Part 1:

Endurance sports require the body to produce energy for a very (and especially “ultra” events) very long time. Thus, it would seem logical that to improve your endurance performance, it would be wise to make your body very efficient and robust at producing energy. One of the key components of achieving this is being selective about the fuel source. The main fuel source that is used directly to produce ATP (i.e. energy) is glucose. In terms of food, the type of foods which contain glucose are carbohydrates. Hence, this is what has led to the hundreds of studies which have looked at the relationship between carbohydrate feeding and exercise.  Out of these studies have come the hundreds of recommendations that tell us to eat carbohydrate foods if we want to improve performance. So glucose = energy, carbs consist of glucose, therefore, eat carbs to produce more energy. Sounds simple right?

Unfortunately, when it comes to ultra endurance, it’s not that simple. There are lots of reasons why so let’s look at them in a bit more detail

  1. Exercise Type/Duration

The majority of these studies typically involve 1-3hrs of exercise and are carried out at medium intensity. There are very studies which have tested athletes exercising for longer than 5hrs. When we look at times to complete ultra endurance events such as ultramarathons, ironman and adventure races, the “exercise” time can be anything from 12 to 24hrs, even longer in some cases. Thus, we cannot fully correlate the findings of the 1-3hr studies with ultra races.

  1. Experimental Conditions

In addition, the majority of these studies are carried out in the laboratory on treadmills and ergometers under controlled conditions. While this is critical to laboratory experiments, it doesn’t replicate the conditions of racing over varied terrain and in the temperatures experienced during races held in mountains and desserts. This brings in a huge psychological component into the equation that is likely (without there being any studies, just my own opinion) to have an effect on physiology. In other words, even if they did conduct studies involving 12hr/24hr runs on the treadmill or riding on stationary bikes, the brain signals which control energy systems, muscle contraction and perception of pain etc, are likely to be different. So even if these 1-3hr studies are referred to as “endurance” investigations, do they really tell us what’s going on inside the body of an ultrarunner doing a 100mile mountain marathon??

  1. Subjects/Participants

As most of these studies are conducted at Universities the volunteers that are put through the study are typically students in their twenties. They are also predominantly male students. In most situations, they will be “untrained” individuals with little experience of a proper exercise regime although there are many studies that have used athletes. However, the majority of the studies are on individuals that have very different characteristics and make-up to a typical ultra endurance athlete. Those competing in ultramarathons and ironman are more likely to be older and have several years of serious endurance training clocked up. So, it is suffice to say that there would potentially be different outcomes between those sets of individuals if we were to compare 10 untrained students compared to 10 experienced ultra endurance athletes.

These points above hopefully explain why the recommendations on the back of sports drinks or even the recommendations in the scientific literature cannot be applied to extreme ultra endurance events. In order to investigate this further, we need to back track a bit to understand how the produces energy. Is it simply one pathway and one system that convert glucose to ATP? Is glucose the only molecule that we can make energy from? The answer to these questions is no and examining all the other energy pathway systems is the key to understanding how ultra endurance physiology works

Energy Systems

Now for the science bit. I don’t want to go into too much detail, but understanding how your body functions is the key to understanding how food works. So very briefly, here are the different ways your body can produce ATP (i.e. energy)

PhosphoCreatine System: ADP +Cr P  –>  ATP

This system is used to produce the first 10-15secs of energy that we need for sprints. Obviously, this is not a system that ultra endurance athletes use very often. It doesn’t require glucose or oxygen and it is fuelled through the use of creatine, a natural molecule produced in the body consisting of 3 amino acids.

Anaerobic System (Glycolysis): Glucose  –>  Pyruvate + ATP + H+

This is a system more familiar to people which burns glucose quickly and coverts it to lactic acid. The result is quick energy, the type needed for high intensity exercise like interval training or hill climbs. This process doesn’t require any oxygen but the trade-off is a large production of hydrogen ions which decrease the pH resulting in muscle fatigue.

Cori-Cycle (The Lactic Acid Cycle): Lactate + ATP   –>   Glucose

Despite what people think, lactate is not the bad guy. The acid builds up and muscle fatigue/pain is mainly caused by an increase in hydrogen ions (which lowers the pH). The lactate produced in the muscle can be recycled in the liver and converted back to glucose. This glucose can then be shuttled to the muscle and used again to produce energy.

