Fasted Exercise: a potentially useful training tool? Tom Aird

The competitive athlete and recreational gym-goer may have different priorities in terms of what they’re looking to get from their workout, but one commonly sought after goal often unites them – improving body composition through a combination of adapting whole body metabolism, increasing lean mass and decreasing body fat %. Recently in both the fitness industry and scientific community, interest in novel nutritional concepts such as intermittent fasting and time-restricted feeding and their subsequent effects on metabolism have increased. A defining feature of these dietary strategies compared with more conventional interventions are not the reliance on monitoring caloric or macro-nutrient intake, but rather the time-frame in which nutrients are ingested. In other words, by reducing the time period of nutritional intake rather than the actual amount, whole body metabolism is improved. The proposed mechanisms by which this occurs is via increased expression of various genes regulating lipid utilisation and metabolism during periods of prolonged fasting, in turn improving whole body metabolism and even overall body composition.

Going one step further than this, combining the fasting nutritional paradigm with exercise to augment both metabolic and performance adaptations is something that has also garnered interest among scientists and practitioners alike in the last number of years. Recent experimental trials have shown that carbohydrate feeding before aerobic exercise blunts the activation of a number of the aforementioned genes regulating lipid metabolism in both skeletal muscle and adipose tissue. Long-term studies have shown that fasted training beneficially up-regulates exercise-induced mitochondrial adaptations in skeletal muscle, compared with fed conditions. These studies, while exploratory, provide promising evidence for the role of fasted exercise as part of an athlete’s training regimen, particularly those who are focused on improving substrate utilisation, mitochondrial adaptation, and whole body metabolism. However, evidence in this subject area remains sparse, particularly with regards to metabolic adaptation in key peripheral tissues such as skeletal muscle and adipose tissue.

In the context of exercise performance, it is well-established that pre-exercise carbohydrate feeding improves performance in a variety of sports. Therefore, athletes may often ask themselves why bother training under fasted conditions when, on competition day, they will likely be competing in a carbohydrate fed state. To provide a more definitive answer to this, and other questions, a recent systematic review and meta-analysis on the effects of fasted vs fed-state exercise on performance and post-exercise metabolism was conducted by members of the PESS department ( Indeed, the findings from this study indicated that while fasted exercise augmented markers of lipid metabolism and mitochondrial adaptation in key peripheral tissues, pre-exercise feeding augmented prolonged exercise performance. However, performance was unaffected during shorter duration (<60 minutes) exercise in fasted vs fed conditions.

The take home message from all of this? Train fasted to promote adaptation, compete fed to maximise performance! In particular, fasted training may prove a useful tool on training days when training volume/intensity is reduced, or when maximal effort/exertion is not required. While it doesn’t have to be as strictly regimented for those who more closely fit the characteristics of the recreational gym-goer, the addition of one or two sessions of fasted exercise into your weekly training schedule may yield benefits in terms of achieving your goals, be they either performance or health-related. The evidence indicates that it certainly won’t hurt!

Tom Aird is a Postgraduate Researcher in the Department of Physical Education and Sports Science at the University of Limerick.  His research interests include investigating the effects of fasted vs fed state high intensity interval training on performance and metabolic adaptations in skeletal muscle.   Contact Tom by email  or view his profile on Linked In.


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