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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 3 of 6:
100 years of low-carb high-fat diets and exercise.
Thomas Solomon PhD.
5th Dec 2020.
So far in this series, you have learned what fat oxidation rates are as well as how and why you might boost them using acute manipulations to your training. Stay with me for another instalment as I throw your brain into the never-ending story that is the fatty mess of low-carbohydrate diets.
Reading time ~18-mins (3700-words).
Or listen to the Podcast version.
Or listen to the Podcast version.
Recent hype and social media glamour will have you believe that a low-carbohydrate diet is a performance athlete’s dream as well as a cure for every disease. But you are smarter than simply believing anecdotes and making a complete lifestyle overhaul based on them.
As I discussed in my last post, doing sessions with a low carbohydrate availability — train low — or going to bed with a low carbohydrate availability — sleep low — are two of the simplest ways you can help increase “fat-burning” during your sessions while also promoting adaptations following your sessions. But a slightly trickier and more aggressive approach is to chronically reduce carbohydrate availability by adhering to a low-carbohydrate, high-fat diet.
Before even considering what such a diet can do for you, ask yourself this:
The purpose of adhering to a low-carbohydrate diet is to reduce carbohydrate availability so that your muscle glycogen stores are always low and so your reliance on fat oxidation is increased during exercise. To do so, you must ensure that you actually are consuming a diet that is low in carbohydrate.
You will probably be familiar with popular info that breaks down diets into percentages of calories (kcals) derived from various macronutrients. A diet that provides 50% of energy (kcals) from carbohydrates, 30% from fat and 20% from protein sounds pretty standard. But the total grams of carbohydrate eaten can be wildly different if the total amount of calories is increased.
Consider two diets, A and B, both providing 50% of energy from carbs, 30% from fat, and 20% from protein. But, diet A is a 3000 kcal diet while diet B is a 2000 kcal diet.
In this case, diet A provides:
The current evidence indicates that to maintain a high carbohydrate availability, you need to eat 5 to 7 grams per kg bodyweight per day of carbohydrate if engaged in a moderate training load, 6 to 10 g/kg/day during heavy training loads, and 8 to 12 g/kg/day during extreme training loads. But, when aiming to create a low carbohydrate availability with a diet, the recommended goal is to eat less than 150 grams of carbs per day to keep muscle glycogen levels low.
If we stay with percentages for a second, “low-carb diets” are often flipped on their heads and called “high-fat diets”, which typically aim to provide at least 60% of energy from fat.
Now let’s consider diet A, the 3000 kcal diet, again while keeping relative protein intake the same but pushing fat up to 60% of kcals.
Diet A 2.0 now provides:
My point here is to be wary of percentages and, should you choose to go down the low-carb diet rabbit hole for creating a low carbohydrate availability, learn to think of your macronutrient intake in daily grams rather than percentages and aim for a low-carb high-fat intake, while always remembering to maintain adequate protein.
So, a low-carb diet is a numbers game — make sure that you know what “low-carb high-fat” specifically means in the context of your total energy intake and in the context of your daily needs, relative to your training load. (For help with macronutrient goals for athletes, you can use my reference tables as a guide, which contains info extracted from the Nutrition and Athletic Performance joint position statement from the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine.)
When removing a macronutrient, this can be challenging.
If you do choose to adopt a low-carbohydrate diet to support your training as a high-performance athlete, there are three critical rules to remember:
Do not jeopardise your energy availability by lowering your total energy intake. By dramatically lowering your carbohydrate intake, by extension, you will also be lowering your total energy intake, i.e. your calorie intake. So, when reducing carb calories, you must increase calorie intake from other macronutrients in order to maintain appropriate energy availability. As an athlete who trains smart, your daily protein intake should already be optimal, so you would typically maintain appropriate energy availability by increasing your fat intake. This way, you will stay healthy and weight stable.
Do not jeopardise your protein intake. If you typically ate protein-containing foods with your carbohydrate-containing meals, don’t cut out both when carb restricting. Protein is a hugely important tool in your recovery and adaptation toolbox, as I have discussed in great detail in a previous post. As I will later discuss, you might even need to increase your protein calories when adhering to a low-carb diet.
Do not jeopardise your micronutrient intake. Many grains contain vital vitamins and minerals. Many cereals are fortified with vitamins. You will need to ensure that you do not become deficient. No matter what dietary practice you wish to follow, healthy eating is key, as I have discussed before.
So, a low-carbohydrate ketogenic diet is also a numbers game and, if you do go down the low-carbohydrate trail, keto or not, make sure you do it sensibly and healthily.
Note: if you have diabetes, you must work closely with your doctor before considering any sudden alteration in your diet. A low carbohydrate ketogenic diet can be used successfully in the management of diabetes but it can also increase the risk of hypoglycemia, especially in athletes. So don’t go fishing in the dark — always seek expert clinical guidance before making changes to your disease management.
