Sprinting on empty: fasted training for better adaptation? dr. brian carson.

Why train fasted?

Training in the fasted state has been practised by athletes for various reasons for centuries. Most early adopters likely pursued fasted training based on the fact that they trained early in the morning and wanted to avoid gastrointestinal discomfort associated with having breakfast just beforehand. More recently, fasted training became an area of interest for coaches, athletes and researchers alike with enhanced adaptation to training and ultimately improved performance being the goal. 

Training in fasted conditions has been shown to alter the fuel sources we use during exercise, with a shift from burning carbohydrate towards fat. As a result or a by-product, endurance training while fasted has been proven to induce favourable skeletal muscle metabolic adaptations compared to carbohydrate feeding, manifesting in improved exercise performance over time. It is hypothesised that by burning more fat during exercise we are training these metabolic systems in the body, and ultimately that helps us enhance our overall energy supply lines and spare our carbohydrate stores for when we need them during performance.

But what about performance during training when fasted?

The research does not suggest we perform on the day of our event in the fasted state, particularly if the duration of the event is greater than 60 minutes (We previously published a Meta-Analysis that showed that performance was not significantly impacted in the fasted state when the session was below 60 minutes in duration). But during training when we are not as concerned about top end times and speeds, doing so in the fasted state is unlikely to have a major detriment to your session and is more focused on the long term pay off in enhanced adaptation to training.

What about high intensity or sprint interval training?

Few studies have looked at high intensity interval training in the fasted state. In a recent study we decided to establish if there was a role in metabolic and performance adaptations to Sprint Interval training (SIT) in the fasted state when compared with pre-exercise feeding. We found that fasted SIT resulted in some additional beneficial adaptations compared to pre-exercise carbohydrate feeding. These adaptations were in the expression of genes associated with the switch between fat and carbohydrate metabolism and also in genes associated with NAD+ metabolism which is an important molecule involved in the conversion of nutrients to energy in the muscle. Despite these metabolic adaptations which we might hypothesis improve the metabolism of the muscle, we did not observe similar differences in either sprint or endurance performance. However, this intervention was over a short period including just 9 sessions of SIT over 3 weeks. We think we would need to extend this to observe differences in athlete performance based on the metabolic adaptations we did observe. I discuss this in more detail in a recent interview you can see here.

Take home message

Fasted training can be successfully and strategically incorporated into your overall programme. Fasted training can enhance metabolic adaptations when compared with pre-exercise carbohydrate feeding. There is no evidence of any downside or negative impact of training in the fated state for either adaptation of performance. Fasted training has been shown to improve endurance performance, but as yet, we do not have any clear evidence that fasted sprint interval training impacts performance.

Aird infographic screenshot

Dr. Brian Carson is a Senior Lecturer in Exercise Physiology in the PESS department at the University of Limerick. His research interests are in the skeletal muscle metabolic adaptations to physical activity, exercise and nutrition. 

Contact: brian.carson@ul.ie   @DrBPCarson   ORCID

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