There is no clear cut answer whether an athlete should train fasted or not. As with most things with nutrition, not one approach suits all. The evidence can guide but practitioners should use an individualised approach to each athletes circumstances.
Fasted training can be very beneficial. Some may choose to train fasted to accelerate training adaptations but it could also be a sub conscious act when an athlete does not fuel adequately out of bad practice or trains before eating in the morning out of convenience. Not every runner will respond well to fasted running though, so every athlete should listen to his or her own body when deciding on a training and nutrition programme.
What is fasted training ?
Fasted training is exercising with low stores of carbohydrate (glycogen). It can also be known as training "low". This can be achieved by excluding carbohydrate intake either pre, during or post training. Over the years, advice has moved away from the atypical chronic "high" intake of carbohydrate to fuel endurance based exercise. We now know that specific phases of training with reduced intake of carbohydrate can enhance the aerobic adaptations that occur in our muscles. The rationale behind this is it triggers mitrochondrial change ( mitochondria are the powerhouses of the cell that convert what we eat into usable energy in the body) meaning an increased ability to use fat as energy. As a result, muscle glycogen is spared in the moderate stages of exercise leaving more in reserves for the later, tougher stages of racing.
So how can we train “low”?
There are various ways an athlete can incorporate training low strategies. This infographic by Prof Asker Jeukendrup (mysportscience.com) shows clearly the ways in which it can be done.
1. Low Carbohydrate diet - a consistent low intake of carbohydrate will mean performing in a depleted state for every session. Energy is mainly provided by fat and protein. Over a long period of time changes in fat metabolism will occur but may also have a negative effect on carbohydrate metabolism and long term performance outcomes.
2. No carbohydrate during recovery - avoiding refuelling with carbohydrate will limit the repletion of glycogen post session.
3. Sleep low - this involves an evening training session and not eating carbohydrate post exercise. Overnight the body will further deplete glycogen stores. A training session will then be carried out pre breakfast in a glycogen depleted state. This will be a tough session!!
4. Training fasted in the morning - this is the most common approach for many athletes. Getting up and training early without any food. The session will be performed in an already depleted state following the over night fast.
5. Long training without carbohydrate - this is when the athlete will do several hours of training without taking additional carbohydrate during exercise. This may cause additional stress to the training load and contribute to additional training affect.
6. Training twice a day - the first training session reduces glycogen levels. Then restricting recovery carbohydrate after the first session will mean training in a depleted state for the second session of the day.
Is fasted training right for you?
Despite it's proven benefits, it isn't right for everyone. International marathon runner Anna Boniface trained 120 miles per week and previously ran a large amount of her training fasted. Unfortunately she suffered a stress fracture on her England marathon debut last year. " I used to think this would make me more fat adapted and more efficient. I would get a buzz from running that far and hard off nothing. I was blissfully unaware of how much stress this was causing my body. Retrospectively, it played havoc with my recovery and put me in a huge energy deficit. That continuous state of stress ultimately had long-term consequences for my health".
This is not uncommon. The problem however does not lie directly with the fasted sessions themselves but the dietary management around them. The adaptations to fasted sessions do not happen immediately. They occur later on when the athlete is well fuelled. Therefore, the athlete needs to pay close attention to nutrient intake and fuelling in and around sessions to ensure the desired training adaptations can occur. Simple steps such as following up fasted sessions with a high carbohydrate and protein meal and maintaining an optimal carbohydrate intake for the remaining training sessions will ensure the athlete progresses. If this does not happen, problems may develop. Athletes can experience many problems such as an impaired immune system and chronic fatigue. All resulting in underperformance in the long term.
Anna B also came to that conclusion "Since working with Alex, during my rehab phase, I have not done any fasted workouts. I notice how much harder I can work in later sessions in the day, where previously I haven't been able to perform as well as due under-fuelling in the morning. I can now work harder in training, recover faster and my performance is better". For Anna to get back to international level, in future months, fasted training will inevitably be part of her training schedule. However, improvements in fuelling around these sessions will critical and a structure will be created to meet her daily training requirements.
Training low is a simple tool you can use to get more out of your training. The results though are not instantaneous. It needs to be managed correctly and it isn't right for every athlete. It has be assessed on a individual basis. Steps need to be taken to ensure that nutrition strategies around fasted training are optimal. If the athlete gets the balance wrong it can catastrophic to progression and can lead to long term problems if not addressed.
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