Eskimo habitually eat a diet of solely fish — a high fat, high protein, close-to-no-carb diet. Many notable stories also document explorers surviving on meat rations (fat and protein) for several weeks, often adopting Eskimo practices. Perhaps the most famous is Lieutenant Frederick Schwatka who went in search of Franklin’s lost party in the Arctic in 1878. (If interested, I can also recommend the excellent Polar review by Ronald Savitt.)
Explorers surviving on a high-fat, high-protein diet do so in harsh conditions but also while engaged in high-volume but low-intensity daily exercise. In the context of endurance performance, you might relate this to the racing demands of ultra-distance athletes.
Like Eskimo and explorers, some famous ultra racers also document “surviving” with few carbs in their diet. One such example is Zack Bitter, a ketogenic chap who runs speedy marathons and currently holds the 100-mile world record in 11:19. Impressive. Yes, indeed he is, but let’s not forget an important caveat: Mr Bitter is very open about the fact that he also loads his diet with carbohydrate leading into his race days and consumes epic amounts of carbohydrate during his races (you can listen to him break down his own ultra training in his Human Performance Outliers podcast, episode 212). And, there is Michael McKnight, the ultra dude who ran 100-miles with no food in 18:40. By the power of Grayskull, that is impressive.
More on these chaps and the nuances of their performances later but, for now, my point is that, yes, humans can survive with few carbs in their diet.
So, how does a chronic low-carb, high-fat diet fit in with what we are learning about exercise energy metabolism and all that jazz? Well, we first need to go back in time...
One of the advantages to living in Denmark for 6-years and working at the University of Copenhagen was being in an environment created by the legacy of the 1920 Nobel Laureate, August Krogh and his colleague Johannes Lindhard. Their Carlsberg-funded work published in 1920, provided comprehensive evidence that recent diet “probably” influences metabolic fuel choice during low-to-moderate intensity exercise as well as people’s ability to perform exercise — confirming some observations made at the turn of the 20th century. August Krogh’s data from 100-years ago showed that people following a low-carb/high-fat diet for 3-days caused RER values during exercise to drop to ~0.8 (indicating high fat oxidation rates and low carbohydrate oxidation rates) compared to 0.95 (low fat oxidation, high carb oxidation) when participants had followed a 3-day high-carb diet. In other words, during low- to moderate-intensity exercise, subjects burned the fuel that was most “available”. Furthermore, participants reported more fatigue during a 2-hour low- to moderate-intensity ride after a 3-day low-carb/high-fat diet compared to when they had followed a high-carb diet. In the summary statements of their 73-page epic, Krogh and Lindhart wrote “All the series agree in showing that work is more economically performed on carbohydrate than on fat. When the work was sufficiently severe, the subjects performed it with greater difficulty on fat than on carbohydrate and became much more tired”.
They wouldn't have guessed it then but many moons on, their work was laying the foundation for the “fueling for the work required” framework, the poetic theory from Prof James Morton on the best approach for optimising endurance training adaptations (which I discussed in my last post).
In 1939, Erik Christensen and Ole Hansen from that same lab in Copenhagen published another seminal (and Carlsberg-funded) paper that lends more credence to the idea thatbeer is awesome a short-term period of low-carb/high-fat feeding may impair performance. (The paper was published in German, so I had to get some help reading this one from my German-speaking wife). “Probably” the best thing about the study was that the participants actually drank Carlsberg for lunch! But, on a serious note, three trained men rode at ~177 watts until fatigue following a 3-day diet providing an average, high, or low-amount of carbohydrate. RER during exercise was always higher following the high-carb than the low-carb/high-fat diet, and all three men rode for longer following the high-carb diet.
Since Christensen and Hansen also documented that blood sugar gradually dropped during prolonged hard work, leading to hypoglycemia (low blood sugar levels), they published a follow-up in 1939 showing that provision of oral glucose prior to or during exercise prevented hypoglycemia developing during exercise. This sounds novel for 1939, but they were actually 15-years behind because at the Boston marathon in 1923, a group at Harvard Medical School had already noted hypoglycemia in some competitors at the finish line and decided to intervene at the following year’s race by providing competitors with a high-carb diet during training, a large amount of carbohydrate 24-hours before the race, and glucose candy during the race. The effect? High carbohydrate availability prevented hypoglycemia and improved finish times.
When you remind yourself that these observations were made 100-years ago, I often find it amusing how often we debate dietary and nutritional strategies and, in doing so, over-complicate something that is inherently simple.
So, historical data showed that recent dietary intake influences fat oxidation rates during exercise, that low carbohydrate availability (low-carb/high-fat diet) impaired exercise capacity, and that high carbohydrate availability (high-carb diet plus pre-race carb-loading plus during-race carb provision during) improved exercise performance. But, 100-years on, where are we now?
