The keto diet may have achieved mainstream popularity as a weight-loss strategy, but it has also piqued the interest of athletes looking to optimize performance as well as body composition.
As you might imagine, this has caused no small amount of pearl-clutching in sports circles. Keto diets require you to strictly limit carb intake—the antithesis of the standard sports nutrition advice.Fueling strength workouts and endurance training sessions without loading up on carbs?! Is it even possible? Safe?
I can personally attest to the power of switching from being an obligate carb-burner to a fat-burning beast. Likewise, I could point to many examples of high-performing athletes who eat a low-carb or keto diet (at least sometimes throughout the year) with great success. There’s KetoGains cofounder Luis Villasenor and “Keto Savage” Robert Sykes—both impressive physical specimens whose physiques are walking answers to the question, “Can you build muscle on keto?” (Yes.) Record-breaking ultrarunner Zach Bitter and Ironman champions Dave Scott and Jan van Berkel use ketogenic and low-carb diets to enhance their training. Virta Health founder Sami Inkinen and his wife Meredith Loring rowed a small boat from San Francisco to Hawaii—2,400 miles in 45 days—on ultra low-carb, high-fat selections like dehydrated beef, salmon, and vegetables, along with fruit, nuts, and olive oil.
But these are all anecdotes. Maybe these athletes are just freaks of nature (and they’re probably blessed with genetic gifts). Just because THEY can do it doesn’t necessarily mean it’s possible for every athlete, nor that it’s advantageous for athletes to limit their carb intake… but I think it is. Or at least it can be.
Why Should Athletes Consider Trying Keto?
Five good reasons to think about doing a Keto Reset if you’re serious about your sport:
1. Being fat-adapted will benefit every athlete, regardless of sport, competitive ambition, and current fitness level.
I’ve long preached the benefits of becoming fat-adapted for athletes:
Access to nearly unlimited energy stores in the form of body fat
Less reliance on carb refeedings before and after exercise
Less reliance on sugary fuels like gels during prolonged workouts, which are a common source of gastrointestinal distress
Often improved body composition
However, many athletes and coaches alike still worship at the altar of carbs. For decades, conventional wisdom has preached that fat is the preferred fuel at low-level, aerobic exercise intensities but that glucose burning predominates once you hit intensities around 60 percent of VO2max. (An imperfect proxy for that is the point at which breathing through your nose only would start to become difficult—a moderate-to-brisk jog for many people.)
Furthermore, say the physiology textbooks, once you hit the upper levels of effort of 85 or 90 percent VO2max, you’re basically burning only glucose. Thus, athletes “need” carbs if they want to go fast or hard. If you don’t eat carbs before, during, and after exercise, you can’t be competitive and won’t reach your full potential. Or so the story goes.
However, more recent studies have called that paradigm into question. Take Jeff Volek and colleagues’ landmark 2016 FASTER study, which looked at elite male ultrarunners and triathletes who had been doing keto for two years on average.1 Compared to similarly trained and fit athletes eating a typical carb-fest, the keto athletes were twice as efficient at burning fat for fuel. They burned more fat at higher exercise intensities than was supposed to be humanly possible. These guys were topping out around 1.5 g/min, whereas the peak fat oxidation rate was thought to be closer to 1 g/min. (Although Stephen Phinney, an author on this paper, had documented the same peak fat oxidation of ~1.5 g/min in keto-adapted cyclists three decades earlier.2)
In short, these guys were the definition of fat-burning beasts, yet—and this is key—their performance on endurance tests was the same as their carb-fueled counterparts. Plus, muscle biopsies showed that both groups had comparable levels of stored muscle glycogen. That means that the fat-fueled athletes did have access to glucose when and if they needed it.
Lest you think the FASTER study was a one-off, a dozen or more other studies have likewise found that when athletes adopt a low-carb, ketogenic diet, their ability to burn fat skyrockets, even at exercise intensities well above the aerobic threshold.3 In short, the evidence is clear: fat is a viable fuel for athletes—with other clear benefits, like…
2. More, and more efficient, mitochondria.
You don’t have to eat a strict ketogenic diet to train your body to use fat for fuel, although it sure does help. The lower your habitual carb intake, the less insulin your pancreas will be releasing on a 24-hour basis (lower insulin AUC, or area under the curve). Fewer carbs plus less insulin equals more fatty acids in circulation, which leads to more fat (and ketones) being used for energy.