Aerobic System (The Krebs Cycle/Citric Acid Cycle): Glucose + O2 –> CO2 + H2O + ATP

This is the main system that our cells use to produce energy. It’s a series of enzyme controlled chemical reactions that use oxygen to breakdown glucose. The first few steps of this reaction involve converting glucose into a smaller carbon chain intermediate. What’s important to note here is that both fats and proteins can be broken down and converted into this exact same intermediate. In other words, as well as pure glucose, our cells can use fats and proteins to fuel the aerobic system. This is a complicated pathway but here is a very simple diagram to illustrate what I mean


Lipolysis is the conversion of triglycerides into glycerol and free fatty acids. These free fatty acids can then be transferred to muscle where they are further broken down through beta-oxidation to prepare them for the Krebs Cycle. The advantage of using fatty acids is 1. The amount we can store is far greater than carbs and 2. They provide roughly twice the amount of energy per gram. So we have lots more of this type of fuel and it produces more energy. The only disadvantage is that because they are much bigger molecules than glucose, they require more oxygen for their combustion. Therefore, fatty acids from the plasma and adipose tissue are oxidised at a higher rate when the intensity is low (i.e. when you can take in more oxygen, i.e. breath more)

ITMG (Intramuscular Triglycerides) Fat Oxidation

So when fats are used to produce energy, they can come from three different locations. 1. Adipose Tissue, where the majority of itis stored 2. Muscle and 3. Blood Plasma. The fat stored in muscle is called Intramuscular Triglycerides. It is this which gives meat its marbled appearance.  As ITMG’s are already present in the muscle, transport and delivery is not an issue. Therefore, the ability to use them is increased especially as exercise intensity increases.

Glycolytic Proteins/Protein Oxidation

Amino acids (such as Leucine, Isoleuncine and Valine) can also be converted into Acetly-CoA (the intermediate that both glucose and fatty acids are converted to) and then used in the Krebs Cycle. It is estimated that 5-10% of energy can come from the oxidation of proteins. However, this is not ideal if amino acids are not plentiful as it means that the source of amino acids will be from muscle tissue. This can be reduced by supplying amino acids and increasing the rate of fat oxidation to spare the use of protein.

Glucose Alanine Cycle

Just like the Cori Cycle, where a waste product is converted back into glucose, the same can be done with amino acids. The amino acids Alanine and Glutamine can be used to convert Pyruvate back into Glucose.

This needs a re-cap

We can produce energy quickly using the PCr and Glycolysis systems. This produces energy that can be used for seconds/minutes at high levels of intense exercise. Aerobic metabolism can then be used to burn glucose, fatty acid and amino acids to produce more ATP for longer periods of time. Glucose can also be remanufactured through the conversion of metabolites by recycling lactate and alanine.

To simplify, there are 3 fuels (glucose, fatty acids and amino acids) that can be used to produce ATP and there are 3 ways of providing glucose to the muscle 1. Consume it 2. Cori Cycle and 3. Glucose-Alanine Cycle.

What can be taken from this?

Ultra endurance is essentially about exercising at low intensity for a long period of time (although there may be short periods where medium to high intensity efforts are required). As ultra endurance events take several hours to complete, optimising the body’s ability to use fatty acids (adipose and ITMG) as the predominant fuel to produce energy can significantly help performance. There are 4 factors that support this 1. Glucose is stored as glycogen in the liver and muscle; however, we only limited stores of roughly 300-400g of glucose which can fuel only approximately 3-4hrs of medium intensity exercise. 2. Once the bodies’ stores are used, glucose needs to be consumed. However, only 1g per hour can be absorbed (for a 70Kg athlete, this equates to 280kcal). Considering we can burn anything between 300-500kcal/hr during an ultra endurance race, we are still unable to meet the energy demand by purely fuelling on carbs. On the other hand, fat is present in large amounts in comparison. A 70Kg athlete with 10% body fat will have approximately 7Kg of stored fat which equates to 69,000kcal. This is theoretically enough to provide energy to fuel several days of exercise.

How does this improve performance?

There are several ways that improving the body’s ability to burn fat as a fuel during ultra endurance exercise.  An important point I want to make first is this; fuel from carbohydrate is still important and some people are not suited to fat adaptation. Also, the emphasis is on ultra endurance events lasting >8hrs. In other words, events where the pace at which you exercise at has to be relatively slow.

