• Mark Turnbull

Fasted Training or ‘Training Low’

Updated: Mar 19



As an athlete or physically active ‘competitive’ person, are you in the slightest bit confused as to whether you should be consuming a diet rich in carbohydrate, or restricting carbohydrate intake and leaning more towards fat and protein as your major nutrient sources? There are certainly some mixed messages out there, so we feel that this topic desperately needs to be discussed in a balanced fashion to clear up any ambiguity.


We’ve known for a long time that carbohydrate is an essential fuel source for endurance performance. Without sufficient carbohydrate available for our working muscles, we can’t perform well in competition and struggle to complete hard training sessions. So, when performance is the goal, it is vital to fuel with carbohydrates before, during and after competition.


Our nutritional approach to competition, where the main goal is to maximise our performance and achieve our athletic potential, isn’t necessarily always the best approach to take when training, where the main goal is to put the body under physiological stress to stimulate it to adapt, recover and become stronger. In this article, we are going to look at how we can use a nutrition strategy called ‘training low’ or ‘fasted training’ to maximise the response we get to selected training sessions.


High Carb, Low Carb and Fat Adaptation


There is a big fight going on in the sports nutrition arena at the moment, with two sides arguing over how endurance athletes should best fuel training and competition. On one side is a group arguing that a low carbohydrate high fat diet is the best approach, whereas the other is arguing that a high carbohydrate approach is best.




When we look at the fuels that our bodies utilise to create the energy for our working muscles, we use two predominate sources, fat and carbohydrate. We have abundant stores of fat as fuel. For example, a relatively lean 70kg athlete with 10% body fat would have in the region of 63,000Kcal of fat stored, which would be enough to fuel a low intensity walk of a few thousand miles without the need to refuel. Carbohydrate on the other hand is a much more finite fuel reserve. A trained athlete who was well fuelled and rested would only have in the region of 2000-3000Kcal of carbohydrate stored within their muscles and liver as glycogen, and this is only enough for as little as 90 minutes of high intensity exercise.



When we look at how our bodies utilise these fuels, as illustrated in the graph above, fat is the predominant fuel source for low intensity exercise, with the relative contribution of fat (white line) decreasing as aerobic power increases. On the other hand, the contribution of carbohydrate (orange line) is relatively low until higher exercise intensities are reached. When we start exercising at above 80% of our aerobic power, we are almost entirely reliant on carbohydrate. Carbohydrate is a much easier fuel source for the body to use at high intensity, as it requires less oxygen to breakdown, so give us a greater economy, but as explained above, it is a finite resource. Through training and nutrition, we can increase the contribution of fat as a fuel source to exercise and this is a key area of interest for many athletes, as the more we can rely on our abundant fat reserves, the longer our glycogen stores will last for, which will delay the point at which we blow up/hit the wall and get the associated catastrophic drop in performance. ‘Blowing up’ or ‘Hitting the wall’ or ‘Bonking’ are all expressions used to describe the painful, debilitating and performance-crippling effects of running out of carbohydrate when exercising!


Low Carbohydrate High Fat Diets (#LCHF) and Fat Adaptation – In a bid to increase fat burning, one relatively extreme approach that some athletes have adopted is to follow a low carbohydrate, high fat diet. By reducing the amount of dietary carbohydrate to <25% of total energy intake, in as little as 6 days, the muscles retool to be able to utilise fat at a much higher intensity. This process is often referred to as ‘Fat Adaptation’. This has benefits in that it allows us to more effectively tap into the abundant reserves of fat stored within the body and use fat at a higher relative exercise intensity.



There are some significant downsides though. Even with a very well trained fat metabolism we can only use fat up to around 60-75% of our aerobic power, which effectively means that we remove our ‘top gears’. By limiting the carbohydrate in our diet to relatively small amounts, we also switch off our ability to utilise carbohydrate, which means that even when we top our glycogen stores back up before competition, we cannot use carbohydrate effectively and our ability to perform high intensity efforts is destroyed. It’s these high intensity efforts that often win us races or allow us to beat our competitors, so despite the proven benefits, typically this leads to a decrease in overall performance. This is reflected in the scientific literature, where to date, no studies have convincingly shown an increase in endurance performance as a result of consuming a low carbohydrate high fat diet. Reducing carbohydrate in the diet may also put you at a greater risk of illness and infection as our immune systems prefer to use carbohydrate as a fuel source.