In 1983, Phinney published the first of the two “classic” studies that are often touted as conclusive evidence that “ketogenic diets work for athletes”. The study examined the effects of a 4-week low-carb (less than 20 g/day) ketogenic study, this time without caloric restriction and in 5 endurance-trained cyclists. The low-carb diet lowered muscle glycogen levels but increased cycling time-to-exhaustion at 62-64% of VO2max with lower carbohydrate oxidation rates. Sounds promising but exercising to failure at 62% of VO2max is very likely below the first ventilatory threshold in trained cyclists and is, therefore, not particularly relevant for an athlete unless they are trying to escape a relentless but slow-moving zombie.
Fortunately, in 1994, Estelle Lambert in Tim Noakes’ lab upped the ante and habituated trained cyclists (but only 5 of them) to a low-carb and a high-carb diet for 2-weeks. Peak power was about the same between diets and time-to-exhaustion at 85% of peak power was not significantly different (despite being 4-minutes shorter, on average, in the low-carb trial), but cyclists rode for twice as long (80- vs. 43-mins) at 50% of their VO2max when habituated to the low-carb diet.
So, the story thus far is that a low-carb diet may cause adaptations that allow athletes to perform for longer when operating at a low- to moderate-intensity. But being able to operate for longer at a low- to moderate-intensity does not help inform training for an endurance athlete racing hard in a marathon or anything shorter.
Is there more evidence concerning higher-intensity exercise?
In 1998, Jørn Helge enrolled 15 folks into a 4-week endurance training intervention with a low-carb or a high-carb diet. The low-carb diet group burned more fat during exercise and had lower muscle glycogen levels but had the same improvement in cycling time-to-exhaustion at 80% of their VO2max as the high-carb group. But, the caveat was that the participants were new to exercise and were consequently adapting to two things — training for the first time and a diet change. Since we know that training has an incredibly powerful stimulus on fitness and performance, the training stimulus may override any potential dietary effect in folks who have never exercised before. So, this data also does not inform training for an endurance athlete.
This and the previously-discussed studies determined exercise performance following an overnight fast with no pre-test meal — not representative of race-day conditions — and performed the time-to-failure at a percentage of people’s pre-intervention VO2max not their newly-improved post-trained VO2max — if you get fitter but exercise at the same relative fraction of your old fitness, your workload will be lower meaning that it will be easier and you will go for longer. We also know that recent food intake massively influences fuel use during exercise and, given that low-carb diets typically decrease muscle glycogen levels when compared to a high-carb diet, you might be wondering:
What happens if you habituate to a low-carb diet but “top-up” muscle glycogen with a last-minute high-carb intake?
To help tease out the answer to this question, in 2000, Louise Burke provided a low-carb/high-fat diet to trained cyclists for 5-days followed by 1-day of high-carb intake before a performance test — a 2-hour ride at 70% VO2max followed by a 7 kJ/kg time-trial after an overnight fast and a no-calorie jelly breakfast (except subjects were not informed about the “no calorie” bit and therefore thought they were eating actual fuel). All the subjects also completed the test following 6-days of or a high-carb/low-fat diet. The outcome? Fat oxidation rates were higher and carb oxidation rates were lower during exercise in the high-fat trial, while muscle glycogen levels and time-trial performances were not different between trials. With this study, we learn a little more and with relevance to athletes, but the fasted time-trial after a 2-hour low-intensity ride is not representative of any race scenario.
So, we need a study with a performance test that actually resembles race day.
In 2006, Estelle Lambert attempted just that and led a more ecologically-relevant study in well-trained cyclists (but only 8 of them). She compared the effects of a 6-day low-carb/high fat diet followed by 1-day of high-carb feeding to a 7-day high-carb diet, finding impaired 1 km and 4 km sprint TT performance but not 100 km TT performance in the low-carb folks. Once again, fat oxidation rates were increased following the fat adaptation protocol. But, high-intensity performance was impaired. A clear pattern had begun to emerge and this prompted Louise Burke and Bente Kiens to write their "nail in the coffin" for low-carb fat adaptation training protocols editorial.
But, since 2006, we can all agree that the low-carb diet and exercise thing has been very popular news. Plus, since that time, there remained a lack of evidence concerning real-life race conditions and Burke followed with her 2015 "did we call the 'nail in the coffin' too soon?" review.
And perhaps we did, since there is an ongoing debate about the utility of high-fat diet keto-adaptation (chronic low-carbohydrate availability) protocols for endurance performance. The sentiments of which are well-summarized in two reviews, one penned by Jeff Volek, Tim Noakes, and Stephen Phinney, the other by Louise Burke.
But keep your wits about you...
The other “classic” study often gets tossed around as conclusive evidence that “ketogenic diets work for athletes”, is the 2015 FASTER study from Jeff Volek and Stephen Phinney and colleagues — “Faster” standing for Fat Adapted Substrate use in Trained Elite Runners. The study compared a group of elite ultra runners who ate a self-selected low-carb ketogenic diet to a separate group of elite ultra runners who ate a self-selected high-carb diet. When brought to the lab, the low-carb diet athletes had higher fat oxidation rates during a 3-hour run at 65% of VO2max than the high-carb runners. No performance test was conducted. I’ll leave you to make your own mind up as to whether that study usefully informs whether you should or should not be “keto” to be a better (ultra) runner.
