Fat adaptation for endurance performance, is it a thing?

There’s been quite a bit of debate in the world of endurance sport over the past few years about whether or not a high fat low carb diet is better for endurance performance due to increased fat oxidation.

When you look at the evidence it seems that carbohydrates are king (1). Even the ISSN guidelines pretty much glaze over the subject of fatty acids, actually going into more detail about the benefits of caffeine than fats (2).

Then, of course, there’s the keto crowd who believe that carbohydrates are responsible for all of man’s evil (9, 10).

If you look on forums and comment sections of various blogs it appears to be a pretty polarising subject and you’re either in the high carb or low/no carb camp. As always, I like to explore the grey areas a bit more and kind of sit on the fence here.

Let’s look at the evidence for and against.

Science says no

The common argument in favour of a high carb diet is that muscle glycogen is the all-important factor where physical performance is concerned and that to maximise both performance and recovery potential you need to load up on as much carbohydrate as you possibly can.

As much as 8-10 g a day (2) and around 60g and hour during events lasting 2-3 hours (1).

That’s a lot of carbohydrates and many people would struggle with that volume, especially if there is a history of gastric intolerance (1).

So, what happens if an athlete tries a low carb approach prior to training?

This has been investigated and the evidence is mixed with Burke et al concluding that low carb diets not only don’t provide a significant performance benefit that they may actually hinder high intensity efforts (3).

However, we often see these studies performed on athletes who are not long term fat adapted, so is 5 days or even 2 weeks enough time to really get a true reading? More on that later.

Ellen Coleman also explored fat loading for endurance sports and came to similar conclusions. It may be useful for ultra-endurance sports, due to the lower intensity efforts being in a fat metabolism zone, but it doesn’t appear to be of much benefit to everyone else (4).

Your muscles need glycogen and anyone who has ever hit the wall or ‘bonked’ knows that glycogen depletion is no laughing matter.

Another study looked at fat oxidation levels and did find that athletes fed a high fat diet were much better at fat oxidation than those on a habitually high carb diet. But, this was only significant in sub-maximal efforts and made no difference during a simulated bike time trial (5).

Besides, fat oxidation happens in everyone (6) and, it appears, that the fitter you are the more efficient you might be at this. In one paper (7) we saw that healthy lean males with a high Vo2max had better 24 hour FO than those with a lower Vo2max so the level of aerobic conditioning clearly plays a big role in improving the ability to metabolise fats. But, none of these athletes were on a low carb diet.

Adding more fuel to this, in 2015 Hetlilid studied well trained versus elite trained runners (8) and discovered that the elite level runners oxidised fats at much higher levels of Vo2max than is commonly accepted (85%). These athletes all consumed a habitual diet of high carbs (60%) so it would be interesting to see what the readings would be on ‘fat adapted’ athletes. Would they have a higher fat max still or would the lack of carbs reduce their ability to train at the same high level of intensity?

So, that’s pretty much that then, isn’t it?

Or is it?

In a 2015 study by Volek et al (9), 2 groups of highly trained ultra-runners were split into low and high carb groups, with the low carb group having been ‘keto-adapted’ for a minimum of 20 months, it was observed that the LC group had much higher rates of fat oxidation.

The tests, however, were performed at unrealistically low intensity levels (64% V02max). Interestingly the LC group had a higher protein intake at 2.1g/kg BW which means that although they did produce more ketones were they technically in ketosis?

Phinney concluded that a protein intake of around 1.5 g/kg but as low as 1.2 g/kg was adequate (10). This is clearly in conflict with the dietary intake of Volek’s study group.

Both groups in Volek’s study showed similar levels of glycogen oxidation leading the authors to hypothesise that pyruvate and the pentos phosphate pathway in combination with lactate conversion compensated for the lack of exogenous carbohydrates in the LC group.

Remember, those previously mentioned trials were largely on very short term adapted athletes so this could explain why so many low carb athletes perform so well, in spite of the evidence and why most athletes who try it short term fail to thrive.