High Carbohydrate Diet – This is the more traditional approach to training nutrition and reflects the approach that many athletes have to competition. All training sessions are completed with the goal of ensuring high carbohydrate availability (i.e. fuelling before/during/after). This allows high volume, high quality training to be completed, with rapid recovery. It replicates the demands of competition, limits the chance of overtraining and helps the gut to adapt to a high carbohydrate intake, reducing the risk of stomach issues during competition. Carbohydrate is also vital for optimal muscle and cognitive function, so this approach helps to ensure high quality training and reduces injury risk. There is also a very large body of evidence to show that high carbohydrate availability can significantly enhance endurance performance. Stellingwerff & Cox (2014) conducted a comprehensive review on the topic (referenced at the end of this article).


However, we need to marry this with the benefits that can be achieved through fat adaptation training, where the diet is very low in carbohydrate and accept that fuelling every session in an optimal way with carbohydrate can lead to a reduction in our ability to utilise fat. Naturally, the body will use carbohydrate in preference to fat, when carbohydrate is readily available.



So, you may be wondering which approach is best for training? Well, as with many things in sports science and life in general, it’s not black and white and the answer lies somewhere within the middle ground. A combination of training with both high and low carbohydrate availability can help us to develop a body that can effectively use both carbohydrate and fat effectively (often referred to as metabolic flexibility) which ultimately will be the best way to enhance our performance and improve as endurance athletes.


For the next part of this article we are going to take a detailed look at exactly what ‘Fasted Training’ or ‘Training Low’ as it’s commonly called, actually is, what the advantages and disadvantages might be, and how you might fit this nutritional strategy into your own training program. It’s highly likely that you may already train in this way to some degree, perhaps by chance rather than design, but a better understanding of it will help you to maximise the benefits and help manage some of the downsides. ‘Training Low’ (TL) by the way, must not be confused with training at low altitudes – it’s about training with low carbohydrate availability.


What is Training Low/Periodised Carbohydrate Intake?


TL is a form of nutritional periodization, much in the same way that your training sessions aren’t the same day in day out, your nutrition shouldn’t be either. Essentially we are looking to fuel specifically for the work required, matching the amount of carbohydrate that we consume to the demands of the training sessions that you are completing. For example, if you are completing a high intensity interval type session, then it needs to be well fuelled with carbohydrate. On the other hand, if you are completing a low intensity training session, you don’t necessarily need to consume lots of carbohydrate and can achieve benefits from carefully limiting carbohydrate intake. This means that although some training sessions would be deliberately focussed at maximising physiological adaptations through adopting a low carbohydrate regimen, others would need to be optimally fuelled to achieve the associated benefits. Competition would always be completed with high carbohydrate availability, as put simply, carbohydrate is king for performance. This is often referred to as the ‘Train Low/Compete High’ paradigm for nutritional periodisation.


TL means exercising with low carbohydrate availability – limiting the fuel’s supply to the working muscles. This can be achieved through a) Starting a training session with low glycogen stores, by depleting/reducing them prior to starting the sessions and/or b) Limiting the consumption of foods containing carbohydrate before/during a training session. Some examples of how this might look in practice include:


Training 6-10 hours after your last meal – often referred to as fasted training.

Completing an endurance training session longer than 60-90 minutes without consuming any/sufficient carbohydrate during the session.

Training before breakfast, having not eaten since the previous day.

Training twice a day and not refuelling with sufficient carbohydrate in-between sessions – it can often be impossible to replenish glycogen stores with less than8 hours between sessions.

Training in the evening and again the following morning with limited/no carbohydrate consumed in between.


Many athletes already inadvertently train/recover in a low carbohydrate state without realising it. We can all hold our hands up to forgetting to consume sufficient fuel on a long training session, missing a meal before training because of a busy schedule, or not refuelling sufficiently after a hard session. To maximise the impact of TL however, it makes sense to recognise when you are and aren’t doing so, to enable you to schedule your training sessions properly and benefit from a correctly periodised and managed approach to the concept.

What are the potential advantages of ‘Training Low’?

Training low is not some magical training technique that will suddenly turn you into a super elite athlete after a couple of sessions, but used appropriately can help you to get a little bit more out of some of your training sessions, which when completed consistently over a period of time, could lead to some improvement in your performance.


