Fat Burning for Ultra Runners

 In Nutrition & Hydration, Running training

Given an ultra takes a lot longer than 2 hours (the approximate limit of our carbohydrates to supply energy) it is clear that fat is going to be a big contributor to our energy supplies.
The less effective we are at using fat for fuel the more carbohydrates we have to consume during a race to fuel our muscles. The more we have to put into our stomachs the more chance of stomach problems. Given stomach upsets are a major causes of DNF’s and below par performances in ultras it makes sense to work on increasing your bodies ability to burn fat.

If you are interested in knowing how much fat you actually burn during exercise then there are ways of testing this. I was invited to do one such test at Jupiter Health based in the Gold Coast . Through measuring CO2 and Oxygen levels via a gas mask during an incremental test to exhaustion it is possible to determine the level of carbohydrate and fat usage during exercise. They also measured VO2max and Anaerobic threshold and then determine heart rate  training zones from that.  They follow up the test with a discussion on your results  and how  you can improve them.

My graph shows a reasonable level of fat burning with fat being the dominant source of energy right up to around 90% of max tested HR but there is defintely some room for improvement. The fat burning line could certainly be higher. I max out at around 8 calories per minute – really effective fat burners can reach 13+ cals per minute.

Thinking about my training and how it relates to the results I know that I wasn’t anywhere near peak fitness when I did the test. When fully fit I am able to do 4 hour runs with 3 x 20 minute hard hill reps all on no breakfast and nothing but water throughout the run. At the time of testing I was only able to manage around 3 hour easy runs without fuel .

So how can we increase our ability to burn fat ?

Training and diet have the largest influence or how much carbs vs fat you burn.


There is plenty of research showing that a high fat diet improves how much fat you can burn per minute and also the intensity at which you can still burn fat as an effective fuel source. A study known as the FASTER study (Fat Adapted Substrate Oxidation in Trained Elite Runners) compared two groups of elite ultra runners . One group on a high fat low carb diet (Fat Protein Carbs 70/20/10) compared to a high carb group (25/15/60). The results show the high fat group were able to oxidise around 1.54 grams of fat per minute compared to 0.67 grams in the high carb group during a progressive treadmill test to exhaustion.

In a 3 hour treadmill test at maximum fat oxidation intensity VO2max the high fat diet athletes fat oxidation rates were around twice as high as the high carb. But maximum carbohydrate burning rates were higher in the high carb athletes.

So you get better at burning what you eat and less effective at burning what you don’t ( i.e. the high fat diet negatively effected carbohydrate burning). If the thought of a high fat diet doesnt appeal you might find some consolation in the fact the benefits seem to reach their peak after 5-10 days of being on a high fat diet and are maintained for 36+ hours after switching back to a normal higher carb diet. One suggestion form the researchers is to finish the 5-10 days around 36 hours before your event and then carbo load so you get the best of both worlds.

However if you aren’t used to it 5-10 days of high fat low carb diet can be a real shock to the system and not necessarily the best thing to do in the week leading into a major race. So whilst its interesting that a high fat diet increases the ability of the body to burn fat, improved race performance is what matters and even if we see results in a lab that a high fat diet increases time to exhaustion it does not mean it will improve race time.

Why? Well  if you aren’t trained to burn fat more effectively  then in a time to exhaustion test consuming nothing but water you’ll exhaust your limited carb stores before someone who is trained to burn fat. But in a race you have the option of taking additional carbs throughout the race. Plus a time to exhaustion test on a treadmill at a predetermined intensity level is vastly different to a 50km + ultra.

We also need to look at all the training that goes into a race performance . Does a high fat diet compromise all the higher intensity sessions needed to maximise aerobic fitness ? Possibly not according to a study by Hulston et al (2) who concluded ” Training with low muscle glycogen reduced training intensity and, in performance, was no more effective than training with high muscle glycogen. However, fat oxidation was increased after training with low muscle glycogen.. ”

So it wasn’t any better but it wasn’t any worse either and had the advantage of increasing fat oxidation .

