Nutritioninterview

DISCOVER THE ULTIMATE FAT-BURNING FOODS THAT 99% MISS | MARK SISSON

with Mark Sisson

35:11·5 May 2023

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TOPICS

Mark Sisson reveals how to unlock your body's fat-burning potential through metabolic flexibility—the ability to efficiently derive energy from stored fat instead of relying solely on carbohydrates. Whether you're an endurance athlete or just trying to perform better and recover faster, Mark breaks down the science of how to train your body to become a fat-burning machine, and explains why the conventional high-carb approach might be holding you back.

Key Takeaways

Metabolic flexibility—training your body to burn fat as efficiently as carbs—allows endurance athletes to reduce dependence on constant carbohydrate intake, recover faster, and maintain performance with less digestive stress.
A 6-8 week 'keto reset' during the off-season, combined with low-intensity training (below aerobic threshold), is the most efficient way to upregulate fat-burning genes and build mitochondrial capacity.
You don't need to deplete glycogen stores to zero or carbo-load aggressively; your body regenerates glycogen in 2-3 days even without excessive carbohydrate intake, and your brain protects you from true depletion through the central governor theory.
Rather than adopting an all-or-nothing approach, age-group and amateur athletes can gradually build metabolic flexibility through intermittent fasting, strategic low-carb days, and reserving higher carbohydrate intake for high-intensity efforts.
Elite endurance training often prioritizes short-term performance over long-term health; sustainable training means cutting overall volume, spending more time in the gym building power, and reducing reliance on excessive daily carbohydrate intake.

Expert Quotes

"If I was someone who was defending my right to eat grains in the face of all this evidence, how many tens of millions of other people must be out there who are wondering what is the cause of their males, what is the cause of their gut issues, what is the cause of their arthritic pain? That's really what prompted me to spread this message to the world."

"You can't go hard every day. All you're doing is practicing to hurt but not getting better, not improving this metabolic conditioning."

"The goal isn't the performance—the goal is performance and sustainability, slash long-term health. Whereas the very best cyclists and runners in the world, their goal is just performance."

KEY QUOTES

“if you're going to be doing a long slog up a hill or you know climbing you know 12 000 feet over a period of 80 mile ride you want to be able to drive most of that energy from stored body fat I can teach you how to do that but it's going to take you retraining your body to build more mitochondria we call it mitochondrial biogenesis”
Mark Sisson — Former world-class triathlete, bestselling author, founder of Primal Kitchen

“it may cost you a season this adaptation right you may get to the point where after one year of doing this you've refined all of your ability to derive this new energy source to store body fat so that now you're getting 92 percent of your energy at say four watts per kilogram or three watts per kilogram from fat where you used to only get 60 or 70”
Mark Sisson — Former world-class triathlete, bestselling author, founder of Primal Kitchen

“it was really the inflammatory diet that I had taken on to fuel all those miles it was really causing the issue so I had arthritis in my feet had tendonitis in my hips I had irritable bowel syndrome that really ran my life I mean I had to kind of orchestrate my day based around where the nearest bathroom was”
Mark Sisson — Former world-class triathlete, bestselling author, founder of Primal Kitchen

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EPISODE TRANSCRIPT3 SECTIONS

