Education for runners and endurance athletes. Learn to train smart, run fast, and be strong.
These articles are free.
Please help keep them alive by buying me a beer:
Buy me a beer.
This article is part of a series:
→ Part 1 — Fat oxidation
→ Part 2 — Fat adapting
→ Part 3 — Low carb diets
→ Part 4 — Low carb & performance
→ Part 5 — Carbs are your friend
→ Part 6 — Carb periodisation
→ Part 1 — Fat oxidation
→ Part 2 — Fat adapting
→ Part 3 — Low carb diets
→ Part 4 — Low carb & performance
→ Part 5 — Carbs are your friend
→ Part 6 — Carb periodisation
Nutritional manipulations for training. Part 5 of 6:
Is carbohydrate a friend you train with regularly or a friend you see from time to time? Understanding the “carb wars” using context and perspective.
Thomas Solomon PhD.
19th Dec 2020.
If you’ve followed this series, you’ve become a bioenergetics nerd and have knowledge of how to acutely increase your fat oxidation rates during exercise. You have also taken the plunge into the 100-year history of low-carb diets and you understand what happens to your economy and performance when habituated to a low-carbohydrate diet. But, you are left with one very important question… Is a low or a high-carb diet your best approach for enhancing your performance? Today, you will find out.
Reading time ~17-mins (3400-words).
Or listen to the Podcast version.
Or listen to the Podcast version.
For the athletes I work with, I understand their needs very well. But I probably don’t know you. This means that I cannot possibly understand the nuances of your needs as an athlete. My purpose with these posts is not to dictate but to help you learn and make your own decisions. Making bold changes to your training, which includes altering your diet, must be well-informed and certainly not implemented on a whim. I always encourage folks to understand their needs and then conduct a cost-benefit analysis for any such change. So, when deciding whether or not to “go low carb” or “be keto”, educate yourself in all possible outcomes.
The first important knowledge bite to be armed with is that,
Such findings have been replicated in runners. Amino acid oxidation was increased following a morning 10 km run when runners had “slept low” (overnight low carbohydrate availability induced by low-carbohydrate feeding) following a high-intensity 10 ✕ 5-min interval session the evening before — also not good.
Collectively, these findings indicate that endurance athletes who train with a low-carbohydrate availability burn more protein during exercise and may, therefore, have increased dietary protein requirements. So, athletes who choose to engage in low-carb dietary practices should be aware of their increased need for daily dietary protein; something you learned all about if you caught my protein article.
The next important knowledge bite to be armed with is to,
Evan Dunfee was one of the world-class athletes enrolled in the first Supernova study who chose to participate on the low-carb/high-fat ketogenic diet. Two weeks after completing the study he had a performance boom, breaking the Canadian 50 km record by 4-minutes! Within the next year, he came 4th at the Olympics, after being only a top-20 contender prior to his participation in Supernova. An anecdotal success that is easy for some folks to assign to “being keto” (for 3-weeks… yes, he did not continue “being keto” after completing the study). And, he was just one of 11 others who completed the study in the low-carb diet group, none of whom had the same performance boom. Dunfee himself does not attribute his success to his “being keto”, he admits he was also training harder in the Supernova camp while surrounded by other world-class athletes — this is highly-motivating — and he believes that because training hard while being “keto” was super tough, it made him appreciate how to train hard thereafter. So, for him, the low-carb diet wasn’t the ergogenic aid but it was the nudge he needed to train harder leading up to the Olympics.
Enticing.
In 2016, Chris Froome tweeted a pic of his low-carb breakfast on a rest day en route to his win at the Tour de France. His sports nutritionist at the time was Prof James Morton, brains behind the “Fuel for the work required” framework, so it was highly unlikely Froome fuelled his win at Le Tour that year on a low-carb diet. But, that didn’t matter because Froome’s avocado and eggs got out and some folks lost their minds, resulting in terrible news stories like, “Low-carb diet propelled Chris Froome to three Tour de France titles”. I’d be curious to know how many cyclists “went keto” as a result of that. Then I wonder how many of them aborted being “keto” when they discovered his actual race diet plan, fully-documented on BBC and openly-discussed on Fuel The Pedal en route to his win at the 2017 Giro D’Italia, where Froome ate more than 1000 grams of carbohydrate on some days — far from low-carb!
Misleading.