Is 5 days or two weeks enough time for all those adaptations to take place in the body? Phinney believes 2-3 weeks is optimal for keto adaptation (10) but is that as important as the other adaptations, especially if it means limiting protein intake?

This begs me to ask, how important is it to be in ketosis in order to see these adaptations and how high can one go with their carb intake before those adaptations switch off? Ketones are a good source of energy for the brain and do have the effect of sparing amino acids but are they really necessary for sporting performance?

There was this though.

Proeyen et al (11) discovered that endurance athletes on a high fat (50% of calories) diet combined with 4 fasted state training sessions per week improved insulin sensitivity, glucose tolerance and, significantly, fat oxidation.

This, it seems, shows that there are indeed some benefits to fasted training and the ‘train low’ method employed by a lot of pro cyclists. However, these individuals were on a 50% fat, 30% CHO and 20% protein diet. So again, we see that adaptations to high fat intake occurred but without the need for excessive carbohydrate restriction.

So, as you can see, the evidence in favour of a low carb diet for performance benefits is sketchy at best so why am I even bothering to examine this?

Anecdotal evidence

There seems to be a pretty good weight of observational and anecdotal evidence to support the theory. Enough, I would argue for us to at least accept that more research is needed here and, indeed, there appears to be more evidence coming out on the train low principle (12).

First of all, there are the stories of the arctic explorers who performed feats of extreme endurance while on a strict diet of fatty seal or reindeer meat (10).

If you follow Chris Froome on Instagram you would have seen his pre-ride breakfast of boiled eggs and avocado and I don’t think anyone can deny that Chris Froome is a high performing athlete.

I personally know a couple of ironman athletes who practice HFLC diets and they are able to go out for 6-7 hour long training rides on nothing more than a few sips of water.

Most of the pro cycling teams now practice a “train low” principle (12). Where a period of glycogen depleted training is performed along with carefully planned fasted state rides in order to develop the fat metabolism. This has led to many of these riders having a very high max, many of whom can now complete an entire tour leg without having to load up on energy gels during the race.

The theory here, which appears to work in practice, is to produce an environment of metabolic flexibility making your body better at using the correct fuel source at the correct time to ensure optimal adaptation to endurance performance.

This seems consistent with some research (11) so, fasted cardio might not be much use for body composition but seems to improve both fat and carbohydrate metabolic adaptations to endurance exercise. Win win!

Of course, the likes of Volek, Phinney, Noakes and co are well known for having an agenda bias and many carb zealots dismiss their hypothensisies. But, if you want to talk about conflicts of interest Asker Jeukendrup (1) is the global senior director of the Gatorade Sports Science Institute. These scientists have to get their funding from somewhere.

"So, in short. Experiment and see what works for you".

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Basically, the scientific evidence implies that there is very little if any benefit from adopting a high fat approach to sports but there is significant enough anecdotal evidence for us to pay attention. All those athletes cannot be lying.

Here’s the kicker, all those fitness experts who work in offices will be up in arms right now, hurling references left right and centre, but they don’t work in the trenches. The evidence gets you so far but there comes a point where you have to adapt and apply that knowledge to the complex human machine. We, after-all, are people dealing with people, not principles on pieces of paper.

Anyway, here’s my take on it. The higher your carb consumption the less fat your body will oxidise (6) resulting in a higher need for exogenous carbohydrate ingestion.

But, we don’t have to restrict ourselves of carbohydrates in order to become ‘fat adapted’ and most athletes who go low carb simply don’t eat enough carbs to enhance their performance. But do we really need to be consuming 10g per kg or more?

So long as you’re keeping your protein intake at at-least 2g/kg BW (2,9) there’s a good chance that Gluconeogenesis will make up for any shortcomings in your carbohydrate intake without leading to catabolism.

This then allows you to go slightly lower on your carbs, especially useful if, like me, you have a history of GI distress, without slipping into the realms of restrictive dieting. Consuming a slightly higher percentage of fats, perhaps as high as 50% but certainly no lower than 30% may help to spare both glycogen and amino acids.

