:
No, I love it. Keep going.

Alyssa Leib:
One other thing to say about the, the low carb, high fat diet is just that adherence is really, really hard. Um, so even if we were in a case where, you know, there was all of this resounding research that said, “Hey, low carb, high fat diet leads to improved performance, no downsides,” um, which is not the case.

But if, if it were, um, it’s not easy to follow, like what we just mentioned, right? Like that is the number of carbs in one banana. Um, you know, and there are– you’re always gonna find some people who are able to kind of pull that off and stick with it in the long term. But for the most part, that’s not gonna happen, and that’s what we see in research.

Um, that’s what I hear kind of anecdotally, um, you know, from people I work with who, who have said, “Hey, I used to do keto.” And I, and I’ll ask them, “Well, you know, what, what happened? Why aren’t you doing keto anymore?” And that’s kind of one of the most common responses I get is like, “It was really hard. Like, I missed bread, I missed pasta.

I couldn’t eat out. I had no flexibility.” So something to keep in mind here as well.

Alyssa Clark:
Also, I mean, I know a lot of people who have done keto, and it really affects your mood and, and how you feel and how you interact with others. Um, full disclosure, had a coach tell me I should try some, uh, low carb, high fat. It was to simulate further along in a race. So I did a hard, hard/medium effort and then didn’t eat carbs or ate as low carb as possible and then went into a five-hour long run the next day.

Did that a couple of times and holy cow. And it was a really bad idea. I’m embarrassed to admit I even entertained the idea. But sometimes when you have someone telling you, uh, “Oh, this will help you improve, improve your performance,” you listen even when you know better. Uh, and I was miserable to be around.

I mean, I w- knew I was miserable. I was miserable to my husband. I didn’t enjoy it. Honestly, the fat, it’s so much fat and just, like, eating things that you just feel heavy and not good. Um, it was awful, and that was literally, like, a probably less than 24-hour period where I was trying to do that. And it was miserable, and I guarantee you my carbs were still higher than they should’ve been.

So I will just say from experience and also s- observing friends who have done keto diets, et cetera, you are not yourself, and it can have really detrimental effects, I think, on relationships, where you’re just kind of a miserable human being too.

Alyssa Leib:
I’m sorry that happened to you, but I, yeah, I’m glad that you realized quickly on that that wasn’t, wasn’t the move. Um, yeah, I mean, and I– you definitely see reports of like keto brain, um, and, you know, carbs are the muscles but also the brain’s preferred source of energy, and so ketones can be used to fuel the brain, but it’s not, it’s not the most efficient way to do it.

So it is going to have some, some detrimental impacts as far as that’s concerned.

Alyssa Clark:
Yes. It’s, uh, yeah. And guess what? It doesn’t, didn’t do anything for my performance. It just made everything really hard. And our training’s already very hard. We don’t need to make it even harder by doing that. We also don’t need to punish our friends and family for our training.

Alyssa Leib:
Very thoughtful of you.

Alyssa Clark:
So, so I think we’ve discussed quite a bit of, uh, why this is not the best idea of a diet. Um, let’s wrap though with practical, practical recommendations for nutrition kind of around these three topics. So the fat oxidation, the fasted training, low carb, high fat. Um, what would you recommend instead that athletes focus on?

Alyssa Leib:
Yeah. So I mean, my first recommendation when it comes to fat adaptation, um, is actually not a nutrition recommendation, but it is just train consistently. Um, do a lot of long, low-intensity aerobic exercise. Um, and the reason why is because during that type of activity, you– it is almost impossible to eat enough calories during that activity to actually make up for everything you’re burning.

And so what happens is then your body is forced to draw on your fat stores, um, as an energy source anyway. And in doing that, it makes your body more efficient at oxidizing fat for energy, because in that situation, there’s really no alternative. Um, you know, if we think about it, and I, I’ve done this math before, and of course I don’t remember the numbers off the top of my head, but, um, if you are going out for, you know, a five-hour long zone two run, um, let’s just estimate that you are burning, you know, let’s say 600 calories per hour.

Um, our bodies can really only tolerate, you know, 250 to 400, maybe 500 calories per hour. So even on the high end of that, you’re still running 100-calorie deficit per hour that has to be fueled from something. And if you’re out for long enough, you’re burning through your body’s glycogen stores, um, so you don’t have that available anymore, and it really does become your body fat as sort of the remaining fuel source.

Um, so just by engaging in a lot of longer training sessions like that, you are automatically training your body to be more efficient at oxidizing fat, um, and doing what it needs to do with a much, much lower risk of injury, et cetera, um, than doing something like fasted training or a low-carb, high-fat diet.

Um, so the best way to train consistently and support your body through training and recovery, um, is Through eating and treating your body right. Um, so what that means is fueling your training by eating carbs. That means eating adequate carbs on a day-to-day basis, as well as fueling with carbs during exercise, so really trying to push that limit.

Um, again, like I said earlier, do I think everybody needs to be shooting for 130 grams per hour? Probably not. Um, but you know, most people with a little bit of training can get their body up to 80, 90 grams per hour. Um, and so if you are, you know, training for something big, you’re regularly engaging in long endurance activity, it really behooves you to find that kind of limit for you and, you know, try to get that intake as high as possible so that you can support better recovery and better training so that you can then become more efficient at oxidizing fat as fuel.

