How Using Fat for Fuel Can Boost Athletic Performance

How Fat Boosts Athletic Performance in lineA few weeks back, I explored the potential benefits using fat as your primary fuel can have on cognitive function. While the strongest research centers on people dealing with age-related cognitive decline and other neurodegenerative diseases, and whether burning fat and ketones can boost cognitive function in healthy adults remains unconfirmed, the totality of the evidence suggests it can provide a benefit. Today, I’ll be discussing a related topic with more solid scientific footing: the effects of fat-adaptation on athletic performance.

Detractors of high-fat, low-carb diets often claim that they’re bad for physical performance. They may offer some help to people with certain forms of brain cancer, they can definitely help obese people lose weight quickly and easily, and the ketogenic diet is the gold standard treatment for epilepsy, but fat-adaptation severely hampers your ability to perform on the field, on the track, and at the gym.

Is this really true, though?

While the effect of fat adaptation on anaerobic performance is unclear, it can actually improve many other measures of physical performance. There may even be cause for anaerobic-centric athletes to get fat-adapted, if only for part of the time.

Let’s dig right into the benefits.

Your energy efficiency improves.

Fat-adaptation makes you better at burning fat and less reliant on muscle glycogen to fuel your efforts. Any amount of work you can accomplish by using mostly/all fat rather than mostly/all glycogen is evidence of efficiency. As the effort increases (with appropriate diet and training over time) and you can still do most of it burning fat, you spare glycogen for even greater efforts or for use far later in the event.

In one study, endurance athletes who’d been on the ketogenic diet for an average of 20 months burned 2.3 times more fat at peak oxidation, burned 59% more fat overall, and did so at higher intensities (70% of VO2max) than endurance athletes on “normal” diets, for whom fat oxidation dropped off and sugar-burning dominated at 54.9% of VO2max. This means that their anaerobic threshold was much higher than conventional athletes; they could go longer and harder using fat and ketones. Glycogen reserves were similar in both groups.

You can access your large reservoir of adipose tissue.

Glycogen’s great to have around for intense activity, but we can’t store much; the most muscle-bound man probably has the capacity for just 600 grams of glycogen in his liver and skeletal muscle tissue. That’s only 2400 calories. Even lean athletes without visible body fat possess tens of thousands of calories worth of clean-burning, reliable adipose tissue. Fat-adapted athletes can burn a greater proportion of body fat before dipping into the relatively scarce and precious pools of muscle and liver glycogen, thus preserving the latter for later use.

You’re not tethered to food.

Back when I was a sugar-burning athlete, I had to keep carbs on my person at all times. Yeah, yeah, I burned every last thing I ate, but I couldn’t go longer than a couple hours without eating. And it wasn’t just the physical burden of eating all the time that got to me. It was also the psychological burden of needing to eat all the time. I felt powerless.

That ended when I became a fat-burning beast. Now, I skip meals. Or the meals skip themselves because I’m just not hungry. When I get that early morning text from a buddy inviting me on a hike, I accept it without worrying about “carbing up” first or loading up my backpack with snacks.

This also means you won’t have to carry several pounds of glucose goo just to make it through a race.

Aerobic exercise “feels easier.”

Since you need less oxygen to do a given amount of work when you are fat adapted, burning fat for fuel makes aerobic activity feel easier. You could be matching or even beating your best times and feel great doing it. If you’re at all like those long-term keto endurance athletes, you might get to the point where close to 70% of VO2max feels downright comfy. This gives you a mental edge. Rather than grit your teeth and power through a grueling session, you can actually enjoy your training.

Plus, if your “discomfort threshold” goes up, you can handle a lot more work before pain and suffering set in.

You’ll build more mitochondria.

Mitochondria are the power plants of our cells. They turn caloric energy—fat, carbs, ketones—into real-world energy, or ATP. The more mitochondria you have, the more ATP you have at your disposal. The best way to spur the creation of new mitochondria is through AMPK activation. AMPK increases in states of energy deprivation. Low-calorie or low-carb diets, low glycogen levels, starvation, ketosis, and certain types of exercise all increase AMPK. Anything that reduces the calories coming in and/or increases the calories going out will probably increase AMPK.

When excess energy overloads the mitochondria’s capacity to convert it in ATP, bad things happen: insulin resistance, generation of reactive oxygen species, and oxidative stress. Having more mitochondria increases your conversion capacity; you can actually handle more energy coming in. And you get more usable energy for fun and important stuff like running sprints or marathons, lifting weights or toddlers, reading books or emails, and being productive at work.

Train low, race high.