Meeting Mark Sisson in Girona

Welcome back to the romance cycling podcast where we're always on the lookout for experts who are pushing the boundaries of health and wellness and today we've got someone truly special a former world-class athlete turned best-selling author who revolutionized the way we approached diet and exercise what sets him apart is his Primal approach to life which emphasizes eating real food moving in ways that our bodies are designed to move a living in harmony with our environment so get ready to be inspired because today we're diving into the world of Primal living with the incredible Mr Marxist hey thanks for having me Anthony Mark I was in a place we spoke briefly offers you know a little bit of my backstory but I remember when I was close to my lowest air in health and fitness and I needed a week to recharge myself and a place I've gone to time and time again when I need to recharge myself it's a little catalonian village called Girona all of our cycling listeners uh know this place if you haven't been out there check it out it's it's amazing and I remember sitting in a cafe there and I filmed one of your early books I think it was your one you published in 2009 your Primal blueprint book and I was frantically transcribing this book on the steps to that cafe and when I look back and I kind of reflect on you know where I was and where I've come to that's one of those inflection points in my journey so I've been kind of weirdly voyeuristically following you for quite a while so it is a bit of an honor to actually chat with you yeah thank you I appreciate that sure you who come from an Endurance Sports background and it's a background that a lot of our listeners will resonate with you know you successful career as a triathlete a runner an Ironman competitor how did those early experience how did that shape your approach to diet and exercise well you know um I was an early um student of performance I mean I'm going back like I was 12 13 years old and I was already reading stuff on on diet and exercise and primarily about longevity I have some weird fascination with living a long life when I was very young and one of the things that got me into running initially was reading Ken Cooper's book on aerobics where Cooper said The more you did that raised your heart rate for the longest period of time the longer you'd live and I'm like okay I can do that so I started running a lot and as I was running a lot uh you know I I of necessity had to figure out a way to fuel all those miles so I started looking into diet and and improving performance I started competing in track and field events I started doing road races as a runner uh and and you know as as you know as an endurance athlete it's all about the clock it's all about the time bringing those times down how could I uh train harder and run faster and I just the fascination with uh the diet was sort of overlane by the idea that I there was only so many miles I could put in my body could only take so many miles of running so what could I do to uh improve those miles and increase the amount of of uh of strain and stress I could put on my body without breaking down and you know you and I think a lot of your listeners will will agree that as an endurance athlete it's a weird it's a weird Pursuit because really what you're doing is managing discomfort all the time right people talk about the pain and it's not really painful unless you get an injury but it's really uncomfortable and and so you know day in and day out you're trying to dig a hole for yourself to see how deep you could put yourself in this hole and emerge later on stronger um in a very sort of nihilistic nichean way that which doesn't kill me makes me stronger and and certainly the endurance athlete is the epitome of this every single day you're managing discomfort and some days it's fun I guess I don't know I mean I think the fun is more like uh it'll be great when it's over um I will have um deserved that point a Guinness uh when I've you know the cold the cold one when I finished my workout but uh going back to the to the um the nutrition part of this so how could I you know how could I recover quickly how could I dig myself out of that hole quickly um with with diet and and supplements or whatever sort of uh nutritional assistance I could give myself within reason and so that really became a a point of interest for me was how could I improve my performance with my diet and exercise and I and I really understood quite early on that that it wasn't just about replenishing carbohydrates every day after the workout so you could go out and do it again tomorrow and and repeat that process every day for 20 years which a lot of us have done and a lot of us still do um so I started to dig into you know the importance of healthy fats and how fat metabolism was really the sort of the primary driver um in in many cases um sort of the main difference between a real uh Elite performing athlete and someone who's just going out there and putting in the miles without regard to how that um that effort is being fueled right so anyway that was that was the original interest and then it just took off and I started applying it across the broad spectrum of of human endeavor from Elite athletes up to people trying to lose 100 pounds through people trying to uh mitigate uh pain and and suffering in a normal life from being you know overweight or having arthritis or having autoimmune diseases uh having type 2 diabetes or metabolic syndrome uh and then that kind of became my uh my main focus was was just the average person what can we do to improve our lives by eating better that's essentially what it came down to where was that switch uh where you decide to share it with other people I know your serial entrepreneur and Soldier company not too recently congrats on that what this quote uh entrepreneurs take responsibility for solving a problem that isn't theirs yeah what point did you go okay I figured this out for myself but now yeah like there's a switch there where you decide okay now I'm going to solve the problem for everyone else yeah uh and that's a good there's no one point but because I the initial early switch was in 1982 when I wrote a book called training uh it was a Runner's World training book and um it was the first real book on training for Triathlon I had done Iron Man I finished fourth I had I have a lot of uh lessons that came out of that one of which was about over training and how we tend to do things uh way more than we should especially in the endurance world we always think more miles is you know somehow is going to add up to a better performance and doesn't always the one I still do this so our friends who are Runners they're still 100 miles