I recently listened to an interview of the former multiple Ironman champion-turned-coach, Dave Scott on the Human Performance Outliers podcast during which he said two things that got my neurons firing in ways that made my facial capillaries dilate. First, “If all athletes switched [to a low-carb diet] they would no doubt see a performance increase” and then, “Look at these former Olympians who are now diabetic” [because of high-carb diets]. These sweeping statements are not scientific evaluations but opinions that confirm his predetermined bias. You could indeed try telling Eliud Kipchoge, Peres Jepchirchir, Jim Walmsley, Camille Heron, and Kilian Jornet that they should switch to a low-carb diet because they will “no doubt see a performance increase”. Sure. You could try that. You could also try telling Olympians to stop eating carbs because just “look at these former Olympians who are now diabetic”. Sure. You could. But, I spent 17 years as a researcher in that very field and there is zero evidence linking an endurance athlete’s diet and their risk of developing diabetes.
Just nuts.
The best way to guard yourself against anecdotes is to heed what you learned as a child, “don’t take candy from a stranger”.
This is an opportune moment to introduce another ultra distance dude, Zack Bitter. Mr Bitter runs speedy marathons and currently holds the 100-mile world record of 11:19. He is a low-carb “keto” guy who is also among the more pragmatic adopters of such dietary approaches — you can listen to him break down his own ultra training in episode 212 of his Human Performance Outliers podcast — he habitually eats a low-carbohydrate diet except when his training load is very high and except when he is leading into a race or during a race, when he uses carbohydrates as a fuel.
When Jim Walmsley and Ellie Greenwood smashed the Western States 100-mile records in 14:09 and 16:47 (a race which has more than 6,000 metres of vertical gain), they were not being “keto” or fasted nor did they do so without consuming any calories during the race. Their records were achieved with high carbohydrate availability.
When Eliud Kichoge and Brigid Kosgei broke the marathon world records in 2:01 and 2:14, they too were not “keto” or fasted or avoiding calories during the race — quite the opposite — they broke the world records with high carbohydrate availability.
For the best on Earth, high performance over any distance appears to be dependent on high carbohydrate availability — moving fast while burning a mix of slow-burning, oxygen-guzzling fuel — fat — and fast-burning, economical fuel — glucose. Except for the “FatMax World Championships”, I know of no high-performance achievement pushing the frontiers of speed that has been nailed with low carbohydrate availability…
Useful anecdote.
So, the final important knowledge bite to be armed with is to,
We humans store a lot of fat.
Fatty acids produce more energy (ATP) per gram than glucose.
Fatty acid metabolism is slower to produce energy than glucose.
Fatty acids are a less economical fuel than glucose, requiring more O2 to produce ATP, meaning glucose produces more ATP per litre of oxygen.
Bioenergetic processes are hardwired in our biochemistry — we cannot change them — we can only influence the relative proportions of fuels being used.
You have loads of fat to use, even if you are very lean. So moving slowly (at a low fraction of your maximal aerobic capacity) for a long time is really not a problem for an endurance-trained athlete. But, when operating at a high intensity, when oxygen availability becomes a limiting factor because you are approaching your VO2max, it is entirely intuitive that your body will try to use the most economical fuel to use less oxygen while continuing to produce ATP at the rate it needs to keep moving forward at high speed with grace (which, incidentally, rhymes with war face). Theoretical bioenergetics suggest this is true; experimental studies show this to be true; and, empirical evidence from the field prove this to be true.
For Zach Bitter, Michael McKnight, Jim Walmsley, Ellie Greenwood, Eliud Kipchoge, and Brigit Kosgei, as well as any other endurance athlete trying to achieve their best times, being a fat-burning monster is essential.
One way to increase fat oxidation rates during exercise is to, quite simply, endurance train (remember: training causes adaptations that increases the amount of fat stored in your muscles, the capacity to deliver fatty acids to muscles, and the amount of fat-burning enzymes in your muscles).
… Another way is to run slowly (remember George Brooks’ crossover concept — at lower fractions of your maximal aerobic capacity, you preferentially burn fat and as you increase your intensity you increasingly rely on carbohydrates to fuel your movement).
… Or you can run before breakfast (i.e. train fasted — low liver glycogen).
… Or you can run having previously glycogen depleted (low muscle glycogen).
… Or you can habitually consume a low-carb/high-fat diet (low muscle glycogen and increased fat-burning enzymes). Many routes to the same goal.
But to be the fastest endurance athlete you can be, in addition to being a fat-burning monster, you also need to be able to burn carbohydrate when you need it. Carbohydrate is your speedy friend.
Anyone who has read the studies comparing the effect of high vs. low-carb diets on endurance performance will be well-versed in both the within-diet and the between-diet variability in performance outcomes.