To utilize those fatty acids and ketones, you need more, and more efficient, mitochondria. Mitochondria are the cellular organelles where fat is metabolized to ATP, the body’s energy currency. Two things that reliably spur mitochondrial biogenesis (the creation of new mitochondria) and make existing mitochondria work better? Exercise and ketogenic diets.45
3. Faster recovery? Yes, please.
I’ve already said that breaking free of the sugar train enhances recovery, a phenomenon that I experienced myself as an athlete. Once I went Primal, the aches and pains I considered an inevitable part of elite-level training virtually disappeared. Countless readers have reported the same over the years. I always chalked that up to a Primal diet being less inflammatory than the Standard American Diet—fewer gut-busting grains, more omega-3s, that sort of thing. That’s true, but there’s more to the story. Ketones also have direct anti-inflammatory properties.
4. Athletes benefit from not carrying around excess body fat.
Keto is a highly effective tool for losing excess fat while protecting lean mass.67 Protein and fat, the core macronutrients of a keto diet, are highly satiating, and ketones themselves tend to suppress appetite. Weight loss can feel almost effortless. This can also become a double-edged sword for athletes, though, for whom proper fueling is paramount. More on this later.
5. All those general health perks.
Let’s not ignore all the other good stuff that happens when you regulate blood sugar and insulin, reduce inflammation, and provide your brain with ketones. For athletes who are trying to train their bodies into well-oiled machines, these can be especially appreciable.
In one study, researchers asked ten highly trained male runners to do a month of keto and a month of eating a typical high-carb diet.8 Three of the athletes had fasting blood glucose in a prediabetic range to start despite being lean and fit. These three also had the most profound response to the ketogenic diet condition, showing the greatest drops in blood glucose and the highest rates of fat oxidation.
In another small pilot study, five athletes did keto for ten weeks. Despite a few hiccups, by the end, “athletes were keen to pursue a modified low-carbohydrate, high-fat eating style moving forward due to the unexpected health benefits [enhanced well-being, … improved recovery, improvements in skin conditions and reduced inflammation] they experienced.”9
Does Keto Improve or Impair Endurance?
All these arguments in favor of keto are all well and good, but some athletes are mostly interested in the bottom line: finishing time and whether they nab a spot on the podium. These folks are taking a bigger gamble by switching up the tried and true carb fueling paradigm in favor of fat—or are they?
Rumor on the streets is that keto hurts high-end power and endurance. Without carbs, you can’t eke out that last little bit that can spell the difference between a top-10 finish or a middle-of-the-pack time. But the data don’t actually back that up. In controlled research studies comparing high-fat, low-carb (HFLC) diets to low-fat, high-carb (LFHC) diets, high-carb sometimes outperforms keto, and keto sometimes outperforms high-carb;10 but the bulk of the evidence finds little difference. One 2021 review, for example, concluded that the two diets were equivalent in 10 out of the 13 studies they analyzed. 11
Other recent reviews reach similar conclusions. Furthermore, the minority of studies that show decrements on keto usually measure endurance performance via brief time to exhaustion tests (Wingate tests, which if you’ve ever tried one, you know are brutal) or repeat sprints. That doesn’t really reflect the type of endurance the average “endurance athlete” is going for. They’re grinding out sessions that take an hour, two hours, half a day at 60 or 70 percent VO2 max, maybe even less. Realistically, most everyday endurance athletes rarely or never reaching for that top-end power anyway.
Why then did keto get a reputation for being “bad” for endurance athletes? Probably because keto-adaptation takes time. Energy, performance, and “oomph” often tank for the first month or two. After that, if you tough it out, energy and performance rebound, and keto athletes do just as well as sugar-fueled athletes.1213 I suspect many athletes quit before the magic happens.
Can You Build and Maintain Muscle on Keto?
Ok, you’re thinking, keto might work for endurance athletes, but what about strength athletes? Is it possible to get strong and ripped without a ton of carbs?
Unequivocally yes, you can build and maintain muscle on keto. Study after study comparing keto to conventional high-carb diets finds no meaningful difference between the two provided that you (1) eat enough food overall, (2) eat sufficient protein to hit your leucine threshold and provide the necessary amino acid building blocks, and (3) deliver the appropriate stimulus in the form of lifting heavy things.141516
Mistakes Athletes Make When Trying Keto
Clearly, it’s possible to be strong and have excellent cardiovascular and muscular endurance without shoveling hundreds of grams of carbs down your gullet each day. Still, I hear from athletes all the time who are struggling in training and competition after going keto. Almost universally, this is a problem with execution, not due to any inherent inferiority with keto itself. These are the most common mistakes I see:
Mistake #1: Not eating enough
Carb restriction and caloric restriction often go hand-in-hand, whether intentionally or not. While you can rely on body fat to make up a deficit, there’s a limit to how much you want to draw on those reserves, especially if you’re already lean.