a week 100 mile a week and I was you know absolutely and I was one of those 100 miles a week for seven years and it was probably 20 miles 30 miles a week more than I should have done given my genetics my body composition my body type Etc et cetera but I but so so what I learned from that Iron Man experience and I went on to become a coach of a professional team and coaching individuals I felt obligated to write down and share it with other uh people who wanted to compete and try last line you know this was not ever going to be a a big money-making effort for me to write a book on training for Triathlon I mean they were you could count the number of triathletes in the world in those days in the maybe tens of thousands certainly in a hundreds of thousands so that you know I started writing books and I I realized that I was someone who when I discovered something and I had something to say about it I wanted to put it out there and and and make it available for the general public to consume um but it was really around two thousand five uh when I I my whole life I'd suffered from irritable bowel syndrome I'd had all these issues many of which had forced me to retire from competition um so it wasn't really as much that the over training although that was part of it it was the inflammatory diet that I had uh taken on to fuel all those miles it was really causing the issue so I had arthritis in my feet had tendonitis in my hips I had irritable bowel syndrome that really ran my life I mean I had to kind of orchestrate my day based around where the nearest bathroom was um not to get too graphic yeah that was really rough and and I um so and I had um gerd gastroesophageal reflux disease I had sinus infections I had I got colds and and and and uh upper respiratory tract infections five or six times a year um because I had no real I had no immune system not just from over training but from from this uh I found I find out later from this diet um I had been writing about carbo loading for years and years and I was still a fan of carbo loading at the time how do you manage your glycogen how do you manage your glucose intake and um at one point you know because so much of my diet was based on grains because it's tough to get a thousand grams of carbs a day without consuming some form of grain products um my wife just said look you're writing about about grains and some of the writing that you're doing is about how antithetical uh grains are to health and you know these anti-nutrients these phytates and these uh and gluten and and and all these other folded proteins that are causing problems in other people why don't you experiment and take 30 days and not eat grains at all yourself and see what happens and it really was uh transformative it was like amazing how my IBS went away almost immediately in my arthritic symptoms cleared up and my skin cleared up and my gerd and I'm I was I was think I just thought to myself God I mean if I was so stubborn in defending my right to eat grains in the face of all this evidence that I've been accumulating and writing as a uh as a as a blogger about this because I had started a Blog authentically to talk about performance and and competing for uh you know using using diet and exercise and and weight training uh you know to uh to enhance your personal performance whether you're an athlete or whether you're not if I was someone who was defending my right to eat grains in the face of all this evidence how many tens of millions of other people must be out there who are wondering what is the cause of their males what is the cause of their gut issues what is the cause of their arthritic uh pain and uh that's really what prompted me that's when I said I need I need to spread this kind of message to the world what is that what makes you want to defend that it's not like it's not what we know that hurts us it's what we know for sure that it just isn't so yeah what's that mentality that makes you want to go into defense mode well I mean the mentality was was more um okay so just to backtrack a little bit again as a serial entrepreneur all the time that I've been competing and and writing it was never really an author um I was I wasn't competing anymore um I was starting businesses so I had uh I I had started a supplement company and I had started a supplement company in the early 90s to provide a safe means for athletes to get the sort of recovery nutrition that they it was it was really difficult to get in a regular diet and so I was putting together these vitamin and mineral formulations um at the time I was also involved in the anti-doping movement with the Olympic program so I was very aware of the abuse of performance enhancing drugs Peds and so I wanted to to offer an opportunity for athletes to have some other means of somehow enhancing their recovery from workout to workout that didn't involve going down that that that dark hole that black hole of PEDs so I started my blog as a means of selling these as a platform to give dispense information about training and and weight training and sleep and sun exposure and all these other things uh to sell the supplements well uh as I got further on down the road there I realized that that much of what caused people their their problems was or were other you know macronutrients um you know the large part of their regular diet and so I my focus went away from writing about micro nutrition and writing about uh something that had to do maybe with supplementing to more what's the larger issue here what's the bigger picture and so I was I was I was writing with the intent of building a platform to sell my supplements but I I sort of shifted when I realized that food was the issue it wasn't the supplementation that could really make a big difference at all if you didn't have the macronutrition right so then it I shifted over the macronutrients and you know as an entrepreneur I was building a platform I was hoping to sell something I mean I um shortly after launching my platform I wrote I wrote the Primal blueprint which you've read I've written 10 books since then uh several have been uh New York Times bestsellers and you know each book opened doorway into exploring what the next level of diet was or the next level of Lifestyle was so then I went from feeling compelled to share this with a few people like you did you know like like you you know your 200 friends that you've gleaned some uh interesting tidbits of information that you want to share to you know expanding it into this podcast well I had the same thing I had I had like I feel so compelled to share this information with the world never really thinking that being a an author was going to make me fabulously wealthy but but the writing of a book is is such