The traditional approach that every training session should always be completed with high carbohydrate and loaded glycogen conditions has been challenged by a number of studies around the world. I recently came across an article by Dr James Morton, the Senior Sports Nutritionist for Science in Sport (SiS) and Senior Lecturer in Exercise Metabolism & Nutrition at Liverpool John Moores University (LJMU).
His lab in Liverpool has collectively shown that carefully scheduled and deliberate periods of training with reduced carbohydrate availability can actually enhance the aerobic adaptations that occur in our muscles as a result of endurance training. They advise that only certain training sessions be completed 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 ways to train low such as training twice per day with no food in between sessions, delaying eating 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 important to note 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 mitocondrial 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.
Perhaps the main limitation to training fasted is the potential decrements in training intensity. 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) is a suitable strategy to prevent muscle protein breakdown and partially restore any impairments in training intensity. 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