Louise Burke’s study, Supernova 1, found that the improvement in 10 km race performance among athletes receiving a high-carb diet during the 3-week training camp was highly consistent: on average, there was a +6.6% improvement for which there was a 90% confidence interval of +4.1 to +9.1%. This means that, based on the observed data in the study, if all endurance athletes were studied (i.e. all folks on Earth that the study was trying to “sample”), 90% of plausible values for performance improvements would be somewhere between 4.1 and 9.1%. Meanwhile, the effect of the low-carb diet was highly-variable: the average change in performance in the study was −1.6% and the plausible range of values (the 90% confidence interval) was predicted to be between −8.5% and +5.3%.
In plain language, Supernova 1 showed that while some low-carb keto folks can improve their performance during an intense training block, most get worse and some get waaaaay worse! Meanwhile, all high-carb diet athletes are likely to get much faster.
If you’ve been following my series, you will know that Supernova was replicated…
Supernova 2 found that the high-carb diet during the training camp improved athletes’ 10 km race time by +4.8% on average, equivalent to 134 s with a confidence interval of 207 to 62 s. On the contrary, the low-carb diet impaired 10 km race times by -2.3% or -86 s with a confidence interval of -18 to -144 s. In non-nerd-speak, Supernova 2 showed that carb-munchers get faster, some waaaaay faster, while keto folks get slower.
Since the Supernova data are publicly available, I was able to do some further analyses. An athlete’s expected chance of having an increase in 10 km race performance is 75 in every 100 people on a high-carb diet and just 10 in every 100 on a low-carb diet, while the chance of a decrease in race performance is 25 in every 100 on a high-carb diet and a massive 90 in every 100 on a low-carb diet. This means that an athlete’s relative risk of worsening their performance following a 3-week intensive training block when following a low-carb ketogenic diet (vs. a high-carb diet) is 4, i.e. they are 4-times more likely to get worse. Whereas athletes following a high-carb diet during a 3-week training camp are 8-times more likely to improve their performance than if they ate a low-carb ketogenic diet.
Such findings were recently confirmed in runners, in whom a 4-week low-carb ketogenic diet impaired running economy (causing them to use more energy and consume more oxygen at submaximal speeds). And, while run time-to-exhaustion at 70% VO2max was not different following a 4-week low-carb or high-carb diet, the authors documented that the ketogenic diet caused a larger range in the change in endurance capacity and greatly increased the chance of decrement in endurance capacity.
Yes, a low-carb diet does work for some but, like the mother of famous ultra-runner Forrest Gump once said, “a low-carb diet is like a box of chocolates... you never know what you’re gonna get”.
And, to put the icing on the cake, in 2020, three systematic reviews of this field were published, concluding that,
“a ketogenic diet does not have a positive or negative impact on physical performance”
and
“findings prohibit definitive conclusions regarding efficacy of the endurance athlete’s ketogenic diet for performance benefit”
and that
“available knowledge demonstrates no clear performance benefit to athletes following a ketogenic diet, with some benefit shown mainly in short duration, vigorous-intensity tests, when weight loss was likely a confounding variable. While many of the trials provided no performance benefit, it is important to note that a ketogenic diet often did not cause a performance decrement, particularly in recreationally trained athletes”.
Now you are ready to tackle that burning question:
In comparison, our bodily store of glucose is very small — about 500 grams (~4 grams circulating in the blood, plus ~100 grams in liver glycogen, and ~400 grams in muscle glycogen), enough to produce around 2000 kcals of energy. This can be munched through pretty quickly during exercise; about 2-hours.
These simple facts make it very clear that it is favourable for every athlete to be able to spare their glucose stores for as long as possible by tapping into their massive fat store because it will never run out during a session or a race.
But, while a high fat oxidation rate during exercise is predictive of an athlete’s ability to perform, it only explains a small part (~12%) of their performance even in ultra-distance races. And, as you also know, fat produces energy more slowly than glucose and is less economical than glucose, meaning that as you run faster (or, more accurately, as you run at a higher fraction of your maximal aerobic capacity, when oxygen delivery to muscles becomes a limiting factor), your muscles will use more glucose and less fatty acids to produce energy.
If you race to compete, most endurance races require that you operate at a high fraction of your maximal aerobic power for several minutes. For example, you will cover a 3000m race at approximately 100% of your velocity at VO2max, a 10 km race at around 90%, and a marathon at around 85% (give or take, depending on who you are and your abilities). So, when racing to compete, you will run at a high-intensity and the predominant fuel used to supply energy will be glucose — the more economical and more rapidly “burnt” fuel.
Consequently, training to compete is focussed on maximising your ability to oxidise carbohydrate when operating at fast speeds and high fractions of your maximal aerobic power.
