Fasted training

Welcome to our nutrition blog by Dr James Morton, Senior Sports Nutritionist for Science in Sport (SiS), where he will be giving advice on nutrition, dispelling fuelling myths and offering his tips every month. If you have a sports nutrition question or area you'd like to see covered in the blog, email

Dr James Morton is the Senior Sports Nutritionist for Science in Sport (SiS), the leaders in endurance nutrition. He has led and collaborated in over 40 research papers and publications and is also Senior Lecturer in Exercise Metabolism & Nutrition at Liverpool John Moores University (LJMU).

In recent years, our laboratory in Liverpool John Moores University (UK) and many others around the world (e.g. Denmark, Australia, Canada, Belgium) have collectively showed that carefully scheduled and deliberate periods of training with reduced carbohydrate (CHO) availability can actually enhance the aerobic adaptations that occur in our muscles as a result of endurance training. This innovative approach to training is, of course, in contrast to the traditional advice that every training session should always be completed with high carbohydrate availability and in glycogen loaded conditions. Nevertheless, because of the well established role of carbohydrate in improving performance, this approach to training has now been termed as the ‘train-low: compete-high’ model, advising that certain training sessions be commenced with reduced carbohydrate availability (so as to maximize training adaptation) but yet competition always be performed with high carbohydrate availability (so as to maximize performance).

There are many strategic approaches to training-low such as training twice per day with no feeding in between sessions, delaying feeding after a single session for several hours or perhaps most practical, simply performing a morning training session in a fasted state i.e. performing your session without consuming breakfast beforehand and limiting intake during exercise to water or low-calorie electrolyte drinks only. It is note worthy that training fasted is not necessarily a dietary approach of refraining from carbohydrate intake. Rather, training fasted simply means that breakfast is consumed after training as opposed to traditional approaches of consuming a carbohydrate rich breakfast before training.

The rationale for training low is based on the premise that it can enhance the mitondrial adaptations of our muscles, which effectively means that we have increased the capacity to use fat as a fuel. As a result, we use less carbohydrate during moderate steady state exercise thereby sparing our muscle glycogen stores for when we need it in the hard parts of the race. However, because on race day we typically consume high doses of carbohydrate in order to maximize performance, it is important that our muscles retain the capacity to utilize it and not just fat. For this reason, we believe that periods of training low should also be practiced alongside periods of training high (where your in-race fueling strategy is practiced) so that on race day itself, you now have a muscle that is well developed to use both fat and carbohydrate as a fuel. In practice, your weekend run (which is usually the longest session) is a good time to practice training high and your in-race strategy whereas training low should be practiced during your shorter sessions during the week.

Perhaps the main limitation to training fasted is the potential decrements in training intensity. Indeed, in a fasted state liver glycogen will be low and depending on the nutritional and training activity the evening before, muscle glycogen may also be compromised. This, in turn, means that blood glucose may become reduced (thus making the exercise seem harder) and the lack of muscle glycogen means that performing hard sessions (especially intervals) will be difficult. Training fasted (especially if muscle glycogen is low) can also lead to a hormonal and metabolic environment that increases muscle protein breakdown and can impair immune function. Training fasted repetitively can therefore lead to many detrimental effects if performed long-term.

For this reason, it is essential to target your fasted sessions to those days when training intensity and duration does not require a significant input from  metabolism e.g. 60 min moderate intensity type activity. Additionally, consuming protein before your session (i.e. a protein only breakfast) and moderate doses of caffeine (approx. 100-150 mg) is a suitable strategy to prevent muscle protein breakdown and partially restore any impairments in training intensity. In real food terms, 3 scrambled eggs and a cup of strong coffee would be suffice.  We must also pay close attention to hydration during our session and in this regard, a low calorie electrolyte drink consumed before and during fasted training will meet your hydration requirements whilst still allowing you to train low.

The low-calorie hydration drink, SiS GO Hydro, is available from Science in Sport (SiS)