Improve Fuel Efficiency

This essentially means that by increasing the efficiency at which you burn fat, you can go for longer and a faster speed. To give an example, let’s say you currently can only rely on fat as your predominant source of fuel while running at 9min/mile. Then, by improving your fat adaptation, you use the same amount of fat as fuel but can run at 8min/mile. This can improve performance through a variety of knock-on effects as discussed below

Carbohydrate Sparing

Simply, by using more fat for your fuel, you use less carbohydrate. This means that your carbohydrate reserves are retained in the muscle for longer where they can be used during periods where you have to increase the pace/intensity e.g. a steep climb, a break-away etc.  A more complex benefit to this is the effect it may have on metabolic fatigue. When the muscle is completely empty of carbohydrate and fat and protein cannot keep up with the ATP demands, an increase in reactive oxygen species occurs. This increases oxidative stress causing structural damage to muscle fibres and the mitochondria. This is essentially what causes the severe muscle damage that ultra endurance athletes experience after several hours with steep ascents and descents.

Protein Sparing

In a similar way to “carbohydrate sparing”, the more the body uses fat as a fuel, the less protein it uses. This may reduce the risk of ammonia build-up which is a by product of amino acid oxidation. Ammonia is linked to metabolic fatigue so the less ammonia the better.

Reduced Risk of GI Distress

It is conceivably possible to rely more on carbohydrate than fat and still perform well. As your glycogen pool empties (after 3-4hrs), glucose from food needs to be supplied to the muscle. The amount needed would have to be close enough to your energy expenditure. So if you were burning 400kcal per hour, you would need to consume roughly 300kcal. This would equate to 70g of glucose (1 sports bar = 30g, 1x 500ml sports drink = 30g). The issue with this is that it can cause GI distress. Consuming 70g of glucose per hour during a race lasting up to 24hrs or longer, is not only impractical but it can cause stomach upsets. Hence, needing less food is desirable.


So are there any studies to prove or support any of these benefits? Well, as I discussed in the introduction, there simply aren’t any performance studies conducted on athletes competing in these ultra endurance events. There are 3, 4 maybe 5hr studies, but as I mentioned, these findings cannot be directly related to what goes on metabolically in an event lasting 24hrs. However, there is some supporting evidence which I will summarise as follows:

  1. Fat adaptation increases rates of fat oxidation and spares carbohydrate (1)

The study conducted by Carey et al, used trained cyclists and the duration of exercise was 5hrs in total. Although there was no improvement in performance, the study did show that the rates of fat oxidation increased. Again, we can’t directly compare these finding to what happens during a much longer event so we can’t draw any definite conclusions. However, it does show that it is possible to improve your fuel efficiency, by increasing the rates of fat burning and limiting the use of carbs. Thus, the benefits discussed above could apply i.e. improved fuel efficiency, carbohydrate sparing, protein sparing and reduced GI distress.

2. Improved Performance (2)

There are one or two studies that actually show how endurance performance can be improved with fat adaptation. The studies showed that there was a varied response amongst the individual athletes – some performed better, some slower, some no difference. They then correlated this with the Respiratory Exchange Ratio (RER). This is a measure of how much fat you burn versus carbohydrate at various intensities. Those that naturally had a better RER value (i.e. burned more fat than carbs), showed an improved performance with the fat adaptation protocol. These findings are possibly what govern this whole concept i.e. it works well for some, not for others. As with all aspects related to health, nutrition, and exercise, no one shoe size fits all and individuals need to find out what works for them.

3. Improved metabolic adaptations (3)

There are a couple of studies which show how fat adaptation improves metabolic adaptations. These are things like upregulated lipid enzymes (carnitine acyltransferase) and transport molecules. Again, this did not make a direct improvement in performance but once more, the exercise study used was very different to the context of an ultra. The exercise protocol used in this particular study was 2.5hr steady cycling followed by a 40km time trial. Thus, we cannot correlate it with a 12 or 24hr event. However, we can postulate that the improved metabolic adaptations can contribute to the benefits previously discussed.

How do we Fat Adapt??

Finally, we get to the crux of the matter – how can nutrition and the way we eat help us to become fat burning machines?!?

That will all be revealed in my next article !


  1. Goedecke JH, Clark VR, Noakes TD, et al. The effects of medium-chain triacylglycerol and carbohydrate ingestion on ultra-endurance exercise performance. International Journal Sports Nutrition & Exercise Metabolism 2005; 15: 15-27.
  2. Carey AL, Staudacher HM, Cummings NK, et al. Effects of fat adaptation and carbohydrate restoration on prolonged endurance exercise. Journal Applied Physiology 2001; 91: 115-122.
  3. Van Proeyen K et al. Beneficial metabolic adaptations due to endurance exercise training in the fasted state. Journal of Applied Physiology 2011; 110: 236 – 245.