But that study used cyclists and a 8 x 5 minute hard 1 minute recovery protocol – whether that still applies to hour long tempo sessions , hard hill repeats and fast finish long runs we can only guess.

Whilst the high fat low carb diet may have some advantages there are many questions remaining on how effective it is. Some researchers suggest that training in a low glycogen state may lead to increased muscle breakdown and overtraining syndrome.

We are a long way from confidently be able to say that a high fat diet is the best for endurance athletes. Almost all elite endurance athletes are on a high carb or more evenly balanced diet as opposed to high fat diet. Even some elite ultra runners that have a predominantly high fat diet still take carbs before key workouts to make sure they maximise the training benefits.

So the research on high far diets for endurance athletes is far from conclusive what about using training to improve our fat burning ability ?

Fortunately we don’t need to eat a high fat diet to improve our ability to burn fat. Research (1) has shown training in a depleted glycogen state does improve our fat burning as well. There are a number of ways to improving fat burning through training.

Before I discuss this it is important to distinguish between low muscle glycogen and consuming no calories in a run. If you start a run with muscle glycogen stores full and consume no calories then unless your  run is at least 2 hours long you will have plenty of glycogen to fuel your training and you are unlikely to improve fat oxidation rates.

1. Slow down
The biggest mistake people make is running the long run at too high an intensity, The higher the intensity the more carbs you burn. To stimulate fat burning you need to run at a low enough intensity that allows the body to obtain enough energy from fat. As your ability to burn fat increases you’ll be able to work at higher and higher intensities and still burn a large percentage of fat. Until then conversation pace – i.e. a pace you can speak in proper sentences without struggling for breath is a good guide.

2. Zero Calorie long runs.

Have your normal pre run snack or breakfast and then head out for your long run using only water. You will need to run for at least 2-3 hours to reduce glycogen levels to a point where they are low and the stimulus for greater  fat oxidation kicks in.

3. Zero Calorie long runs on no breakfast.

Wake up and have only water or tea/coffee and then head out on your long run consuming only water. This is marginally harder than option 2 but you still need to run for several hours before you can say muscle glycogen levels are low.

4. Back to Back runs

Do a high intensity tempo session that depletes your glycogen stores in the evening and follow up with a long run the next  morning  with no breakfast  before or calories  during the run. The less carbs you have after the higher intensity  run the lower your glycogen levels will be and the more reliant on fat you will be in the morning after run.

5. Low carb eating the day before a long run

If you run long on Saturday then on Friday reduce your carbs so when you start your Saturday long run you already have reduced glycogen supplies in your muscles.

6. Low carb eating combined with carb depletion run.
This is the same as option 3 but with minimal carbs all day so the hard evening sessions uses up anything left making sure you are starting from very low levels for the morn run

As you can imagine the 6th option is substantially more difficult than the first option.

Start with option 1 and when you feel comfortable doing 3-4 hour runs on no calories then drop the pre run snack/ breakfast. Once that feels comfortable you can progress through the other options.

However options 5 and 6 are particular challenging and unless your fat burning systems are extremely well developed most people will find that the long run suffers the next day either from a markedly reduced speed or even inability to complete it. So you need to weigh up whether you need to take it to the next level or not. For most people options 1,2,3 and 4 will significantly improve their fat burning capabilities and its not necessary to go the options 4 and 5.

What about high intensity sessions. Do you need to have carbs to get the most out of these sessions ?

To start with unless you are on a low carb diet or have had minimal carbs the day before its likely you will have enough glycogen to complete a typical high intensity tempo or interval session with no added carbs. But lets say you did have low glycogen ( e.g. you ate nothing but salad for a few days) , the research is interesting with some studies showing that training in low glycogen states can elicit the same fitness gains as in a high glycogen state even if the workout intensity isn’t as high due to a lack of glycogen (2).

So whilst those workouts may be tough , not as fast and you may not be able to complete as many reps the training benefit may be the same. However the confidence gained from doing a good high intensity session may not be there if you are struggling to run times you know should feel a lot easier. I wouldnt recommend it unless you are very well fat adapted.