Clue: it does not.
Many of the studies I have discussed have focussed on “fat-burning” but many of them have also measured the size of the muscles’ glucose store, finding that muscle glycogen is lower following a low-carb/high-fat diet. The other common finding is that adaptations of muscle enzyme levels during training change in favour of either preferential fat or preferential carbohydrate use during exercise depending on the type of habitual diet being eaten. One key observation is that low-carb/high-fat diets decrease muscle levels of pyruvate dehydrogenase (or PDH).
What the hell is PDH?
Well, after glucose is broken down to pyruvate through the steps of glycolysis, PDH is the key enzyme that allows pyruvate to enter the mitochondria to produce ATP. Therefore, a low-carb/high-fat diet causes an adaptation that reduces your muscle’s ability to use glucose as a fuel source. While this may not be a problem at low-intensities, at high-intensities (e.g. >80% VO2max), when you are trying to rock your socks off, you need glucose to rapidly and economically produce ATP to help you move fast — recalling what you learned about bioenergetics in my previous post — pound-for-pound, fat uses more oxygen to produce ATP than glucose and produces ATP at a slower rate than glucose.
Low-carb/high-fat diets increase fat oxidation rates during exercise at low-, moderate-, and high-intensities.
Low-carb/high-fat diets decrease muscle glycogen levels and reduce the levels of enzymes involved in glucose-burning metabolic pathways.
A low-carb/high-fat diet can improve endurance time-to-exhaustion at low-intensities (≤70% VO2max) — helping you evade a relentless but slow-moving zombie — but does not improve (and sometimes impairs) performance at high-intensities (≥80% VO2max) — you will not do well in a manic-zombie apocalypse.
There is often a highly-variable endurance performance response to a low-carb/high-fat diet.
The variability may partly result from the tiny numbers of participants included in most studies.
The performance tests are rarely ecologically-valid — they are typically conducted after an overnight fast and often involve a ride- or run-to-exhaustion at a fixed sub-maximal (often low-)intensity.
So, although having a higher maximal fat oxidation rate during exercise and achieving it at a higher fraction of your VO2max would be expected to improve performance, muscle glycogen levels are always decreased following a low-carb/high-fat diet unless a day of high-carb feeding replenishes glycogen on the day prior to performance testing. Therefore, even if you can spare glucose for longer after adapting to a low-carb diet, your muscles’ glucose store (glycogen) will be smaller and will therefore not last as long when it is called upon to move your ass more quickly.
It is also important to remember that increased fat oxidation rates during exercise are not synonymous with improved performance — as we already learned, max fat oxidation rates only explained 13% of performance outcomes in Ironman triathlon. Furthermore, increased fat oxidation rates are a natural consequence of low carbohydrate availability — when we move, energy (ATP) has to come from somewhere — if glycogen is low, we must burn fat.
Up to this point, given the evidence available, it is probably becoming quite evident that it is not possible to boldly conclude that a low-carb/high-fat diet offers a performance advantage to an endurance athlete.
But, to truly test the hypothesis, we need a well-controlled trial examining the effects of habituation to a low-carb vs. a high-carb diet on ecologically-valid outcomes (race performance) in a large group of highly-trained endurance athletes.
Thanks to Louise Burke and her team, we don’t have to wait… But you do, because that is the story for next time.
Until then, don’t make any drastic changes to your diet just yet; keep training smart and learn more soon...
As I discussed in my last post, doing sessions with a low carbohydrate availability — train low — or going to bed with a low carbohydrate availability — sleep low — are two of the simplest ways you can help increase “fat-burning” during your sessions while also promoting adaptations following your sessions. But a slightly trickier and more aggressive approach is to chronically reduce carbohydrate availability by adhering to a low-carbohydrate, high-fat diet.
Before even considering what such a diet can do for you, ask yourself this:
What is a low-carb diet?
First of all, remember something very important that you learnt last time — all training manipulations that are designed to increase fat oxidation rates reduce the amount of carbohydrate available for energy production. The same is true no matter whether you are acutely manipulating carbohydrate availability in and around your sessions or whether you are chronically reducing carbohydrate availability in your daily diet.The purpose of adhering to a low-carbohydrate diet is to reduce carbohydrate availability so that your muscle glycogen stores are always low and so your reliance on fat oxidation is increased during exercise. To do so, you must ensure that you actually are consuming a diet that is low in carbohydrate.
You will probably be familiar with popular info that breaks down diets into percentages of calories (kcals) derived from various macronutrients. A diet that provides 50% of energy (kcals) from carbohydrates, 30% from fat and 20% from protein sounds pretty standard. But the total grams of carbohydrate eaten can be wildly different if the total amount of calories is increased.