One of the most important adaptions to endurance training is an increase in the number of our mitochondria and this occurs through a process called ‘mitochondrial biogenesis’. Mitochondria are basically the engine of the muscle cells, responsible for burning both fat and carbohydrate. The more mitochondria that we have, the better our exercise capacity (i.e. with more mitochondria we can perform more work over a shorter period). Although nobody has ever won an Olympic games through having the most mitochondria, it’s a key part of our physiology that can help our performance.



The stress associated with exercise leads to our muscles sending out signals (or ‘gene expression’ as its referred to in science). These signals dictate how the muscles respond and adapt to that sessions. In the case of training with low carbohydrate availability, the increased stress response from not having carbohydrate available as a fuel, leads to an increase in signals associated with mitochondrial biogenesis and means that training low results in a greater amount of mitochondrial biogenesis than training with a high carbohydrate availability. This effectively means that you get more bang for your buck when it comes to training with low carbohydrate availability. Several studies have now shown that completing selected sessions with low carbohydrate availability can lead to positive training adaptations associated with fat metabolism, (of which mitochondria biogenesis is a key one), to a greater degree than training with high carbohydrate availability for all sessions. Several studies listed at the end of this article support these findings (Bartlett et al. 2013; Hulston et al., 2010; Lane et al. 2015; Morton et al., 2009; Van Proyen et al. 2011; Yeo et al., 2008).


It is important to note, that although very important, the increase in mitochondria is only one of many adaptations associated with endurance training and if we were to train low all the time, we wouldn’t get any of the key changes associated with utilising carbohydrate from ‘Training High’ (TH). It is therefore important to balance both high and low carbohydrate training and to ensure high carbohydrate availability on competition day to ensure optimal performance. It runs deeper than this too, because failure to perform high intensity endurance and anaerobic interval sessions as part of your periodised training plan will leave you ill-prepared for competition. The key to TL is to tap the benefits of mitochondrial biogenesis without destroying your ability to burn carbohydrate and your form in general!


There is now emerging scientific evidence that periodised TL sessions not only lead to an increase in specific training adaptations, but importantly, can also lead to increases in endurance exercise performance when balanced with TH sessions. Two studies by Marquet et al, published in late 2016, have shown performance improvements in relatively short time frames, in moderately trained athletes, as a result to including some TL sessions in combination with some TH sessions, as part of a structured training plan. Interestingly, these studies have compared two separate groups of athletes with identical training loads and carbohydrate intakes, the only difference between them being the timing of the feeding of carbohydrate. So, one group completed all the sessions with high carbohydrate availability whilst the other performs some training with high and some with low carbohydrate availability. Simply by adjusting the timing of carbohydrate intake in a structured fashion, these researchers were able to improve performance in these moderately trained athletes in as little as 1-3 weeks.


TL is a relatively new and exciting topic in the sports science research, but being a fledgling area of investigation means that a lot of questions remain unanswered. Very little is known about how best to structure this type of training into an athlete’s schedule overall. With this in mind, it stands to reason that TL must be very carefully managed, if considered at all, particularly for athletes who may already train low ‘accidentally’ because of heavy training loads. At the end of this article, we will of course communicate our recommendations based on our assessment of the available information.


What are the disadvantages of TL?


There are several disadvantages to completing TL sessions, which include:


Training Intensity – Training low makes sessions feel much harder than training with high carbohydrate availability, so even a relatively easy session can be perceived as being very hard! Because the training experience can feel so unpleasant, motivation can drop and training can end up feeling like a chore rather than a pleasant developmental experience. It also makes it very hard/impossible to complete intense training sessions, so it makes sense to only complete relatively low intensity training sessions when training low.


Protein Breakdown – Training low leads to an increase in the breakdown of muscle protein for use as a fuel (ironically, your body converts protein into carbohydrate at an energy cost. It’s called ‘gluconeogenesis’). If fasted training is performed frequently, this can accumulate into a significant loss of muscle, which will have a negative effect on your performance.


Immune Function – Performing unaccustomed, long duration or high intensity training sessions in a low carbohydrate state will put your immune system under significant stress, leaving you at much greater risk of illness and infections, which will clearly set back your training.


Recovery – Once you deplete your body’s stores of glycogen, it can take up to 24-48hours consuming a high carbohydrate diet to fully replenish them. If you are completing a training session with low glycogen stores and have limited time before your next high intensity training session where carbohydrate is needed, a fasted session could compromise your performance during the following session.


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