So, for those of you who train and race to compete — i.e. training to race as fast as you can over a certain distance — on race day the clock is ticking and prizes are for the first person across the line. This also includes folks who truly race to smash ultra-distance races — just watch Jim Walmsley attacking Western States from start-to-tape or Pau Capel “givin’ it large” at UTMB. There are no medals for who produces the highest fat oxidation rate during the race; being able to produce as high a speed as possible and to operate at as high a fraction of your maximal aerobic capacity for as long as possible are key! To do so, carbohydrate is your speedy friend with whom you train with pretty regularly. Because having glycogen stores full will keep you ready to support your high energy demands and produce high power outputs when you need to.
But…
Many folks don’t care about running economy or training to race as fast as they can over a certain distance. Your goal might be to complete, not compete, a race. Others among you may only be interested in the enjoyment of being able to sustain long, slow plods. Some of you might even be interested in completing ultra-distance races (not smashing them like Jim Walmsley). If you fall into any of those camps, being able to sustain as high a fraction of your maximal aerobic capacity as possible is less important. In that case, carbohydrate could be a friend you see from time to time. When racing at slower speeds or, more accurately, at lower fractions of your maximal aerobic capacity, your muscles have a lesser need to oxidise carbohydrate and can more-greatly rely on the less rapidly “burnt” and abundantly-stored fuel — fat — to produce energy. If you are just trying to “complete” a marathon or ultra-distance event, you might only be moving at 50 to 60% of your maximal aerobic capacity; so, your training only needs to prepare you to sustain a low-intensity and less carbohydrate-demanding speed. This could be achieved with a low-carbohydrate (possible ketogenic) diet. But, also remember that training to complete a race is also easily achieved eating simply and healthily with a moderate or high amount of carbohydrate in your daily diet.
It is, in essence, actually quite simple. If you want to choose between low- and high-carbohydrate availability, learn to understand your context — is carbohydrate your speedy friend with whom you need to train with regularly, or is carbohydrate a friend you only need to train with from time to time? Consider your goals and what is important to you, make a cost-benefit analysis, and you will know what to do.
It is common to hear endurance athletes ask, “Will you get faster on a low-carb diet?”. This was eloquently-debated by Tommy and Nick the Bubble in Lock Stock and Two Smoking Barrels.
My point being that there is no binary answer to, “is a high-carb diet better than a low-carb diet?”.
In this series, I have discussed many different routes to increasing fat oxidation rates during exercise by reducing your carbohydrate availability:
Train low — a glycogen-depleting session in the morning followed by another session later the same day with no (or low) carbohydrate intake in between the sessions.
Sleep low — an evening session followed by no (or low) carbohydrate intake at dinner, followed by a pre-breakfast session the following morning.
Live low — daily low-carb intake with either a low-carb ketogenic diet (less than 50 grams of carbs per day), a high-fat diet (80% of daily calories coming from fat), or a non-ketogenic low-carb, high-fat diet (15–20% of daily calories coming from carbs and 60–65% from fat. How very complex!
As an endurance athlete with new knowledge and perspective, you can re-formulate and ask yourself a better question, “how can I train smart with carbohydrates to perform like a freaking legend?”. You are now armed with multiple approaches to help you “fat adapt” by manipulating your carbohydrate availability — choose them wisely and keep training smart...
Image Copyright © Thomas Solomon. All rights reserved.
The first important knowledge bite to be armed with is that,
Low carbohydrate availability will probably increase your protein needs.
Low carbohydrate availability caused by a short-term low-carbohydrate diet, reduces whole-body net protein balance during and following a moderate-intensity workout due to increased whole-body protein oxidation, when compared to a short-term high carbohydrate diet. In other words, low carbohydrate availability causes more protein to be “burnt” to produce fuel during exercise and in the recovery period that follows — not good.Such findings have been replicated in runners. Amino acid oxidation was increased following a morning 10 km run when runners had “slept low” (overnight low carbohydrate availability induced by low-carbohydrate feeding) following a high-intensity 10 ✕ 5-min interval session the evening before — also not good.
Collectively, these findings indicate that endurance athletes who train with a low-carbohydrate availability burn more protein during exercise and may, therefore, have increased dietary protein requirements. So, athletes who choose to engage in low-carb dietary practices should be aware of their increased need for daily dietary protein; something you learned all about if you caught my protein article.
The next important knowledge bite to be armed with is to,
Beware of anecdotes.