Consider two diets, A and B, both providing 50% of energy from carbs, 30% from fat, and 20% from protein. But, diet A is a 3000 kcal diet while diet B is a 2000 kcal diet.
In this case, diet A provides:
375 grams of carbs
(50% × 3000 kcals ÷ 4 kcals/gram)
100 grams of fat
(30% × 3000 kcals ÷ 9 kcals/gram)
and 150 grams of protein
(20% × 3000 kcals ÷ 4 kcals/gram)
Meanwhile, diet B provides:
(50% × 3000 kcals ÷ 4 kcals/gram)
100 grams of fat
(30% × 3000 kcals ÷ 9 kcals/gram)
and 150 grams of protein
(20% × 3000 kcals ÷ 4 kcals/gram)
250 grams of carbs
(50% × 2000 kcals ÷ 4 kcals/gram)
67 grams of fat
(30% × 2000 kcals ÷ 9 kcals/gram)
and 100 grams of protein
(20% × 2000 kcals ÷ 4 kcals/gram)
So, while the percentage of carb calories are identical for these two diets, their absolute carbohydrate content is very different — diet B has two-thirds the amount of carbs than diet A and is, therefore, lower in carbohydrate.
(50% × 2000 kcals ÷ 4 kcals/gram)
67 grams of fat
(30% × 2000 kcals ÷ 9 kcals/gram)
and 100 grams of protein
(20% × 2000 kcals ÷ 4 kcals/gram)
The current evidence indicates that to maintain a high carbohydrate availability, you need to eat 5 to 7 grams per kg bodyweight per day of carbohydrate if engaged in a moderate training load, 6 to 10 g/kg/day during heavy training loads, and 8 to 12 g/kg/day during extreme training loads. But, when aiming to create a low carbohydrate availability with a diet, the recommended goal is to eat less than 150 grams of carbs per day to keep muscle glycogen levels low.
If we stay with percentages for a second, “low-carb diets” are often flipped on their heads and called “high-fat diets”, which typically aim to provide at least 60% of energy from fat.
Now let’s consider diet A, the 3000 kcal diet, again while keeping relative protein intake the same but pushing fat up to 60% of kcals.
Diet A 2.0 now provides:
150 grams of carbs
(20% × 3000 kcals ÷ 4 kcals/gram)
200 grams of fat
(60% × 3000 kcals ÷ 9 kcals/gram)
and 150 grams of protein
(20% × 3000 kcals ÷ 4 kcals/gram)
The new 3000 kcal diet A with its 60% of kcals coming from fat meets is right on the money at 150 grams of carbs. But, if diet A had a larger total caloric intake, say 5000 kcals to meet the energy demands of an endurance athlete engaged in a high training load, then at 60% fat, this high-fat diet munching athlete would be eating carbohydrate in excess of the 150 grams per day goal — 200 grams to be precise (20% × 5000 kcals ÷ 4 kcals/gram) — and would not, therefore, actually be eating a low-carbohydrate diet.
(20% × 3000 kcals ÷ 4 kcals/gram)
200 grams of fat
(60% × 3000 kcals ÷ 9 kcals/gram)
and 150 grams of protein
(20% × 3000 kcals ÷ 4 kcals/gram)
My point here is to be wary of percentages and, should you choose to go down the low-carb diet rabbit hole for creating a low carbohydrate availability, learn to think of your macronutrient intake in daily grams rather than percentages and aim for a low-carb high-fat intake, while always remembering to maintain adequate protein.
So, a low-carb diet is a numbers game — make sure that you know what “low-carb high-fat” specifically means in the context of your total energy intake and in the context of your daily needs, relative to your training load. (For help with macronutrient goals for athletes, you can use my reference tables as a guide, which contains info extracted from the Nutrition and Athletic Performance joint position statement from the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine.)
But, what is a ketogenic diet? (aka, a keto diet)
Not all low-carbohydrate diets are created equally. Adhering to a low-carbohydrate diet does not mean you are in a state of ketosis, or “keto”. To achieve nutritional ketosis, that is raising blood ketones to around 1 to 2 mmol/L, you should aim to consume less than 50 grams per day of available carbohydrate (not including fibre). And, to stay healthy and weight stable, you must do this while consuming an appropriate total caloric amount to maintain a healthy energy availability.When removing a macronutrient, this can be challenging.
If you do choose to adopt a low-carbohydrate diet to support your training as a high-performance athlete, there are three critical rules to remember:
Do not jeopardise your energy availability by lowering your total energy intake. By dramatically lowering your carbohydrate intake, by extension, you will also be lowering your total energy intake, i.e. your calorie intake. So, when reducing carb calories, you must increase calorie intake from other macronutrients in order to maintain appropriate energy availability. As an athlete who trains smart, your daily protein intake should already be optimal, so you would typically maintain appropriate energy availability by increasing your fat intake. This way, you will stay healthy and weight stable.
