Firstly, what is fasted Cardio?
Fasted cardio is the process of completing aerobic exercise in a fasted-state. A fasted-state being on an empty stomach, particularly after an overnight fast.
This theory has been popularised over the last 10 years or so. Many fitness professionals adhered to the hypothesis that fasted-state cardio can lead to greater fat loss than fed-state cardio. The key claim for fasted-state cardio being a superior aid to fat loss, is that when you fast overnight and subsequently train upon waking, the body is depleted of glycogen (the stored form of glucose) – which is it’s primary source of energy. This leads to the body using the next best energy source in the body, which is stored fat.
Sounds good, right? However:
Whilst it is correct that during fasted-state exercise the body will utilise more stored fat as an energy source than glycogen, the body is dynamic and constantly adapting. Thereby, to simply state that fasted-state cardio is superior because it burns more fat than fed-state cardio would be short sighted and unscrupulous.
The result of this small time sample is soon made obsolete. That’s because later in the day the body will compensate for the usage of fat stores through greater carbohydrate utilisation throughout the rest of the day. This means that when total caloric expenditure (calories burned) is the same, fat-loss is the same during a 24-hour period wether fasted-state cardio or fed-state cardio has been performed.
Research says:
This hypothesis has been tested many times within studies. For example, Brad Schoenfeld et al (2014, NIH:25429252) tested two experimental groups over 4 weeks to measure changes in weight and fat mass between a group performing fed-state aerobic exercise versus a group performing fasted-state aerobic exercise.
The significant variables were controlled, with the participants following meal plans to induce a caloric deficit, receiving nutritional counselling for dietary adherence and performing an hour of steady-state cardio 3 days per week.
Both groups showed a significant loss of weight and fat mass from their starting point, but there were no significant group differences measured in the outcome. The findings suggest that body composition changes and overall fat loss following aerobic exercise are similar regardless of wether or not the individual is fasted or not prior to exercise.
Conclusions:
If fat-loss is the goal, then performing exercise is equally beneficial when performed in a fed-state or a fasted-state. If you prefer to exercise in a fed-state or a fasted-state, then you can continue to do so either way without missing out on any significant benefits.
But if muscle-building is the goal, then resistance training can be negatively impacted by training in a fasted-state if glycogen levels are already low.
Fasted-state cardio may be beneficial for you if you feel more energetic without eating before a workout, or if you are on a tight schedule.
Lastly, it was previously stated that when you fast overnight and subsequently train upon waking, the body is depleted of glycogen. Whilst this is mostly true, sleep doesn’t use much glycogen. It is likely that sleep will burn through the glycogen stores in the liver, but it won’t make much difference to the glycogen stored in the muscles.