A good strategy is to backload your carbs, during training periods. This way you get all the adaptive benefits of a LCHF diet but you also have sufficient glycogen storage to ensure that you don’t suffer during the higher intensity efforts and recover well after training or races.

Combine some fasted state training or, at least, training in a low glycogen state and you will really start to see the benefits by increasing fat oxidation and extended time to fatigue. This seems to be the preferred method of Team Sky who are known for low carb training camps and including early morning fasted rides during the training camps where they are clocking lots of base mileage.

We all adapt differently and some people adapt extremely well to a low carb diet and still perform well. One guy in Volek’s study showed extraordinary adaptation to a keto style diet (9) but most struggle and if you go keto it pretty much has to be a lifestyle otherwise it just won’t work. So no more slices of nan’s chocolate cake for you.

Now, if you’re a carb eating machine and the prospect of riding for 6 hours and consuming your body weight in gels or jelly beans doesn’t bother you then why change anything? Millions of athletes have excelled over the years doing just that so stick to Mr Jeukendrupp’s tried and tested guidelines (1). But, if the thought of consuming the recommended quantity of carbs and all that sugar fills you with dread this might just be the way for you, it’s not all about carb loading. Some people don’t digest simple carbohydrates well and if you have any history of IBS or simply feel sluggish on a high carb intake a moderate carb approach might suit you better.

Ensure adequate protein intake to avoid catabolism and, if you do any higher intensity training efforts you may need to cycle in some extra carbs to ensure you’re training hard enough to attenuate the required adaptations.


On a train low day you have a few options:

  • Eat a normal balanced meal the night before
  • If you’re having a pre-training breakfast something like a 3-4 egg omelette, a protein shake or anything along those lines will do
  • If fasting, then simply ensure you’re well hydrated and may benefit from a shot of caffeine (1,2). Espresso is ideal
  • There may be some benefit to increasing sodium during low carb periods so make sure you salt your food (9,10)

Obviously pack a gel, energy bar or some fruit in case of emergencies but the idea here is to push the envelope to test your levels of endurance. Of course, you’ll want to build this up gradually and by-all-means cycle low and high training days to see what works best for you.

Afterwards you can have a good meal and get those carbs in to assist muscle recovery.

So, in short. Experiment and see what works for you but for goodness sake do so during your training schedule and not on race day!

If you are an endurance athlete and are interested in exploring your options or want to better develop your fat metabolism for long sustained efforts I am still taking on clients.


  1. Asker E. Jeukendrup (2015) Nutrition for endurance sports: Marathon, triathlon, and road cycling. Journal of Sports Sciences
  2. Kreider et al (2010) ISSN exercise & sport nutrition review: research & recommendations. JISSN
  3. Burke et al (2005) “Fat adaptation” for athletic performance: the nail in the coffin? J Appl Physio
  4. Ellen Coleman, MA, MPH, RD, CSSD (2010) Fat Loading for Endurance Sports. Nutritiondimensions.com
  5. Andrew L Carey et al. Effects of fat adaptation and carbohydrate restoration on prolonged endurance exercise. J Appl Physics 91: 115-122, 2001
  6. Dr-Fo (2015) Fat oxidation – exercise, muscle. World of Sports Sciences
  7. Robinson et al (2015) Maximal fat oxidation during exercise is positively associated with 24-hour fat oxidation and insulin sensitivity in young, healthy me. J Appl Physio
  8. Hetlelid et al (2015) Rethinking the role of fat oxidation: substrate utilisation during high-intensity interval training in well-trained and recreationally trained runners. http://bmjopensem.bmj.com
  9. Volek et al (2015) metabolic characteristics of keto-adapted ultra-endurance runners. Metabolismjournal.com
  10. Stephen D Phinney Ketogenic diets and physical performance (2004). Bio Med central
  11. Proeyen et al (2010) et al. Training in the fasted state improves glucose tolerance during fat-rich diet. The Journal of Physiology
  12. Taylor et al (2012) Protein ingestion does not impair exercise-induced AMPK signalling when in a glycogen-depleted state: implications for train-low compete-high.European journal of applied physiology