Do not jeopardise your protein intake. If you typically ate protein-containing foods with your carbohydrate-containing meals, don’t cut out both when carb restricting. Protein is a hugely important tool in your recovery and adaptation toolbox, as I have discussed in great detail in a previous post. As I will later discuss, you might even need to increase your protein calories when adhering to a low-carb diet.
Do not jeopardise your micronutrient intake. Many grains contain vital vitamins and minerals. Many cereals are fortified with vitamins. You will need to ensure that you do not become deficient. No matter what dietary practice you wish to follow, healthy eating is key, as I have discussed before.
Note: if you have diabetes, you must work closely with your doctor before considering any sudden alteration in your diet. A low carbohydrate ketogenic diet can be used successfully in the management of diabetes but it can also increase the risk of hypoglycemia, especially in athletes. So don’t go fishing in the dark — always seek expert clinical guidance before making changes to your disease management.
Can humans survive with such few carbs in their diet?
Absolutely they can.Eskimo habitually eat a diet of solely fish — a high fat, high protein, close-to-no-carb diet. Many notable stories also document explorers surviving on meat rations (fat and protein) for several weeks, often adopting Eskimo practices. Perhaps the most famous is Lieutenant Frederick Schwatka who went in search of Franklin’s lost party in the Arctic in 1878. (If interested, I can also recommend the excellent Polar review by Ronald Savitt.)
Explorers surviving on a high-fat, high-protein diet do so in harsh conditions but also while engaged in high-volume but low-intensity daily exercise. In the context of endurance performance, you might relate this to the racing demands of ultra-distance athletes.
Like Eskimo and explorers, some famous ultra racers also document “surviving” with few carbs in their diet. One such example is Zack Bitter, a ketogenic chap who runs speedy marathons and currently holds the 100-mile world record in 11:19. Impressive. Yes, indeed he is, but let’s not forget an important caveat: Mr Bitter is very open about the fact that he also loads his diet with carbohydrate leading into his race days and consumes epic amounts of carbohydrate during his races (you can listen to him break down his own ultra training in his Human Performance Outliers podcast, episode 212). And, there is Michael McKnight, the ultra dude who ran 100-miles with no food in 18:40. By the power of Grayskull, that is impressive.
More on these chaps and the nuances of their performances later but, for now, my point is that, yes, humans can survive with few carbs in their diet.
So, how does a chronic low-carb, high-fat diet fit in with what we are learning about exercise energy metabolism and all that jazz? Well, we first need to go back in time...
Historical legends delved into this field 100-years ago.
Having taught undergrad and postgrad courses at several universities and having supervised truckloads of bachelor, masters, and PhD students through their theses, I have made one clear observation: the older I get, the better the younger generation is at staying on top of emerging data but the poorer is their recollection of historical data. When discussing this topic, younger folks are often blown away when I pull a paper out of my metaphorical arse that was published before their great grandparents were born — the effects of low-carb/high-fat feeding on exercise metabolism were studied many moons ago.One of the advantages to living in Denmark for 6-years and working at the University of Copenhagen was being in an environment created by the legacy of the 1920 Nobel Laureate, August Krogh and his colleague Johannes Lindhard. Their Carlsberg-funded work published in 1920, provided comprehensive evidence that recent diet “probably” influences metabolic fuel choice during low-to-moderate intensity exercise as well as people’s ability to perform exercise — confirming some observations made at the turn of the 20th century. August Krogh’s data from 100-years ago showed that people following a low-carb/high-fat diet for 3-days caused RER values during exercise to drop to ~0.8 (indicating high fat oxidation rates and low carbohydrate oxidation rates) compared to 0.95 (low fat oxidation, high carb oxidation) when participants had followed a 3-day high-carb diet. In other words, during low- to moderate-intensity exercise, subjects burned the fuel that was most “available”. Furthermore, participants reported more fatigue during a 2-hour low- to moderate-intensity ride after a 3-day low-carb/high-fat diet compared to when they had followed a high-carb diet. In the summary statements of their 73-page epic, Krogh and Lindhart wrote “All the series agree in showing that work is more economically performed on carbohydrate than on fat. When the work was sufficiently severe, the subjects performed it with greater difficulty on fat than on carbohydrate and became much more tired”.
They wouldn't have guessed it then but many moons on, their work was laying the foundation for the “fueling for the work required” framework, the poetic theory from Prof James Morton on the best approach for optimising endurance training adaptations (which I discussed in my last post).
In 1939, Erik Christensen and Ole Hansen from that same lab in Copenhagen published another seminal (and Carlsberg-funded) paper that lends more credence to the idea that
Since Christensen and Hansen also documented that blood sugar gradually dropped during prolonged hard work, leading to hypoglycemia (low blood sugar levels), they published a follow-up in 1939 showing that provision of oral glucose prior to or during exercise prevented hypoglycemia developing during exercise. This sounds novel for 1939, but they were actually 15-years behind because at the Boston marathon in 1923, a group at Harvard Medical School had already noted hypoglycemia in some competitors at the finish line and decided to intervene at the following year’s race by providing competitors with a high-carb diet during training, a large amount of carbohydrate 24-hours before the race, and glucose candy during the race. The effect? High carbohydrate availability prevented hypoglycemia and improved finish times.