Practical applications in racing.

Once you are well fat adapted in training runs you should notice that in races your carbohydrate requirements are reduced. We are often recommended to obtain around 60 grams of carbs per hour for maximum performance but for many that amount gives our stomach problems so if through becoming better fat adapted you can reduce that to 30-40 grams it makes it far easier for your stomach .

Note you should still do some long runs using race nutrition to determine what works best for you and to try out alternatives so if on race day you have problems you have a plan B to fall back to.

Which diet is best ?

Whilst I am still of the belief that if your event is 2 hours or less a high carb diet is probably going to give the the best performance outcomes it becomes a lot less clear when we go longer. Personally I am not a fan of the low carb high fat diet – being a vegetarian would make that very difficult I am also of the opinion that a more balanced diet is a better option. Gone are the days when I would eat 6000 calories a day and 85% of that carbohydrates. Today my diet contains a lot less bread, rice pasta and a lot more vegetables , nuts and legumes.

Adjusting your diet throughout the training cycles to include some periods of higher carb intake paired with moderate length harder sessions and some periods of lower carb intake paired with longer slower sessions may be the optimal strategy and this is a new area for research that hopefully we’ll understand more about it and how to apply it over the next 5-10 years.

1) Manipulating Carbohydrate Availability between Twice-Daily Sessions of High-Intensity Interval Training Over 2 Weeks Improves Time-Trial Performance
International Journal of Sport Nutrition and Exercise Metabolism
Andrew J.R. Cochran* , Frank Myslik* , Martin J. MacInnis* , Michael E. Percival* , David Bishop* , Mark A. Tarnopolsky* , Martin J. Gibala*

2) Training with low muscle glycogen enhances fat metabolism in well-trained cyclists.Med Sci Sports Exerc. 2010 Nov;42(11):2046-55.
Hulston CJ1, Venables MC, Mann CH, Martin C, Philp A, Baar K, Jeukendrup AE.

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Showing 6 comments
  • Dan Irwin

    Nice post Andy : ) Always helpful to hear some the science rigour behind the theories.

  • nick

    Good article Andy, what does the research say about how much protein is metabolised from muscle when there is insufficient glycogen and fat? Is there a risk that training without sufficient energy / food just leads to muscle loss?

    • Andy DuBois

      You will break down some muscle tissue – but the amount is small as the body prefers breaking down fat for energy rather than muscle and if you are on a low carb diet then you’ll be consuming higher protein levels which will compensate up for any muscle loss.
      If you aren’t on a low carb diet then given your diet still contains adequate protein the muscle loss experienced would be insignificant.

      The question then is where does this protein breakdown come from – muscles you are using or muscles we aren’t using – i.e. if body breaks down its small proportion of muscle from say chest muscles when running then thats not a bad thing – its not helming us run anyway – if its from quads then not so good but so far I havent seen any evidence that the muscle loss we are talking about is any way significant

  • Leo N

    Wow Andy
    How goos and well explained this article is !

    Manu things to change in my case to improve

  • Johanna Meyer

    Hi Andy,

    newbi here! Very interesting article.

    Given your >>Fuel burning profile<>1. Slow down<<, my question would be how far should you slow down in terms of heartrate measured in bpm to train your aerobic system most effectively.

    Sorry for my bad english, I'm alien ;-).

    An answer would be much appreciated. Looking really forward to it.


    • Andy DuBois

      Like most things it depends – better fat burners can work a high aerobic levels and still burn most energy from fat , someone who is very carb dependant will need to keep the pace very easy to enhance fat burning .
      So a heart rate of anywhere from 50-85% of max Heart rate but thats doesnt help you much I suspect . Tif you have never done any runs without calories during or if you always eat before you run then a ball park figure is 65-70% of max heart rate but for some people may need to be lower .
      You should be able to get through the run without feeling hungry at all , if you do then you went too fast. If you cant maintain pace then you went too fast .

      Hope thats a bit more helpful

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