Anecdotes can be enticing. But anecdotes can also be misleading. And some anecdotes are just nuts.Evan Dunfee was one of the world-class athletes enrolled in the first Supernova study who chose to participate on the low-carb/high-fat ketogenic diet. Two weeks after completing the study he had a performance boom, breaking the Canadian 50 km record by 4-minutes! Within the next year, he came 4th at the Olympics, after being only a top-20 contender prior to his participation in Supernova. An anecdotal success that is easy for some folks to assign to “being keto” (for 3-weeks… yes, he did not continue “being keto” after completing the study). And, he was just one of 11 others who completed the study in the low-carb diet group, none of whom had the same performance boom. Dunfee himself does not attribute his success to his “being keto”, he admits he was also training harder in the Supernova camp while surrounded by other world-class athletes — this is highly-motivating — and he believes that because training hard while being “keto” was super tough, it made him appreciate how to train hard thereafter. So, for him, the low-carb diet wasn’t the ergogenic aid but it was the nudge he needed to train harder leading up to the Olympics.
Enticing.
In 2016, Chris Froome tweeted a pic of his low-carb breakfast on a rest day en route to his win at the Tour de France. His sports nutritionist at the time was Prof James Morton, brains behind the “Fuel for the work required” framework, so it was highly unlikely Froome fuelled his win at Le Tour that year on a low-carb diet. But, that didn’t matter because Froome’s avocado and eggs got out and some folks lost their minds, resulting in terrible news stories like, “Low-carb diet propelled Chris Froome to three Tour de France titles”. I’d be curious to know how many cyclists “went keto” as a result of that. Then I wonder how many of them aborted being “keto” when they discovered his actual race diet plan, fully-documented on BBC and openly-discussed on Fuel The Pedal en route to his win at the 2017 Giro D’Italia, where Froome ate more than 1000 grams of carbohydrate on some days — far from low-carb!
Misleading.
I recently listened to an interview of the former multiple Ironman champion-turned-coach, Dave Scott on the Human Performance Outliers podcast during which he said two things that got my neurons firing in ways that made my facial capillaries dilate. First, “If all athletes switched [to a low-carb diet] they would no doubt see a performance increase” and then, “Look at these former Olympians who are now diabetic” [because of high-carb diets]. These sweeping statements are not scientific evaluations but opinions that confirm his predetermined bias. You could indeed try telling Eliud Kipchoge, Peres Jepchirchir, Jim Walmsley, Camille Heron, and Kilian Jornet that they should switch to a low-carb diet because they will “no doubt see a performance increase”. Sure. You could try that. You could also try telling Olympians to stop eating carbs because just “look at these former Olympians who are now diabetic”. Sure. You could. But, I spent 17 years as a researcher in that very field and there is zero evidence linking an endurance athlete’s diet and their risk of developing diabetes.
Just nuts.
The best way to guard yourself against anecdotes is to heed what you learned as a child, “don’t take candy from a stranger”.
But some anecdotes are useful.
In the spring of 2020, the ultra runner, Michael McKnight, ran 100-miles without eating in 18:40. Impressive, right? Yes, of course, it is. But, let’s think for a second... 18:40 is about 2-hours slower than McKnight’s best 100-miler of 16:32, during which he maintained high carbohydrate availability by regularly feeding, and nearly 7-hours slower than the 100-mile world record of 11:19.This is an opportune moment to introduce another ultra distance dude, Zack Bitter. Mr Bitter runs speedy marathons and currently holds the 100-mile world record of 11:19. He is a low-carb “keto” guy who is also among the more pragmatic adopters of such dietary approaches — you can listen to him break down his own ultra training in episode 212 of his Human Performance Outliers podcast — he habitually eats a low-carbohydrate diet except when his training load is very high and except when he is leading into a race or during a race, when he uses carbohydrates as a fuel.
When Jim Walmsley and Ellie Greenwood smashed the Western States 100-mile records in 14:09 and 16:47 (a race which has more than 6,000 metres of vertical gain), they were not being “keto” or fasted nor did they do so without consuming any calories during the race. Their records were achieved with high carbohydrate availability.
When Eliud Kichoge and Brigid Kosgei broke the marathon world records in 2:01 and 2:14, they too were not “keto” or fasted or avoiding calories during the race — quite the opposite — they broke the world records with high carbohydrate availability.
For the best on Earth, high performance over any distance appears to be dependent on high carbohydrate availability — moving fast while burning a mix of slow-burning, oxygen-guzzling fuel — fat — and fast-burning, economical fuel — glucose. Except for the “FatMax World Championships”, I know of no high-performance achievement pushing the frontiers of speed that has been nailed with low carbohydrate availability…
Useful anecdote.
So, the final important knowledge bite to be armed with is to,
Remember that carbohydrate is your speedy friend.
Remember what you have learned about energy storage and bioenergetics:
We humans store a lot of fat.
