If you have ever tried to lose weight through exercise, there is a good chance that someone has told you that the best way to do it is thorough fasted cardio. Fasted cardio is a training method which has stirred up divisive opinions, so what is it and what is the basis of those opinions?

What is Fasted Cardio?

Fasted cardio generally represents exercise of a moderate intensity, carried out between 30 minutes to an hour on exercise equipment such as a bike, treadmill, cross trainer or rower, on an empty stomach, after an approximate 8 hour fast (from sleep) prior to the first meal of the day.

What are the Benefits of Fasted Cardio?

The general thinking regarding the benefits of fasted cardio is that some individuals believe that more fat is burned by the body as fuel and that fat is burned more effectively from cardiovascular exercise following a fast. There is certainly some rational thinking to support this theory. Following consumption of food, the hormone insulin is released. Increased insulin within the bloodstream has been show to restrict the metabolism of fats (lipolysis). The stress hormone cortisol is also generally at its greatest first thing in the morning following a fast. Since cortisol is catabolic, it is thought by some individuals that these high levels promote the metabolism of fat. The body also has lower glycogen stores following a fast, meaning that once they are completely depleted, the body will use fat stores for energy.

Fasted cardio has also been shown to restrict appetite for the remainder of the day and increase the body’s capacity to sore glycogen in the muscles meaning that it may be effective to endurance athletes and those who engage in endurance activity.

The Fasted Cardio Counter-argument

Opposed to the number of supporters of fasted cardio is an equal amount if not greater amount of detractors. The opposition cite that the metabolism of fat (lipolysis) is not synonymous with the oxidation of fat (using it for energy). Moreover, whilst there is evidence that more fat is metabolised during fasted cardio, research conducted on fasted and non-fasted groups has shown no difference in the amount of fat oxidised during exercise between the groups, with the exception of exercise carried out for over 90mins.

The principle that fat must be used for energy once glycogen has become depleted represents the basis for many popularised diets. As a preference, the body will use glucose for energy. When glycogen levels have become exhausted, the body enters ketosis whereby fat stores are used to produce ketones which are used in the place of glucose. Whilst moderate intensity cardio represents an effective way of reducing body fat, if the body is not producing ketones, it requires glucose in order for the oxidation of fat to take place. Whilst an overnight fast will deplete glucose levels, they will not be depleted to the level of ketosis. It is more likely that the lower glycogen stores will result in fatigue resulting in performance deficits and subsequent fat burning.

Additionally, whilst fasted cardio has been shown to suppress the appetite, it has been shown that the consumption of food prior to cardio results in more energy being burned for the remainder of the day. Biomarkers for the oxidation of fats have shown that it is greatest following non-fasted high intensity training, and that oxidation of fat is lowest following moderate intensity fasted cardio.

Fasted Cardio and Losing Muscle

A frequent argument against fasted cardio is that it results in the loss of muscle tissue. During exercise muscle is in a catabolic state, and may be used for energy when glycogen and blood glucose levels are depleted. Research has shown that more nitrogen is excreted from muscles during fasted cardio than in non-fasted cardio. Given that nitrogen excretion is a common biomarker for muscle catabolism, it is indicative that fasted cardio may induce greater muscle breakdown than non-fasted cardio. Additionally cortisol, which is at its greatest following a fast, is also implicated in contributing to muscle catabolism. Therefore many individuals carry out cardio following a protein or amino acid shake in order to prevent muscle loss.

Fasted Cardio Summary

For many people, fasted cardio represents an effective method of reducing body fat, but for other people results will be poor. The best advice would be to break the fast from sleep by consuming a protein or essential amino acid shake prior to cardio in the morning, in order to prevent muscle protein breakdown and support post-workout fat oxidation. Remember that there is no one form of exercise that is ideal for everyone, and the best protocol is one to fit your own unique set of characteristics, so it is important to find the best type of exercise to fit you.