When you remind yourself that these observations were made 100-years ago, I often find it amusing how often we debate dietary and nutritional strategies and, in doing so, over-complicate something that is inherently simple.
So, historical data showed that recent dietary intake influences fat oxidation rates during exercise, that low carbohydrate availability (low-carb/high-fat diet) impaired exercise capacity, and that high carbohydrate availability (high-carb diet plus pre-race carb-loading plus during-race carb provision during) improved exercise performance. But, 100-years on, where are we now?
The modern-day story of low-carb, high-fat diets.
Until 1980, low-carb diet and exercise interventions were short, lasting only 3- to 5-days — long-term studies were needed. Then stepped in Stephen Phinney who, in 1980, demonstrated for the first time that a long-term (6-week) low-carb (less than 10 g/day) ketogenic diet could improve endurance performance — increased walking time-to-exhaustion at 75% of VO2max. But the details of the study are important: Phinney used a hypocaloric diet and the 6 young folks with obesity whom he studied lost 11 kg, on average. Therefore, hauling ass uphill on a treadmill would have become a lot easier and the outcomes are not particularly informative for weight-stable athletes.In 1983, Phinney published the first of the two “classic” studies that are often touted as conclusive evidence that “ketogenic diets work for athletes”. The study examined the effects of a 4-week low-carb (less than 20 g/day) ketogenic study, this time without caloric restriction and in 5 endurance-trained cyclists. The low-carb diet lowered muscle glycogen levels but increased cycling time-to-exhaustion at 62-64% of VO2max with lower carbohydrate oxidation rates. Sounds promising but exercising to failure at 62% of VO2max is very likely below the first ventilatory threshold in trained cyclists and is, therefore, not particularly relevant for an athlete unless they are trying to escape a relentless but slow-moving zombie.
Fortunately, in 1994, Estelle Lambert in Tim Noakes’ lab upped the ante and habituated trained cyclists (but only 5 of them) to a low-carb and a high-carb diet for 2-weeks. Peak power was about the same between diets and time-to-exhaustion at 85% of peak power was not significantly different (despite being 4-minutes shorter, on average, in the low-carb trial), but cyclists rode for twice as long (80- vs. 43-mins) at 50% of their VO2max when habituated to the low-carb diet.
So, the story thus far is that a low-carb diet may cause adaptations that allow athletes to perform for longer when operating at a low- to moderate-intensity. But being able to operate for longer at a low- to moderate-intensity does not help inform training for an endurance athlete racing hard in a marathon or anything shorter.
Is there more evidence concerning higher-intensity exercise?
In 1998, Jørn Helge enrolled 15 folks into a 4-week endurance training intervention with a low-carb or a high-carb diet. The low-carb diet group burned more fat during exercise and had lower muscle glycogen levels but had the same improvement in cycling time-to-exhaustion at 80% of their VO2max as the high-carb group. But, the caveat was that the participants were new to exercise and were consequently adapting to two things — training for the first time and a diet change. Since we know that training has an incredibly powerful stimulus on fitness and performance, the training stimulus may override any potential dietary effect in folks who have never exercised before. So, this data also does not inform training for an endurance athlete.
This and the previously-discussed studies determined exercise performance following an overnight fast with no pre-test meal — not representative of race-day conditions — and performed the time-to-failure at a percentage of people’s pre-intervention VO2max not their newly-improved post-trained VO2max — if you get fitter but exercise at the same relative fraction of your old fitness, your workload will be lower meaning that it will be easier and you will go for longer. We also know that recent food intake massively influences fuel use during exercise and, given that low-carb diets typically decrease muscle glycogen levels when compared to a high-carb diet, you might be wondering:
What happens if you habituate to a low-carb diet but “top-up” muscle glycogen with a last-minute high-carb intake?
To help tease out the answer to this question, in 2000, Louise Burke provided a low-carb/high-fat diet to trained cyclists for 5-days followed by 1-day of high-carb intake before a performance test — a 2-hour ride at 70% VO2max followed by a 7 kJ/kg time-trial after an overnight fast and a no-calorie jelly breakfast (except subjects were not informed about the “no calorie” bit and therefore thought they were eating actual fuel). All the subjects also completed the test following 6-days of or a high-carb/low-fat diet. The outcome? Fat oxidation rates were higher and carb oxidation rates were lower during exercise in the high-fat trial, while muscle glycogen levels and time-trial performances were not different between trials. With this study, we learn a little more and with relevance to athletes, but the fasted time-trial after a 2-hour low-intensity ride is not representative of any race scenario.