a cathartic experience and and kind of gels all the other thinking around it in in my life and I suspect it's with other authors as well that you keep you keep wanting to increase the uh the knowledge base and share that with the world ultimately what happened is 2014 um I had been writing so much about food and had gotten such great feedback on how my ideas about uh eliminating certain foods and and you know eating real food and avoiding processed foods avoiding sugar avoiding grains uh avoiding industrial seed oils the results the benefits have been so overwhelming for so many hundreds of thousands if not millions of people that I realized it just my I had an aha moment this I'm writing so much about food and what makes food interesting especially when you cut out all of the when you eliminate all the foods that that you know so many of us would love to eat but can't because they're bad for us they come down to a fairly small list of foods right meat fish foul eggs nuts seeds vegetables a little bit of fruit maybe some starchy tubers what makes the difference is what you put on the food how you prepare it the methods of preparation the herbs the spices the sauces the dressings the toppings I said I'm going to make a food company where all I make are things you put on food and that was the impetus for Primal kitchen and how we started initially with a with a avocado oil-based mayonnaise which was at the time and still is probably the healthiest you know avocado oil is the healthiest oil you can use of the available oils and so we launched this product based on partly um you know certainly wanting to build a business and make a ton of money but also wanting to offer something that didn't exist in the marketplace that gets back to your you know find a problem that doesn't exist and solve it um and you know the problem existed but I think much of the world didn't want to acknowledge that that problem existed they were looking for a way to cling to their uh regular store-bought sauces and dressings and and and toppings and we sort of disrupted uh that whole uh Arena if you will in in the food World by offering uh a very clean label with a product that also tasted really good a lot of your writings and when you've spoken on podcasts you speak very eloquently on science backed on the ketogenic diets this is still quite a departure for a lot of people listening to this podcast and you know the terminology standard American dice you know a portion of our audience here in the US but you know somewhere across the UK and Mainland Europe the standard American diet when I reference it as for this conversation I'm talking more about like a a standard athlete diet someone you know they're not binging out on a full Irish every morning they're having quite a balanced diet across the macros if you're talking to somebody right now and they're on that diet can you briefly explain to me the benefits of moving to a more ketogenic focused on it well I mean for an athlete the ideal scenario is would you like to burn more fat would you like to derive more energy from your own stored body fat when you're on a 200 mile bike ride or 100 mile bike ride and not have to rely so much on gel packs and carbohydrate Laden drinks to get you through any sort of of effort would you like to reduce your body fat level and hence the weight on the bike or the weight on the Run of necessity just by going cruising through the day burning off your own stored body fat so it gets down to a level that's still healthy but but you know maybe in the single digits if you're an athlete would you like to recover quickly more quickly from uh hard workouts if the answer is yes and obviously yes the the sort of counterintuitive approach is to spend some amount of your training uh in a carb starved state in other words force your body to learn how to burn fats by withholding carbohydrates from your body the body will adjust to uh you know a lot of different scenarios in which the macronutrient input can change um but if you keep feeding your carbohydrates not just every day but every meal through through your training um the body says look I don't I don't need to tap into my carbohydrate I mean I don't need to tap into my fat stores because I'm burning carbs all the time and I've always got a ready supply of glycogen in the muscles I've got glycogen in the liver and the fact that I'm going to eat every couple of hours when I'm not training and then eat every Cup every 15 or 20 minutes or 30 30 minutes when I'm training the body just has no reason to tap into its its fat stores has no reason to build the metabolic machinery to take fat out of storage and and combust it in the cells so you have to create that need and this goes back to my my original premise which you know we all have this genetic ability to derive energy from any substrate whether it's the the glucose in your bloodstream the glycogen stored in your muscles or your liver the fat stored on your body the fat on your plate of food the ketones that your liver makes in the absence of of carbs and so the the argument to to eat a ketogenic diet and by the way isn't to eat keto all the time it's to use a ketogenic diet to achieve what we call metabolic flexibility to be able to um to fine-tune your fuel access to the point that you can derive energy uh from whatever substrate is available based on what you're doing at the time so if you're going to be doing you know hard Sprints you want to be you're going to be fully glycolytic your body's great at doing that you you really don't lose that ability but if you're going to be doing a long slog up a hill um or you know climbing uh you know 12 000 feet over a period of uh 80 mile ride you want to be able to drive most of that energy from stored body fat I can teach you how to do that but it's going to take you retraining your body to build more uh mitochondria we call it mitochondrial biogenesis you're going to train your body to derive this this energy from stored body fat and it's going to require a little bit of a uh maybe an uncomfortable shift in how you eat and when you eat but at the end of the day it's worth it one of the reasons like you're seeing more ketogenic athletes now you're seeing a lot of you know Ketone Esters being used in races and in training but for the longest time I told you know I told some top pro athletes I said look it's it may cost you a season this adaptation right you may you may get to the point where after one year of of doing this you've refined all of your your ability to derive this uh new energy source to store body fat so that now you're getting 92 percent of your energy at um say four watts per kilogram uh or three watts per kilogram from fat where you used to only get 60 or 70.

