• 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 d