Fatty acids produce more energy (ATP) per gram than glucose.
Fatty acid metabolism is slower to produce energy than glucose.
Fatty acids are a less economical fuel than glucose, requiring more O2 to produce ATP, meaning glucose produces more ATP per litre of oxygen.
Bioenergetic processes are hardwired in our biochemistry — we cannot change them — we can only influence the relative proportions of fuels being used.
For Zach Bitter, Michael McKnight, Jim Walmsley, Ellie Greenwood, Eliud Kipchoge, and Brigit Kosgei, as well as any other endurance athlete trying to achieve their best times, being a fat-burning monster is essential.
One way to increase fat oxidation rates during exercise is to, quite simply, endurance train (remember: training causes adaptations that increases the amount of fat stored in your muscles, the capacity to deliver fatty acids to muscles, and the amount of fat-burning enzymes in your muscles).
… Another way is to run slowly (remember George Brooks’ crossover concept — at lower fractions of your maximal aerobic capacity, you preferentially burn fat and as you increase your intensity you increasingly rely on carbohydrates to fuel your movement).
… Or you can run before breakfast (i.e. train fasted — low liver glycogen).
… Or you can run having previously glycogen depleted (low muscle glycogen).
… Or you can habitually consume a low-carb/high-fat diet (low muscle glycogen and increased fat-burning enzymes). Many routes to the same goal.
But to be the fastest endurance athlete you can be, in addition to being a fat-burning monster, you also need to be able to burn carbohydrate when you need it. Carbohydrate is your speedy friend.
“But low-carb works for me.”
That is fine, if it works.Anyone who has read the studies comparing the effect of high vs. low-carb diets on endurance performance will be well-versed in both the within-diet and the between-diet variability in performance outcomes.
Louise Burke’s study, Supernova 1, found that the improvement in 10 km race performance among athletes receiving a high-carb diet during the 3-week training camp was highly consistent: on average, there was a +6.6% improvement for which there was a 90% confidence interval of +4.1 to +9.1%. This means that, based on the observed data in the study, if all endurance athletes were studied (i.e. all folks on Earth that the study was trying to “sample”), 90% of plausible values for performance improvements would be somewhere between 4.1 and 9.1%. Meanwhile, the effect of the low-carb diet was highly-variable: the average change in performance in the study was −1.6% and the plausible range of values (the 90% confidence interval) was predicted to be between −8.5% and +5.3%.
In plain language, Supernova 1 showed that while some low-carb keto folks can improve their performance during an intense training block, most get worse and some get waaaaay worse! Meanwhile, all high-carb diet athletes are likely to get much faster.
If you’ve been following my series, you will know that Supernova was replicated…
Supernova 2 found that the high-carb diet during the training camp improved athletes’ 10 km race time by +4.8% on average, equivalent to 134 s with a confidence interval of 207 to 62 s. On the contrary, the low-carb diet impaired 10 km race times by -2.3% or -86 s with a confidence interval of -18 to -144 s. In non-nerd-speak, Supernova 2 showed that carb-munchers get faster, some waaaaay faster, while keto folks get slower.
Since the Supernova data are publicly available, I was able to do some further analyses. An athlete’s expected chance of having an increase in 10 km race performance is 75 in every 100 people on a high-carb diet and just 10 in every 100 on a low-carb diet, while the chance of a decrease in race performance is 25 in every 100 on a high-carb diet and a massive 90 in every 100 on a low-carb diet. This means that an athlete’s relative risk of worsening their performance following a 3-week intensive training block when following a low-carb ketogenic diet (vs. a high-carb diet) is 4, i.e. they are 4-times more likely to get worse. Whereas athletes following a high-carb diet during a 3-week training camp are 8-times more likely to improve their performance than if they ate a low-carb ketogenic diet.
Such findings were recently confirmed in runners, in whom a 4-week low-carb ketogenic diet impaired running economy (causing them to use more energy and consume more oxygen at submaximal speeds). And, while run time-to-exhaustion at 70% VO2max was not different following a 4-week low-carb or high-carb diet, the authors documented that the ketogenic diet caused a larger range in the change in endurance capacity and greatly increased the chance of decrement in endurance capacity.
Yes, a low-carb diet does work for some but, like the mother of famous ultra-runner Forrest Gump once said, “a low-carb diet is like a box of chocolates... you never know what you’re gonna get”.
