Low carb, high fat diets, attempting to re-write genetics.

The biggest challenge facing every endurance athlete is how to keep going at the fastest possible speed without fatiguing before the finish line.  Here I’m focusing on all levels of triathlon from sprint right up to Iron distance.  The speed changes but one thing remains constant and that is in order to perform at your best it’s more important to be good at metabolising carbohydrates versus fats.

So, can we pinpoint the moment where the racing nutrition waters got muddied?  Not really.  Like most things it works in a cycle.  We’re in that point of the cycle were Low Carb, High Fat (LCHF) diets are the trendy thing to do.  Don’t worry it will pass and most likely come back again.

On paper there’s a definite appeal to LCHF and how it might be applied to racing and it has some other supplementary benefits as well such as weight loss.  However, on the road it doesn’t work and there are many scholarly and recent real-world examples that prove so.

What is the appeal of LCHF?

To understand the basis of the LCHF camp you need to know a little bit about how the body produces energy.  The two main things you need to get your head around are the two major energy pathways glycolysis and lipolysis.  Two pathways that produce ATP (adenosine tri phosphate) which is the energy our muscles use to fuel our racing and training. 

Here's the basics

Glycolysis.  This is the energy pathway that metabolises carbohydrates (CHO) to produce ATP.

Lipolysis.  This is the energy pathway that metabolises fat to produce ATP.


Two avenues that produce the same outcome.  The production of ATP.  Or do they?  This is where you need to understand where the LCHF camp is coming from.  And it makes sense on paper but in the real world it doesn’t work. 

To keep it simple lipolysis (fat adaptation) produces FAR more ATP than does glycolysis. As you can see in the diagram above the breakdown of triglycerides into free fatty acids produces a whopping 400+ ATP, whereas CHO only produce approximately 36 ATP through the glycolysis pathway.  On paper it’s very appealing for an endurance athlete to want to learn how to maximise their usage of fats in training and racing.  “If I can use more fats, I have more energy, I can go for longer without bonking.” 

Secondly, fat adaptation has a practical appeal as far as race fuelling is concerned. If we are metabolising fat and producing this 400+ ATP then in theory we don’t need to carry much other than water and electrolytes during a race.  It means for something like a 70.3 or Ironman we don’t have to ingest as much fuel into the body during the race as we are already carrying it in our body so therefore issues such as upset stomachs are avoided. 

And lastly, switching to LCHF does have some short term gains in weight loss.  As we are stripping the body of an essential macro nutrient we are reducing the energy part of the equation which leads to some very quick noticeable changes in body composition. 

And therein lies the appeal of LCHF or “fat adaptation”.  If I were the marketing guru behind LCHF my slogan would be “more energy, simplified race nutrition and look fabulous.”


Now that we’ve discussed fairy tales and unicorns, its time for a reality check.  LCHF does not work for racing. Full stop, underline, italicised in some big ass font!  It is more important to train the body to absorb more carbohydrates so we can race fast versus fat adapting to race slow. I could fill the NSA’s server with scientific examples and proof as to why LCHF doesn’t work and I’ll give you a few examples below. 

“it is more important to train the body to absorb more carbohydrates so we can race fast versus fat adapting to race slow.”

The big problem is the way fats and carbs are broken down to create that energy and it lies in their chemical structures.  Fat is harder to break down then carbohydrates.  When exercising fats are broken down at much lower training intensities than CHO.  You can train your body to increase the use of fats to fuel exercise however no clear benefits to performance have been
demonstrated 1.  At best LCHF gives the athlete a “tantalizing feeling2.”

In racing or high intensity exercise there comes a crossover point where your body seeks to metabolise simpler substrates to fuel its activity, ie: carbohydrates.  And the reality is quite simple.  If you are low on CHO stores in your muscles and liver then you will slow down.  If you want more scholarly proof you only need to read the testing outcomes of studies where LCHF diets were used. 

In one such case “the increased rates of fat oxidation result in reduced economy (increased oxygen demand for a given speed) at velocities that translate to real-life race performance in elite race walkers. In contrast to training with diets providing chronic or periodised high carbohydrate availability, adaptation to an LCHF diet impairs performance in elite endurance athletes despite a significant improvement in peak aerobic capacity3.”

For a recent practical example of the failures of LCHF have a watch of Lionel Sanders’ recent Ironman Mont Tremblant race report 4.  Lionel openly admitted to neglecting his carbohydrate loading plan pre-race as a part of the reason for having a bad race.  On the flipside look at athletes who are at the top of their game at the moment.  Daniela Ryf, Lucy Charles, Sebastian Kienle etc. all sponsored by Red Bull, which is essentially a sugar and caffeine company.

So let's just put this issue to bed shall we?  Keep the LCHF diet to the weight loss arena and let's stop demonising CHO.  If you wanna go fast embrace the carbs.  To race fast it is more important to train the body to absorb more carbohydrates so we can race fast versus fat adapting to race slow.


1 Effect of fat adaptation and carbohydrate restoration on metabolism and performance during prolonged cycling, Louise M. Burke, Damien J. Angus, et al. Journal of Applied Physiology Volume 89, issue 6, December 2000.

2 Louise M. Burke https://www.youtube.com/watch?v=Fh0EqE8zUOI “High Fat Low Carbohydrate diets?”31 July 2016.

3 Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers, Louis M. Burke, Megan L. Ross et al.  The Journal of Physiology, Volume 595 Issue 9, 1 May 2017

4 Lionel Sanders, https://www.youtube.com/watch?v=EO6Gp3Yp6Y8, “A Tough Day at Ironman Mont Tremblant,” 19 August 2018.


  • dqzzzzqpuo

    Muchas gracias. ?Como puedo iniciar sesion?

  • dYWqrTly




Leave a comment