Periodizing for Metabolic Flexibility

But but now you're getting more efficient at the at the long distance but you maybe lose your top end for a while eventually you get your top end your power back your maximum sustained power but it might take more than a season I don't think many athletes are willing to give up a season you know what I mean to to to fully adapt to this but those who have and have fully adapted over say a year and a half two years find now that it's much more efficient much more easy to maintain their body mass maintain their maximum sustained power output um improve their watts per kilogram over time and be able to get through longer races and multi-day races in the case of cycling with you know less discomfort because they train themselves to become more metabolically flexible and less dependent on primarily carbs as this uh source of emergency fuel does that make sense yeah so for endurance athletes we're not really looking to bring someone across in an ideal scenario to a ketogenic diets we're looking to bring them across to having this ability to now use multiple fuel sources as you call it metabolic flexibility ability now I think people in in the same way that you broke down food and it's actually very simple there's only a couple of foods or you know a list a short list of foods we actually eat and it's how we combine those I think training is much the same you know we have days off our recovery days we have endurance days we have interval days and we have race days if we're getting started and trying to build this Machinery to create metabolic flexibility is there like a setup or you would use where you're like okay let me intermittent fast for 16 or 18 hours or whatever your protocol is on rest days and endurance days but let me fuel the carbohydrates on interval days or race days or how would you yeah that's a great that's a great app yeah so what I would do is I start with a pull out and look at the season and say all right I'm gonna spend two months where I'm I know I'm not going to race and I'm going to be I'm willing to um to not do um glycolytic work right I'm just going to be doing uh um like heart rate of uh maybe 180 minus you know minus your age as a max heart rate which is for a lot of people is like you get really really a low heart rate right so especially if you're in the 30s or 40s it's like don't explain me don't exceed 130 beats a minute for two months on on the bike well in conjunction with the diet which you're doing and I would say for a lot of these athletes who are serious that's when you you go full keto for six weeks or eight weeks I call that a keto reset and that's really the most efficient way to to shift your metabolism and prompt the up regulation of these genes that recognize there's not going to be a lot of carbs there's not going to be a lot of glucose so we're going to really become efficient at Burning uh fats and the brain is going to become more used to and efficient at Burning ketones in the absence of glucose um and so in the off season I would say six to eight weeks of a full-on keto program with uh a limit on the amount of glycolytic work that you're going to doing I would say in many cases probably not at all unless let's just say you went to the gym and you go to the gym and you're doing um legs you know you're doing uh presses or lunges or uh or squats or deadlifts all of which uh would serve any cyclist and any Runner uh very well to be doing this in the off season and sure to build you build this metabolic Machinery into that you build uh some periods of intermittent fasting and then as you shift into uh you say okay now I've done the work I've done the base work I've I've built some of this metabolic Machinery now I want to employ it uh I want to I want to start to do some intervals I want to start to do some some um you know some speed work some maybe short time trials and and with that then you start to do judiciously introduce carbs so you say well tomorrow I'm going to be doing a day at the track or I'm going to be doing a day uh in the Hills where I'm really going to be going high intensity yeah 150 grams 200 grams of carbs the night before would be would would serve you well to replenish the glycogen reserves but understand that you know the body doesn't stop making glycogen when you're keto it still builds glycogen the only reason that we ever did carbo loading was because we had this weird notion that we had to go hard every single day and so we're if we're going to deplete glycogen in one workout today if I want to do this again tomorrow I gotta I gotta overfill or try to overfill the glycogen stores well if you didn't work hard every day and that's another thing we've learned through through the years about training you can't go hard every day I mean all you're doing is practicing to hurt but not getting better not improving this metabolic conditioning so if you're going to go um and you know you're not going to go hard after tomorrow's glycolytic day you're going to do a day of of uh of distance and you're going to pull that heart rate way back the body keeps making glycogen it just takes two days or three days to refill the stores but it doesn't stop make a difference has there been like a muscle yeah yeah it's not that much it's like no so so if you said if if multi-day training was not in the program and I wanted to see how long it took me to build my glycogen stores back if on day one I deplete them fully with a glycolytic workout and then day two I I was very good about keeping my heart rate below my Max aerobic function so I'm you know below that aerobic threshold or an anaerobic threshold I was really good about solid about doing that and all I was going to do was burn fat on that workout or that day three days later you would say okay the total amount of glycogen in the body is now uh 475 grams of liver and muscles included and maybe the most it could have been if I'd overfilled my my carbo loading aggressively and intensively the most it could have been is 600 grams throughout the body but now it's 475.