And, to put the icing on the cake, in 2020, three systematic reviews of this field were published, concluding that,
“a ketogenic diet does not have a positive or negative impact on physical performance” and
“findings prohibit definitive conclusions regarding efficacy of the endurance athlete’s ketogenic diet for performance benefit” and that
“available knowledge demonstrates no clear performance benefit to athletes following a ketogenic diet, with some benefit shown mainly in short duration, vigorous-intensity tests, when weight loss was likely a confounding variable. While many of the trials provided no performance benefit, it is important to note that a ketogenic diet often did not cause a performance decrement, particularly in recreationally trained athletes”.
×
![]()
Now you are ready to tackle that burning question:
For endurance athletes, is a high-carb diet better than a low-carb diet?
As you are now well-aware, we humans, even the leanest of us, store a lot of fat — about 10 kg in a 65 kg person with 15% body fat, enough to produce around 100,000 kcals of energy. That is enough energy to run for about 4-days straight or enough to sit down doing nothing for about 50-days.In comparison, our bodily store of glucose is very small — about 500 grams (~4 grams circulating in the blood, plus ~100 grams in liver glycogen, and ~400 grams in muscle glycogen), enough to produce around 2000 kcals of energy. This can be munched through pretty quickly during exercise; about 2-hours.
These simple facts make it very clear that it is favourable for every athlete to be able to spare their glucose stores for as long as possible by tapping into their massive fat store because it will never run out during a session or a race.
But, while a high fat oxidation rate during exercise is predictive of an athlete’s ability to perform, it only explains a small part (~12%) of their performance even in ultra-distance races. And, as you also know, fat produces energy more slowly than glucose and is less economical than glucose, meaning that as you run faster (or, more accurately, as you run at a higher fraction of your maximal aerobic capacity, when oxygen delivery to muscles becomes a limiting factor), your muscles will use more glucose and less fatty acids to produce energy.
If you race to compete, most endurance races require that you operate at a high fraction of your maximal aerobic power for several minutes. For example, you will cover a 3000m race at approximately 100% of your velocity at VO2max, a 10 km race at around 90%, and a marathon at around 85% (give or take, depending on who you are and your abilities). So, when racing to compete, you will run at a high-intensity and the predominant fuel used to supply energy will be glucose — the more economical and more rapidly “burnt” fuel.
Consequently, training to compete is focussed on maximising your ability to oxidise carbohydrate when operating at fast speeds and high fractions of your maximal aerobic power.
So, for those of you who train and race to compete — i.e. training to race as fast as you can over a certain distance — on race day the clock is ticking and prizes are for the first person across the line. This also includes folks who truly race to smash ultra-distance races — just watch Jim Walmsley attacking Western States from start-to-tape or Pau Capel “givin’ it large” at UTMB. There are no medals for who produces the highest fat oxidation rate during the race; being able to produce as high a speed as possible and to operate at as high a fraction of your maximal aerobic capacity for as long as possible are key! To do so, carbohydrate is your speedy friend with whom you train with pretty regularly. Because having glycogen stores full will keep you ready to support your high energy demands and produce high power outputs when you need to.
But…
Many folks don’t care about running economy or training to race as fast as they can over a certain distance. Your goal might be to complete, not compete, a race. Others among you may only be interested in the enjoyment of being able to sustain long, slow plods. Some of you might even be interested in completing ultra-distance races (not smashing them like Jim Walmsley). If you fall into any of those camps, being able to sustain as high a fraction of your maximal aerobic capacity as possible is less important. In that case, carbohydrate could be a friend you see from time to time. When racing at slower speeds or, more accurately, at lower fractions of your maximal aerobic capacity, your muscles have a lesser need to oxidise carbohydrate and can more-greatly rely on the less rapidly “burnt” and abundantly-stored fuel — fat — to produce energy. If you are just trying to “complete” a marathon or ultra-distance event, you might only be moving at 50 to 60% of your maximal aerobic capacity; so, your training only needs to prepare you to sustain a low-intensity and less carbohydrate-demanding speed. This could be achieved with a low-carbohydrate (possible ketogenic) diet. But, also remember that training to complete a race is also easily achieved eating simply and healthily with a moderate or high amount of carbohydrate in your daily diet.
It is, in essence, actually quite simple. If you want to choose between low- and high-carbohydrate availability, learn to understand your context — is carbohydrate your speedy friend with whom you need to train with regularly, or is carbohydrate a friend you only need to train with from time to time? Consider your goals and what is important to you, make a cost-benefit analysis, and you will know what to do.
What can you add to your training toolbox?
There are no such things as bad questions but there are questions that need to be reformulated.It is common to hear endurance athletes ask, “Will you get faster on a low-carb diet?”. This was eloquently-debated by Tommy and Nick the Bubble in Lock Stock and Two Smoking Barrels.
Tommy: “Ahh, no can do.”