Um, so as part of all of this, it means eating enough to actually meet your needs, um, you know, not trying to train in a calorie deficit. Um, this is a conversation I have all the time, but, you know, nutrition for weight loss and nutrition for performance are totally contradictory things. Um, and doing both of them at once is not going to result in positive outcomes, you know, probably for either of those things.

Um, you know, in addition to all of this nutrition stuff, it’s how can you sleep enough? How can you manage stress? How can you prioritize rest and recovery? All of those good things that can help you, you know, do it today and then get up tomorrow and do it again. Um, and ultimately, you know, none of this stuff is sexy, and, you know, I think that is why low-carb, high-fat diets and fasted training and intermittent fasting get so much attention, because they, they’re sexy.

I mean, they’re, you know, it’s this very concrete thing that you can go do and put work into. Um, and that does make it really appealing, but ultimately, you know, the basics, the basics are really unsexy, and they are things that if you’re not doing it, if you’re trying to only do the hacks, like you’re, you’re not gonna have a good time.

You’re not gonna get where you want to be. You’re not gonna improve as an athlete. You’re more likely to get injured. You know, the, the hack is that there is no hack, and you really have to focus on all these basics.

Alyssa Clark:
That could summarize pretty much all of endurance training in a nutshell is there’s not, there’s not a hack to building an endurance base. But the beautiful thing is if you build it right and you support it, you have it for a very long time, and you can do so many things off of that base, but it takes a lot of work to build that base correctly and to have a solid foundation.

Alyssa Leib:
Yeah. No, and you’re, you’re so right. I mean, that’s, you know, for whether it’s health or performance, whatever you care about, it’s just like how can you do the basics? And then once you have the basics, you can think about adding in some of those sexy things to gain that edge. But you have to have the basics before that edge can actually be found.

Alyssa Clark:
Absolutely. Yeah, you can’t skip to step 34 when you’re still doing one, two, and three right, or trying to.

Alyssa Leib:
Yeah, absolutely. It’s not gonna be sustainable. It’s not going to have the impacts that, that you want it to have.

Alyssa Clark:
Absolutely. So any last pieces of advice you’d like to touch on? Also, if people want to get into contact with you about nutrition coaching, um, yeah, love to hear, hear from you.

Alyssa Leib:
Yeah. I think, you know, one more thing, and I was thinking about this as we were talking about, um, fasted training. Um, you know, and I realized that most of the downsides that I gave were sort of the result of not eating enough. And so I guess just to get ahead of, you know, somebody saying, “Okay, well, what if you do fasted training and then you do make up for it afterwards?”

Um, I just wanted to mention this idea called within-day energy deficit, um, which is basically this idea, uh, where you, you have sort of your overall energy balance, um, over the course of, like, 24 hours. But researchers have actually gone in and started looking at, you know, one hour at a time and seeing, okay, are you in an energy surplus or a deficit here?

So classic example of this is you do a run, a 60-hour, or 60-hour, oh my gosh, a 60-minute run fasted. During that time, your body is using a lot of energy, but you’re not taking anything in. So during that hour, you are in an energy deficit. Um, and what new research has started to find is that the more hours you spend in a deficit, that’s associated with negative health and performance outcomes.

So even if you do eat enough after that fasted training session, um, in order to meet your total needs for the day, by definition, you have spent some time during that day in an energy deficit, and that can still result in those negative outcomes that we talked about. Um, so I just wanted to make sure I mentioned that because, you know, fasted training, primarily we do see it detrimentally because it results in lower energy intake overall, but it doesn’t necessarily have to, and there’s still some downsides to it.

Alyssa Clark:
Gotcha. No, that’s, that’s useful to put out there. Um, awesome. And then if people wanna work with you as a nutritionist.

Alyssa Leib:
Yeah. Um, so kind of two ways to do it. Um, nutrition consultations, kind of like a one-time get your very specific or very general questions answered. Um, and then otherwise nutrition coaching, so ongoing monthly type things. Um, I should know this URL off the top of my head, but if you go to our website…

Alyssa Clark:
Yeah, or put it, put it in the show notes.

Alyssa Leib:
Exactly. Um, yeah. And yeah, I’m also, like, open to feedback or just, like, interesting comments, so yeah, feel free to reach out with anything.

Alyssa Clark:
Awesome. And we have, I believe all three articles are now published on the website where you can see, um, or read if you would like more about our, uh, research and, um, just kind of conclusions that we’ve drawn on these three topics. We do have those published on the website. I will link them in the show notes as well.

So thank you, Alyssa, for coming on. This was awesome.

Alyssa Leib:
Yeah, of course. Always happy to talk about fun nutrition topics, and I, I mean, I, I really think this is one of the more important things to come out of the nutrition space in the last couple of years.

Alyssa Clark:
Could not agree more. So thank you for listening to the Uphill Athlete Podcast. If you can rate, review, and subscribe on your favorite podcast platform, that really helps us to help more athletes. It’s not just one, but a community. We are Uphill Athlete.

Uphill Athlete:
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