Fat isn’t an instant performance boost. No one’s eating a stick of butter before the race and claiming superpowers. Fat-adaptation for athletic performance is a long game: by improving substrate utilization, building new mitochondria, increasing metabolic flexibility, and increasing the amount of fat burned for a given intensity, athletes enjoy a bigger engine.

This is the “train low, race high” paradigm gaining prominence in the endurance world. By training with low glycogen levels, you get better at functioning without glycogen. When race time rolls around, you carb up and get the best of both worlds: enhanced fat oxidation and plenty of glycogen.

That’s how Zach Bitter, a fat-adapted ultramarathoner, trains. He spends most of his training time on a high-fat diet, but carbs up for and during races. Seems to be working okay for him, if you consider running 7-minute miles for 12 hours straight and beating the US 100 mile world record “okay.”

Note the distinction between training and competition. I’m not suggesting that Lebron James go on an epilepsy-worthy keto diet during the NBA Finals or that Michael Phelps fast before an Olympics relay. I am suggesting—and James would probably agree, as he’s gone low-carb paleo to drop body weight during off seasons—that athletes can benefit from training while fat-adapted and low in glycogen even if they intend to “race” while high in glycogen.

Okay, getting fat-adapted can clearly benefit endurance athletes, but what about maximal intensity—anaerobic activity that demands carbohydrate? It remains to be seen. While we haven’t seen many NBA point guards, NFL running backs, CrossFit Games runners-up, or Grand Slam winners going ketogenic during the season, wait for it. I’ve heard some very interesting things from top-level athletes and the people who work with them about high-end power and intensity on long-term high-fat diets. No studies (yet), just whispers.

We do have studies where fat-adapted athletes beat sugar-burning athletes at high-intensity intervals due to their increased ability to burn fat and retain glycogen. Critics might say the intensity of those intervals wasn’t quite high enough—six 4-minute intervals on an incline treadmill with two minute rest in between—but that’s moving the goalposts.

In another study, trained adults followed a normal diet (moderate carbs, moderate fat, moderate protein) for seven days followed by a low-carb diet (54% fat, 5% carb, 35% protein) for seven days. After both arms, the researchers subjected participants to a series of physical tests: handgrip strength, max rep bench press, 1 rep max in squat and bench, vertical leap, and Wingate test. These were legitimate tests of maximal strength and power output. The Wingate test, for example, consists of 4 x 30 second all-out sprints on a stationary bike set to the highest resistance. Puke buckets are standard protocol. Yet despite losing more body weight after the low-carb arm, the participants suffered no hits to their performance. 

We know that elite gymnasts can live and train on a high-protein, high-fat ketogenic diet for 30 days, lose body fat and body weight, gain lean mass, and maintain their physical performance.

I know I feel great playing my Ultimate games, usually on an empty stomach, sometimes with a scoop of Primal Fuel, occasionally with some exogenous ketones, always fat-adapted. Three hours of slower, steadier movement punctuated with bursts of all-out sprints aren’t a problem.

Most of you folks have different priorities. Training to compete isn’t your day job. You want to get stronger, faster, fitter, and look better naked. You want to be more physically capable. You want to keep up with your kids or grandkids. How can fat-adapted training help non-elites?

It’ll build up some extra mitochondria to take the load off your body, reduce oxidative stress and insulin resistance, and improve your capacity to burn energy.

It’ll make those weekend bike rides/climbs/Ultimate games a lot easier—and more enjoyable.

You won’t have to pack two pounds of trail mix just to finish a hike.

It’ll help you burn some more body fat.

You’ll conserve glycogen for when you really need it.

The physical performance of every human being on this planet, from elite athletes to hobbling grandpas, can benefit from spending some time and exercising in a fat-adapted, glycogen-depleted state. Why don’t you give it a try?

If you want to get really good at this stuff, check out Primal Endurance.

Thanks for reading, everyone. I’d love to hear about your experiences with fat-adaptation down below. How has it impacted your training and performance?

Prefer listening to reading? Get an audio recording of this blog post, and subscribe to the Primal Blueprint Podcast on iTunes for instant access to all past, present and future episodes here.

Primal Kitchen Ranch


About the Author

Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.

If you'd like to add an avatar to all of your comments click here!

40 thoughts on “How Using Fat for Fuel Can Boost Athletic Performance”

Leave a Reply

Your email address will not be published. Required fields are marked *

  1. I don’t like the new website – looks nice, but too many clicks to get to the blog…

    1. It’s only one click, then, just as with the old site, a click on the “read more” to see the whole article. Is that really two many clicks? Seems kind of nitpicky. There is always some resistance to a major site revamp. I, for one, like how similarly stories and subjects are grouped together; makes it a lot easier to find things. Give it a chance.