Nick the Greek: “What's that? A place near Kathmandu? Meet me halfway, mate.”
And, that they did…
Nick the Greek: “What's that? A place near Kathmandu? Meet me halfway, mate.”
My point being that there is no binary answer to, “is a high-carb diet better than a low-carb diet?”.
In this series, I have discussed many different routes to increasing fat oxidation rates during exercise by reducing your carbohydrate availability:
Train low — a glycogen-depleting session in the morning followed by another session later the same day with no (or low) carbohydrate intake in between the sessions.
Sleep low — an evening session followed by no (or low) carbohydrate intake at dinner, followed by a pre-breakfast session the following morning.
Live low — daily low-carb intake with either a low-carb ketogenic diet (less than 50 grams of carbs per day), a high-fat diet (80% of daily calories coming from fat), or a non-ketogenic low-carb, high-fat diet (15–20% of daily calories coming from carbs and 60–65% from fat. How very complex!
×
![]()
Disclaimer: I occasionally mention brands and products but it is important to know that I am not affiliated with, sponsored by, an ambassador for, or receiving advertisement royalties from any brands. I have conducted biomedical research for which I have received research money from publicly-funded national research councils and medical charities, and also from private companies, including Novo Nordisk Foundation, AstraZeneca, Amylin, A.P. Møller Foundation, and Augustinus Foundation. I’ve also consulted for Boost Treadmills and Gu Energy on their research and innovation grant applications and I’ve provided research and science writing services for Examine — some of my articles contain links to information provided by Examine but I do not receive any royalties or bonuses from those links. These companies had no control over the research design, data analysis, or publication outcomes of my work. Any recommendations I make are, and always will be, based on my own views and opinions shaped by the evidence available. My recommendations have never and will never be influenced by affiliations, sponsorships, advertisement royalties, etc. The information I provide is not medical advice. Before making any changes to your habits of daily living based on any information I provide, always ensure it is safe for you to do so and consult your doctor if you are unsure.
If you find value in this free content, please help keep it alive and buy me a beer:
Buy me a beer.
Share this post on your social media:
Want free exercise science education delivered to your inbox? Join the 100s of other athletes, coaches, students, scientists, & clinicians and sign up here:
About the author:
I am Thomas Solomon and I'm passionate about relaying accurate and clear scientific information to the masses to help folks meet their fitness and performance goals. I hold a BSc in Biochemistry and a PhD in Exercise Science and am an ACSM-certified Exercise Physiologist and Personal Trainer, a VDOT-certified Distance running coach, and a Registered Nutritionist. Since 2002, I have conducted biomedical research in exercise and nutrition and have taught and led university courses in exercise physiology, nutrition, biochemistry, and molecular medicine. My work is published in over 80 peer-reviewed medical journal publications and I have delivered more than 50 conference presentations & invited talks at universities and medical societies. I have coached and provided training plans for truck-loads of athletes, have competed at a high level in running, cycling, and obstacle course racing, and continue to run, ride, ski, hike, lift, and climb as much as my ageing body will allow. To stay on top of scientific developments, I consult for scientists, participate in journal clubs, peer-review papers for medical journals, and I invest every Friday in reading what new delights have spawned onto PubMed. In my spare time, I hunt for phenomenal mountain views to capture through the lens, boulder problems to solve, and for new craft beers to drink with the goal of sending my gustatory system into a hullabaloo.
Copyright © Thomas Solomon. All rights reserved.
I am Thomas Solomon and I'm passionate about relaying accurate and clear scientific information to the masses to help folks meet their fitness and performance goals. I hold a BSc in Biochemistry and a PhD in Exercise Science and am an ACSM-certified Exercise Physiologist and Personal Trainer, a VDOT-certified Distance running coach, and a Registered Nutritionist. Since 2002, I have conducted biomedical research in exercise and nutrition and have taught and led university courses in exercise physiology, nutrition, biochemistry, and molecular medicine. My work is published in over 80 peer-reviewed medical journal publications and I have delivered more than 50 conference presentations & invited talks at universities and medical societies. I have coached and provided training plans for truck-loads of athletes, have competed at a high level in running, cycling, and obstacle course racing, and continue to run, ride, ski, hike, lift, and climb as much as my ageing body will allow. To stay on top of scientific developments, I consult for scientists, participate in journal clubs, peer-review papers for medical journals, and I invest every Friday in reading what new delights have spawned onto PubMed. In my spare time, I hunt for phenomenal mountain views to capture through the lens, boulder problems to solve, and for new craft beers to drink with the goal of sending my gustatory system into a hullabaloo.
Copyright © Thomas Solomon. All rights reserved.