Noakes and the Central Governor

You know those are not rate limiting factors in any way shape or form you want to understand that if you ever read any of Tim Knox work you know brilliant and he was the man for 30 years he was the go-to guy I was like okay I gotta go see what note says about carbohydrate management and glycogen stores and all of that but he came up with this this notion years ago the central Governor theory of the brain which said when you hit the wall when you Bonk it is not that you have run out of muscle glycogen it's that your brain said no in a survival in a sort of a surviving protective mechanism time to pull over the side of the road uh because we're going to uh start to you know cannibalize muscle tissue we're going to eat into um reserves that we can't afford to eat into it's the brain that says pull over it's not the legs that run out of that run out of glycogen so in a fully depleted glycogen state for an athlete who's like gone to the wall and bumped they're still 150 or 175 grams of glycogen left dispersed throughout the liver and the muscle well maybe not so much liver but the muscles so you can't get the zero you don't deplete the glycogen reserves down to zero you just take it from maybe 550 down to 200 or maybe 150. so that's only 300 grams of glycogen that you have that you have at your disposal efficiently and effectively at your disposal which is only 1200 calories worth of work right so unless you are taking in continuous IVs or or you know pulling filling up with with glucose gels through a ride or throughout a ride or eating a bar you don't have that much access to carbohydrates so the real rate limiter is how how much fat can you burn at that rate of output at that 2.4 watts per kilogram up to three and a half four watts per kilogram how much can you shift the amount of energy coming from fat stores away from carbohydrate stores which has the effect of of unburdening your stomach of having to take in 60 grams of carbs every hour just to make up the difference right so so back to the to the training aspect of this and training your body to become more efficient and more effective at Burning at Burning fats it also becomes more effective the brain becomes more reliant on the ketones and says you know what if we run out of glucose I'm not going to panic I'm not going to send a ton of cortisol to the liver to to try and strip a couple of amino acids from the muscles to create more glucose the brain becomes more Adept and in comfortable burning the ketones that are produced by the liver in this in this absence of glucose and that's where this this whole back of notes the central Governor theory of the brain and you say well if it if I can if I can orchestrate my diet and my training so that my brain doesn't start getting wonky and it caused me to get sleepy and pull over the side of the road but the brain can kind of bypass this because it now is comfortable with ketones then I can I can extend my time to exhaustion um judiciously by the use of this this fat burning this new fat burning superpower that I've that I've brought onto myself is there a Pareto Principle version of this and I stopped like what your new book is a move towards making it that's a hardcore example like asking somebody you know unless they're a super studious and they're digging into the research for themselves and they've a hundred percent facing this asking somebody to shelve one year of their season take two months out for keto it's maybe a plunge for a lot of people I'm wondering is there a Pareto version where we can say okay you're in season you want to get some of the benefits of building this metabolic flexibility but you don't want to go you know full jumping out on a plane with no parachute how can they integrate it in sort of a graded manner yeah I mean I think what I just described is probably a very doable process that that um you know you you can get closer to it's not a it's not binary right it's not it's not all or nothing you can get better at it at this sort of fat burning Reliance through any measure of shift toward a lower carb higher fat dietary regime regimen you know it's sure I think if you're an age group athlete or you're a you know you're a you know a cat a cat two cat three whatever Cat four cyclist and you wanna you get a Long window ahead of you because part of this is sustainability right I see a lot of athletes that burn out and I was one I burned out because of my Reliance on carbs you know had I had I known what I know now about this metabolic flexibility and and also how much time I should have spent in the gym building uh Power and and what we call maximum MSP maximum sustain power um I would have cut my miles down I probably would have extended my career my you know Elite career as by a number of years so if you're if you're looking for uh the um secret to doing this over a long period of time it absolutely is found in gradually um refining your metabolic flexibility to be able to um be more efficient and put more miles in with less effort with less you know put higher higher power in with less effort recover faster um so so any amount of uh shifting toward this metabolic flexibility and there are ways of doing it you can do it again in the off season you you mentioned metabolic flexibility I mean you mentioned uh intermittent fasting that's certainly one way to sort of prompt the body to say look there's not a lot of of food around even to get through a a non-training day let's call upon our fat burning um superpowers let's upregulate all these fat burning enzymes and pull them out of storage and and uh and pull fat out of storage and combust it um that's another way of doing this is it doesn't have to be just through manipulating the diet with with um Draconian reduction in carbohydrate it can be done with a with a combination of that and with um and with intermittent fasting I wrote a book a few years ago called uh was probably 10 years ago now called Primal endurance and so any of your listeners who want to kind of get a real full primer on that uh Primal endurance was my answer to people who said hey Mark I'd love to go do a marathon but I don't want to hurt myself I don't want to you know I don't want to sacrifice my health in pursuit of this kind of bucket list item right because I'm not a big fan of running marathons and I did you know I did 50 of them in my career uh initially as a marathon but I'm not a fan of doing it but if you got but my response was okay if you insist on doing this I will show you how to do it in a way that's Kinder and gentler than the than the historic uh you know training programs that that uh everybody from my generation embarked upon and and in large part suffered from because of the lack of of uh knowledge of the details of how the body actually well I think you've hit the nail on the head there because it's this Jewel Focus which 99 of our listeners here the goal isn't the performance they might say the goal is performance but if you step back and you analyze it the goal is performance and sustainability slash long-term Health whereas if you look as I get a chance to chat on the podcast due to very best cyclists and runners in the world their goal isn't sustainability a lot of time their goal is just performance and that carbohydrate intake you know 60 was a one I would have for years said 60 grams per hour it's gone north of 120 at the moment in the world of war cyclist at the very top and for me even trying that out training 120 grams of carbohydrates place a havoc on your gastro system like there's no sustainability around that if you look at a 10-year period of that sort of carbohydrate intake yeah I had no idea there was it was there was ramping up from the 60 60 was the sort of starting point for long you know Iron Man events and things like that but certainly multi-day events where you're trying to meter out your output over not just a a three-hour event but over um multi-day four hour and five hours six hour events

