It has long been thought that training in a fasted stated would elicit better results for fat loss. The method has been popularized by the world of boxing and body building. But does fasted cardio pose additional benefit to reducing body fat compared to fed cardio? It is a well-known fact that pre-exercise carbohydrate intake mediates aerobic performance improvement by way of limiting the onset of fatigue. This is due to the ability of the body to maintain preferential substrate use of carbohydrate stores and thus yield rapid ATP turnover, maintaining performance levels for longer, especially at higher intensities where aerobic oxidation is less involved.

It has long been postulated that exercising in the fasted state would shift this carbohydrate dependence to fat oxidation and thus translate into a higher rate of fat loss. This theory holds some truth, exercising in the fasted state does in fact cause a shift to greater levels fat oxidation, especially when low muscle glycogen levels are present. But does this translate to greater rates of fat loss?

Exercise in fasted state interestingly brings about adaptations that favor fat oxidation, improves insulin sensitivity, glucose tolerance and increases various lipolytic enzymes. And the confusion between the “why” of doing fasted cardio has certainly increased the popularity of the misconception that fasted cardio is superior.Controversy exists with the claim that this increase in fat oxidation would translate into the better rate of body fat loss. The evidence in quality research is lacking.

Why is this?

Research Methods

Often in research of fed and fasted trials, the experimental design is highly flawed and only weak conclusions and associations can be made from the results obtained. One of the major flaws being the absence of dietary control with respect to energy intake differences between subjects. When isocaloric conditions are not present between subject groups, then valid conclusions cannot be drawn on whether fed is better than fasted for fat loss.

There is research available which shows greater rates of fat loss with participants of fasted conditions compared to fed conditions but the major confounding variable has not been accounted for (energy balance).

Self-reported or unreported dietary intakes are inappropriate methods for the question at hand. Some subjects could be consuming far less energy than they are expending which is driving the rate of fat loss and vice versa. Increases in fat oxidation do not translate to increases in fat loss. Why? Fat loss only occurs when a person is in a net negative energy balance.

Energy Balance & The First Law of Thermodynamics

Body fat loss is a result of shifting energy balance to favor expenditure over intake and thus driving a net negative energy balance. Regardless of fed vs fasted exercise, fat loss will only occur when this is the case. The first law of thermodynamics states that energy cannot be created or destroyed, it can only be transformed from one form to another.

Metabolic Flexibility

The human body is very adaptable and in particular to the demands imposed on it. This why the theory of fasted exercise makes sense. Doing fasted cardio increases fat oxidation acutely But what happens after the exercise and when feeding occurs? The human body is constantly adjusting its use of substrate for fuel.

This increase in fat oxidation is compensated for later in day with a rise in carbohydrate utilization. Therefore fat loss must be looked at using a wide lens, over days, not hours. Increases in fat oxidation from fasted exercise can be neutralized by an increase in thermic effect of exercise by way of pre-exercise eating. Exercise trials whereby high intensity exercise is performed shows a reverse trend, increased carbohydrate utilization during exercise and then a post exercise rise in fat oxidation.

Fat Balance

Fat balance is the difference between fat storage and fat utilization. Fat balance can help better describe why increases in fat oxidation do not translate to increases in fat loss, especially when dietary fat intakes are greater. Many charlatans argue that by increasing fat intake, fat substrate utilization will increases and again this will translate into greater rates of fat loss. These charlatans are often of the combined opinion that fasted exercise will aid in this fat loss.

However, this theory is flawed.

Yes, when fat intake increases, fat utilization will also increase. The body adapts to the fuel provided. But this does not mean fat loss will occur, as fat storage will be higher because of increase in intakes. Just as when higher carbohydrate intakes shift fat oxidation to lower rates, fat storage lowers! For fat loss to occur, the energy expended must be greater than intake.

This further highlights that the breakdown of fat:carb intake does not matter much for fat loss.

Take Home Message

Fasted exercise is a valuable training tool when looking to chase certain training adaptations, however, when it comes to fat loss, fasted exercise is not superior. Fat loss occurs as a result of perturbing energy balance in favor of expenditure over intakes. It is better to train fed if that is what makes you feel and perform better.

The dietary and training structure should suit both your goals and likes.