So, we need a study with a performance test that actually resembles race day.
In 2006, Estelle Lambert attempted just that and led a more ecologically-relevant study in well-trained cyclists (but only 8 of them). She compared the effects of a 6-day low-carb/high fat diet followed by 1-day of high-carb feeding to a 7-day high-carb diet, finding impaired 1 km and 4 km sprint TT performance but not 100 km TT performance in the low-carb folks. Once again, fat oxidation rates were increased following the fat adaptation protocol. But, high-intensity performance was impaired. A clear pattern had begun to emerge and this prompted Louise Burke and Bente Kiens to write their "nail in the coffin" for low-carb fat adaptation training protocols editorial.
But, since 2006, we can all agree that the low-carb diet and exercise thing has been very popular news. Plus, since that time, there remained a lack of evidence concerning real-life race conditions and Burke followed with her 2015 "did we call the 'nail in the coffin' too soon?" review.
And perhaps we did, since there is an ongoing debate about the utility of high-fat diet keto-adaptation (chronic low-carbohydrate availability) protocols for endurance performance. The sentiments of which are well-summarized in two reviews, one penned by Jeff Volek, Tim Noakes, and Stephen Phinney, the other by Louise Burke.
But keep your wits about you...
The other “classic” study often gets tossed around as conclusive evidence that “ketogenic diets work for athletes”, is the 2015 FASTER study from Jeff Volek and Stephen Phinney and colleagues — “Faster” standing for Fat Adapted Substrate use in Trained Elite Runners. The study compared a group of elite ultra runners who ate a self-selected low-carb ketogenic diet to a separate group of elite ultra runners who ate a self-selected high-carb diet. When brought to the lab, the low-carb diet athletes had higher fat oxidation rates during a 3-hour run at 65% of VO2max than the high-carb runners. No performance test was conducted. I’ll leave you to make your own mind up as to whether that study usefully informs whether you should or should not be “keto” to be a better (ultra) runner.
Clue: it does not.
Many of the studies I have discussed have focussed on “fat-burning” but many of them have also measured the size of the muscles’ glucose store, finding that muscle glycogen is lower following a low-carb/high-fat diet. The other common finding is that adaptations of muscle enzyme levels during training change in favour of either preferential fat or preferential carbohydrate use during exercise depending on the type of habitual diet being eaten. One key observation is that low-carb/high-fat diets decrease muscle levels of pyruvate dehydrogenase (or PDH).
What the hell is PDH?
Well, after glucose is broken down to pyruvate through the steps of glycolysis, PDH is the key enzyme that allows pyruvate to enter the mitochondria to produce ATP. Therefore, a low-carb/high-fat diet causes an adaptation that reduces your muscle’s ability to use glucose as a fuel source. While this may not be a problem at low-intensities, at high-intensities (e.g. >80% VO2max), when you are trying to rock your socks off, you need glucose to rapidly and economically produce ATP to help you move fast — recalling what you learned about bioenergetics in my previous post — pound-for-pound, fat uses more oxygen to produce ATP than glucose and produces ATP at a slower rate than glucose.
What can you add to your training toolbox?
The 100-years of studies I have summarised make several important observations:
Low-carb/high-fat diets increase fat oxidation rates during exercise at low-, moderate-, and high-intensities.
Low-carb/high-fat diets decrease muscle glycogen levels and reduce the levels of enzymes involved in glucose-burning metabolic pathways.
A low-carb/high-fat diet can improve endurance time-to-exhaustion at low-intensities (≤70% VO2max) — helping you evade a relentless but slow-moving zombie — but does not improve (and sometimes impairs) performance at high-intensities (≥80% VO2max) — you will not do well in a manic-zombie apocalypse.
There is often a highly-variable endurance performance response to a low-carb/high-fat diet.
The variability may partly result from the tiny numbers of participants included in most studies.
The performance tests are rarely ecologically-valid — they are typically conducted after an overnight fast and often involve a ride- or run-to-exhaustion at a fixed sub-maximal (often low-)intensity.It is also important to remember that increased fat oxidation rates during exercise are not synonymous with improved performance — as we already learned, max fat oxidation rates only explained 13% of performance outcomes in Ironman triathlon. Furthermore, increased fat oxidation rates are a natural consequence of low carbohydrate availability — when we move, energy (ATP) has to come from somewhere — if glycogen is low, we must burn fat.
Up to this point, given the evidence available, it is probably becoming quite evident that it is not possible to boldly conclude that a low-carb/high-fat diet offers a performance advantage to an endurance athlete.
But, to truly test the hypothesis, we need a well-controlled trial examining the effects of habituation to a low-carb vs. a high-carb diet on ecologically-valid outcomes (race performance) in a large group of highly-trained endurance athletes.
Thanks to Louise Burke and her team, we don’t have to wait… But you do, because that is the story for next time.
Until then, don’t make any drastic changes to your diet just yet; keep training smart and learn more soon...
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.
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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.