      1. My problem is the font. Too small and the paper thin grey text on white is a real pain to read. I understand the new design is “on trend” but some trends are stupid. Artsy-fartsy text is one of them – and I’m a web designer. Beef up the fonts and use black.

  2. Great post! I’m definitely not an athlete…just someone who wants to feel great, have tons of energy and not have to worry about gaining weight. I have been fat adapted for years now and it has made a huge difference in how I feel. My energy levels are so much more stable. I feel more focused. And even though I LOVE food, I’m not thinking about it constantly the way I used to. I have no idea what my macros are, but I make sure to include healthy fat with every meal. I always start with 2 tablespoons of coconut oil blended into my coffee, along with some collagen. I find consuming enough fat and protein really makes a difference with cravings for sweets as well.

  3. Great article! I’m curious to see more long term studies come out. Seems like the evidence is pointing in the direction of fat adaptation being the way to go.

  4. Who doesn’t love more mitochondria? That’s why I make sure to get my sprints in every week. 😉

  5. The savings in trail mixes alone warrants a good go at getting fat-adapted for physical endurance feats!

  6. Makes sense. The more you can dig directly into fat, rather than glycogen, as your fuel source, the more fat you’ll burn and the more calories you’ll have at your disposal. A win-win if you’re trying to both stay lean and go hard for longer periods of time.

  7. Love Primal Endurance. Adjusting to a fat-based regimen has been a huge benefit for me. I’m so glad you’re talking about the endurance issue. It’s one of the most fervent areas of high-carb allegiance.

  8. Have to agree with hawt (above) comment about the new website. It looks good and I’m sure we will all get used to it but I sure liked the old one better and it was much more user friendly.

  9. The biggest notice for me is my ability to teach 5 exercise classes on Saturday on nothing more than a cup of coffee and some mineral water. Granted I do not do the whole workout but I have to be very upbeat and I do so without any help. On really hot days I drink some coconut water but it is amazing to go so long on no fuel. I also tend to hike on Sundays now (my day off) for 1-2 hours always on an empty stomach. I really love the clarity that comes with it. Best of all is the eggs and bacon with a bowl of fruit for breakfast afterwards.

    And for the record the new website is very nice and easy to use, change is just hard sometimes!

  10. In my experience of long distance running, I’ve found many benefits to fat adaption. Once I became fat adapted, I noticed that I could run longer without fuel, run on an empty stomach, and feel more satiated throughout my workouts. In addition, I’ve noticed an increase in my overall strength and ability to perform as an athlete.

  11. Hmmm. 5 years being Primal and doing intense body weight exercises fasted, and being fat adapted, and Mark says, “you’ll conserve glycogen for when you really need it”. The thing is, I go hard about 3 times a week and I’ve NEVER needed it, or at least I have not even come close to bonking. This is coming from a 2lbs. trail mix, Power Bars, emergency rations in the daypack, etc. so I wouldn’t crash with hypoglycemia. Maybe another post is needed on “when do we really need it”

  12. I used to live on the route of an annual semi-marathon. Besides hating the inconvenience, there was all that litter of carb gels and who knows what. Why can’t Americans live and function w/o “products.” It’s a disease

  13. I like how you didn’t emphasize the need for testing urine, blood, or breath ketones.

    A lot of times people get caught up in that and feel like it is a vital part to a ketogenic diet, when in reality they’re just optional adjuncts.

  14. I’m fat adapted, eat a low carb keto diet and workout fasted. No problems whatsoever. I continue to get better, setting new PRs. Am never hungry and often have to remind myself to eat. I’m doing keto for my brain, however. The physical fitness benefits are just extra freebies.

  15. I struggle with morning fasted workout so much! I get so light headed and tire much easier. What am I doing wrong?

    1. Maybe nothing. Are you fat adapted yet? Have you tried an afternoon or evening, fasted workout? It’s possible to find out with an N=1 experiment. Some people may not do well with a fasted workout. Good luck!

  16. I love the freedom of needing to eat only 2 meals per day. And, I too love to hike on an empty stomach and never take snacks. So freeing and I feel great as long as I stay primal.

    1. I agree. As I became more fat adapted I no longer needed a snack before surfing. I get up, drink a bunch of water and some green tea, then go surf for two hours. When I come home may also do some intervals on my spinner bike or some planks before I eat. And i don’t feel ravenous when I eat either. Just normal hungry. It really is freeing to not have to shove carbs down every two hours because I’m so hungry.

  17. I read “Primal Endurance” when it was released and am starting another 12 week aerobic base building phase. I can through in the odd IF 18~30 hour long IF here and there, so I feel I’m fairly fat-adapted.