FREQUENTLY ASKED QUESTIONS

What foods help your body burn fat instead of carbs?+

Mark Sisson recommends prioritising healthy fats such as avocados, nuts, olive oil, fatty fish, and eggs to train the body to use stored fat as its primary fuel source. Reducing refined carbohydrates and processed foods forces the body to become metabolically flexible, meaning it can efficiently switch between fat and glucose for energy rather than depending solely on carbohydrate intake.

What is metabolic flexibility and how do you achieve it?+

Metabolic flexibility is the body's ability to efficiently burn stored fat for fuel rather than relying on a constant supply of carbohydrates. Mark Sisson argues you achieve it by reducing carbohydrate dependence, increasing healthy fat intake, and avoiding the habit of refuelling with carbs immediately after every workout. Over time, the body adapts to access its own fat stores for energy, which supports fat loss and sustained endurance.

Do endurance athletes need carbs to perform well?+

Mark Sisson, a former elite Ironman triathlete, argues that most endurance athletes overtrain and overrely on carbohydrates, which stalls fat adaptation and harms long-term performance. Once the body becomes fat adapted through a lower-carbohydrate, higher-fat diet, it can draw on its substantial stored fat reserves during prolonged exercise, reducing the need for constant carbohydrate fuelling during training and competition.

How long does it take to become fat adapted on a keto or primal diet?+

Most people require between four and twelve weeks of consistently low carbohydrate intake before the body reliably shifts to burning fat as its dominant fuel source. Mark Sisson notes that athletes with a history of high-carbohydrate training may take longer to adapt, as the body must rebuild the enzymatic machinery needed to efficiently oxidise fat during both rest and exercise.

