Why You Should Avoid Fasted Training

If you are an endurance athlete, you have probably heard of the term 'fasted training. More recently, there has been a rise in athletes practising 'Intermittent Fasting'. While both have been heralded as advantageous to performance and health, is that really the case? Is fasted training and intermittent fasting beneficial, or should we be avoiding them?

What is Fasted Training?

Fasted training is when an athlete performs a training session in a fasted state, usually first thing in the morning.

Studies have shown that, while there may be some potential benefits to this, these sessions need to be performed at your easy / recovery pace, for no more than 60 minutes and ideally no more than twice a week.

What is Intermittent Fasting?

Intermittent Fasting (IF) was first made popular as a means of weight loss in 2012 by BBC Broadcast Journalist Michael Mosley when he published his book, “Eat Fast, Live Longer”. Since then, there have been numerous others who have followed in his footsteps, including high profile athletes.

There are two main versions of IF.

The first one involves a prolonged period of fasting with a specific ‘window’ in which to consume food. The most well known is 16:8, where the individual consumes their meals within an 8-hour window, and then fasts for 16-hours. The timing is up to the individual, but it needs to be maintained. So, if they choose to eat between 12pm-8pm, then they would fast from 8pm until 12pm. While this is the most popular, new variations have been introduced such as 12:12 or 14:10.

The second involves 5:2 where you eat “normally” 5 days of the week, and during the remaining 2 days of the week your intake is limited to just 500 calories a day. Similar to the time restricted version of IF, different versions of this form of IF have also arisen including alternate day fasting. This involves eating as normal on alternate days and limiting calorie intake to 500 calories on the other days.

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Beyond weight loss, the additional benefits boasted by following an IF model include slowing the ageing process, reduced oxidative stress, improved body composition and improvement in metabolic biomarkers, which is why so many athletes have also started to employ this approach around their training.

Is there any scientific evidence to suggest either practice, especially when it comes to performance gains?

Fasted training is very popular for individuals who partake in endurance events. There is a belief that if you train in a depleted state - especially carbohydrate depleted - then your body improves its ability to use more fat for fuel which can be advantageous for maintaining pace in longer events. However, while this makes sense, it is not such a simple equation.

In fact, the issue when employing fasted training in longer training sessions is that it has the tendency to add stress to the system and elevate your stress hormones, particularly cortisol and sometimes prolactin. While the body can cope with a small amount of stress, when these stress hormones stay chronically high they have a negative impact on both health and performance.

In women, this response is particularly sensitive. In general it is advised that females should avoid fasted training and fasting all together as it can have severe negative consequences to your reproductive hormones. Low reproductive hormones affect your health, lowers mood, impacts body composition and decreases bone health, as well as performance including reduced capacity to train, poor adaptation from performance, poor balance and higher risk of injury.

Professionally, I have huge concerns about either approach for anyone who is physically active.

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Nutrition is integral to both health and performance. In order to get the most out of your performance, it is more important to ensure you fuel this appropriately, which means fueling properly before, potentially during (depending on how long your session is), and then immediately after for your recovery. Sufficient nutrition, particularly in daily energy intake, is necessary for the hormonal cascade needed not just for progression in your performance, but also to ensure that you prevent metabolic injury which in the long run can lead to the body down regulating and preserving energy, which in turn impacts numerous processes in the body including body composition.

It is also important to appreciate that the human body is biologically biassed towards energy balance. So, there is a real concern that in some individuals who may be susceptible to developing dysfunctional eating behaviours, large periods of time fasting may then lead to periods of time of eating to excess, resulting in difficult thoughts and emotions which then set the scene for more restrictive behaviours and the start of binge / purge / restrict cycle.