    My question is this: will performing the occasional “7-minute Workout” or even Primal Blueprint Fitness workout negate my aerobic base building efforts if my heart rate jumps above aerobic max? Can I do pushups/pullups/whatevers until my heart rate jumps above 180-age and not have it hinder my aerobic base building efforts while still training for strength gain?

  18. I’ve been doing fasted cardio and workouts since college football (because the trainers goal was to make you puke each workout). I’ve currently been training for a marathon and would say I’m close(ish) to being fat-adapted. I don’t eat or drink goo during my long runs (last one was 16 miles) and I haven’t really had any setbacks or issues. This stuff works.

  19. Great article. I’ve been working on this for myself for a while. I currently use Generation UCAN for long runs and am training for a marathon. I’ve been fine so far on runs up to 15 miles. But am worried about the actual race and my 18 miler this upcoming weekend. What would you recommend that is easy to carry and eat while running? I think the full will take me around 4 hr 15 mins. This is my first one without relying on gels.

    1. I’ve been using UCAN for a few years now and in my experience, use 1.5 servings before and then again midway through your race. Doe the same for all runs over 16 to get used to drinking it while running.I usually mix it a little thick to fit in a small bottle to carry. Adjust your dosage according to how you feel and how low carb your diet is. UCAN will “catch” your falling blood sugar levels as you run so if you normally need higher levels, just increase the amount of UCAN. HTH!

  20. If I understand, already from 70% of VO2 energy use will already be of glycogen. If it is, is very low, that is, moderate activity ever reaches this level. High intensity heart rate belt. So I didn’t understand well what is the dividing line between use of Carbo-Grease

  21. I find bigger meals way me down. When I’m fat adapted. I can get through a day with nothing but an avocado and never lose energy. A full stomach takes energy to process.

  22. Mark…what do you think of the research by Dr. Cyrus Khambatta regarding the high ingestion of carbohydrates?

  23. All good…even great. However, how does one walk that fine line between keto-trained and over worked, especially where the adrenals are concerned as far as hypoglycemic conditions that are brought about by moderately intense training. I seem to run into this too often. Sleep suffers and no doubt, I am compromising just gotten gains and fitness. The betwixing thing is that I am quite adapted to the gluconeogenis process but when it comes to training, perhaps I’m just a glutton for punishment and in turn, simply am not eating enough calories?

    1. I’ve been struggling with what you said for sometime. I completely exhausted my adrenals two years ago and developed hypoglycemic conditions, panic attacks, anxiety, and heart palpitations. It’s taken a year to repair my body while also completely humbling me during recovery. Right now, it seems once I begin to increase intensity and try new exercises, I have set-backs again and again. I’m looking for an answer on how I can improve and evolve with this nagging problem still surfacing. I understand my threshold has decreased, but how can make any gains training/exercising when it has a direct impact on me and not to mention recovery at work all day can be daunting if I’m not careful.

      1. Yeah, sounds a lot like me; effortlessly thin, consistent energy for workouts (perhaps too much), immune system inherently stronger that it had been years ago yet some days at work, I feel almost miserable and despondent. I attribute this to A) lack of quality sleep and B) a caloric deficit since I have noticed this “black cloud” is especially pronounced after training sessions. So, are you simply eating enough after workouts? I have also noticed that this pall abates after a few good meals.

        Now, as far as the sleep thing, I am relatively perplexed. This is mainly what concerns me since ketogenesis seems to affect it whether or not i work out. It could also be that I am not consuming enough calories on a daily basis in general. Perhaps this is where the occasional “carb refeed” is important….

  24. Would “train low, race high” apply when competing in a combat sport, like Brazilian Jiu-Jitsu where there’s a mix of steady-state and bursts? I’m competing this weekend and eat lower carb most of the time while training. Would there be an advantage to eating say a sweet potato on the morning of competition?

  25. Mark great post! Speaking of “Whispers”, are you hearing anything regarding U.S. military and NASA looking at Ketosis/fat adaptation training?

  26. Hi, reading your article way back was very inspiring as I am was sugar-addicted, talentfree endurance runner who strived and strives to better his halfmarathon times.

    So I am trying to stay in ketosis since a year or so. I am quite lean (about 10% bodyfat). My blood ketones are in the lower range at .5 to .7 mmol/l.

    Now, the last two days I ate a lot of healthy and not so healthy carbs and I thought this must/should be the end of ketosis for some days.

    But when I controlled my blood ketones today, right after waking up, my blood ketones were at .3 mmol/l. Although not in ketosis, this is a pretty normal value for me this early of the day.

    Does anyone has ideas why the carbloading the last two days didn’t throw me out of ketosis?