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Key Takeaways

Metabolic flexibility—training your body to burn fat as efficiently as carbs—allows endurance athletes to reduce dependence on constant carbohydrate intake, recover faster, and maintain performance with less digestive stress.

A 6-8 week 'keto reset' during the off-season, combined with low-intensity training (below aerobic threshold), is the most efficient way to upregulate fat-burning genes and build mitochondrial capacity.

You don't need to deplete glycogen stores to zero or carbo-load aggressively; your body regenerates glycogen in 2-3 days even without excessive carbohydrate intake, and your brain protects you from true depletion through the central governor theory.

Rather than adopting an all-or-nothing approach, age-group and amateur athletes can gradually build metabolic flexibility through intermittent fasting, strategic low-carb days, and reserving higher carbohydrate intake for high-intensity efforts.

Elite endurance training often prioritizes short-term performance over long-term health; sustainable training means cutting overall volume, spending more time in the gym building power, and reducing reliance on excessive daily carbohydrate intake.

Expert Quotes

"If I was someone who was defending my right to eat grains in the face of all this evidence, how many tens of millions of other people must be out there who are wondering what is the cause of their males, what is the cause of their gut issues, what is the cause of their arthritic pain? That's really what prompted me to spread this message to the world."

"You can't go hard every day. All you're doing is practicing to hurt but not getting better, not improving this metabolic conditioning."

"The goal isn't the performance—the goal is performance and sustainability, slash long-term health. Whereas the very best cyclists and runners in the world, their goal is just performance."

KEY QUOTES

“if you're going to be doing a long slog up a hill or you know climbing you know 12 000 feet over a period of 80 mile ride you want to be able to drive most of that energy from stored body fat I can teach you how to do that but it's going to take you retraining your body to build more mitochondria we call it mitochondrial biogenesis”
Mark Sisson — Former world-class triathlete, bestselling author, founder of Primal Kitchen

“it may cost you a season this adaptation right you may get to the point where after one year of doing this you've refined all of your ability to derive this new energy source to store body fat so that now you're getting 92 percent of your energy at say four watts per kilogram or three watts per kilogram from fat where you used to only get 60 or 70”
Mark Sisson — Former world-class triathlete, bestselling author, founder of Primal Kitchen

“it was really the inflammatory diet that I had taken on to fuel all those miles it was really causing the issue so I had arthritis in my feet had tendonitis in my hips I had irritable bowel syndrome that really ran my life I mean I had to kind of orchestrate my day based around where the nearest bathroom was”
Mark Sisson — Former world-class triathlete, bestselling author, founder of Primal Kitchen

FREQUENTLY ASKED QUESTIONS

What foods help your body burn fat instead of carbs?+

What is metabolic flexibility and how do you achieve it?+

Do endurance athletes need carbs to perform well?+

How long does it take to become fat adapted on a keto or primal diet?+

DISCOVER THE ULTIMATE FAT-BURNING FOODS THAT 99% MISS | MARK SISSON

Key Takeaways

Expert Quotes

KEY QUOTES

Meeting Mark Sisson in Girona

Periodizing for Metabolic Flexibility

Noakes and the Central Governor

FREQUENTLY ASKED QUESTIONS

NEVER MISS AN EPISODE

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Key Takeaways

Expert Quotes

KEY QUOTES

Meeting Mark Sisson in Girona

Periodizing for Metabolic Flexibility

Noakes and the Central Governor

FREQUENTLY ASKED QUESTIONS

NEVER MISS AN EPISODE

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Key Takeaways

Expert Quotes

KEY QUOTES

1.Meeting Mark Sisson in Girona

2.Periodizing for Metabolic Flexibility

3.Noakes and the Central Governor

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Key Takeaways

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KEY QUOTES

1.Meeting Mark Sisson in Girona

2.Periodizing for Metabolic Flexibility

3.Noakes and the Central Governor

FREQUENTLY ASKED QUESTIONS

NEVER MISS AN EPISODE

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Meeting Mark Sisson in Girona

Periodizing for Metabolic Flexibility

Noakes and the Central Governor

Meeting Mark Sisson in Girona

Periodizing for Metabolic Flexibility

Noakes and the Central Governor