Mistake #2: Not supplementing electrolytes
Nine times out of ten, when an athlete complains about headaches, low energy, muscle fatigue, cramps, or brain fog, they need more electrolytes. Sodium especially, but also potassium and magnesium.
Mistake #3: Not giving it enough time to work
Many of the most-cited studies supposedly showing that keto “hurts performance” or “doesn’t work” for athletes have ludicrously short adaptation periods—like less than a week. It takes minimally three to four weeks for the process to really get going. Athletes, who require a lot of energy to sustain their training, may need several months to feel totally normal again.17
Mistake #4: Going keto at the wrong time
Because it takes time to adapt, I recommend that athletes who are brand new to keto, or who have been away for a long time, save a Keto Reset for the off-season. Wait until you can reduce the volume and/or intensity of your training as needed. Don’t completely switch up your diet a month before your A race. You will almost certainly regret it.
Mistake #5: Fearing carbs
Carbs are not the enemy here. I’ve said over and overagain that athletes who “burn and earn” carbs can and should replenish them—but that they should opt for Primal carb sources, and they should, in my opinion, strive to find the minimum effective dose that supports their training load (even if they choose to exceed it sometimes).
While a more sedentary person typically needs to limit intake to 30 to 50 grams of carbs per day to stay in ketosis, hard-charging athletes can probably consume several times that. They’ll still spend much of their time in ketosis because those carbs are used for fuel immediately during their workouts and for replenishing glycogen stores after.
To find your personal carb tolerance, use a blood or breath meter to measure your ketones at different carb intakes.
Mistake #6: Adding back carbs too soon
Hard-charging athletes might ultimately prefer a targeted or cyclical keto approach where they titrate carbs up or down depending on the volume or intensity of their current training cycle. In fact, many successful “low-carb” elite athletes reportedly consume hundreds of grams of carbs per day when they’re really pushing their training. While that might sound like a lot, it’s still considerably less than their conventional peers who might consume two, three, even four times that amount. It’s all relative. Plus, low-carb athletes might still be in ketosis even consuming a couple hundred grams of carbs per day since they are regularly depleting glycogen.
I have no problems with using carbs strategically, but I recommend waiting until you’re fully keto-adapted and feeling “normal” again on keto before experimenting with a targeted or cyclical keto approach. Otherwise, you’re just delaying the adaptation process.
What about Female Athletes? Can Keto Work for Them?
Yes, but with caveats.
Premenopausal females’ bodies are more attuned to dietary restriction, and relative energy deficiency is already a significant problem for high-level female athletes. I’d exercise caution here. Female athletes who are interested in keto must be very conscious of their overall food intake and be alert for signs that they are restricting too much. These include decreased energy or motivation to train, sleep issues, hair loss, or menstrual irregularities.
For the same reason, I wouldn’t recommend that (premenopausal) female athletes also engage in intermittent fasting alongside carbohydrate restriction. Choose one or the other.
The Bottom Line
The majority of the evidence finds keto to be just as effective for endurance and strength athletes as a conventional high-carb diet. Plus, with keto, you get all the anti-inflammatory, fat-burning, recovery-supporting perks.
Yes, there is a chance that you might lose some of your maximal power, speed, or strength, especially during the adaptation period. For most people, that seems a worthy sacrifice. The vast majority of people who toe the starting line of a half marathon or ironman triathlon aren’t there to win. They want to finish in a time that is respectable for them and not be totally wrecked after. Unless you’re being paid to be at the absolute top of your game, competing with the best of the best in a sport demanding all-out strength or speed, trading a little bit of top-end power for all the benefits of being a fat-burning beast is a good deal. Nay, a great deal.
When you feel better, sleep better, and recover more quickly, you’re also able to train more efficiently. It’s that much easier to motivate yourself to lace up your shoes and get out there. Everything feels easier and more enjoyable. Isn’t that what you ultimately want? To enjoy your sport? I know there are some masochistic athletes out there who are in it for the pain and the grind, but I think most of you are in it for fun, health, and camaraderie with other athletes.
And remember, you don’t need to be keto full time to reap the benefits.
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.