‘Fasted cardio’, or exercising on an empty stomach, has come into mainstream media recently as the new exercise trend. Advocates of fasted cardio claim that it can help burn more calories and cause faster weight loss, while critics say that it is no better than traditional exercise.

We take an inside look at both answers to the question, “does exercising on an empty stomach cause faster weight loss?”

“Yes.”

Several studies, including one from the UK published in the British Journal of Nutrition, has found that when participants had fasted before taking part in cardiovascular exercise in the morning they burned 20% more fat than when they had eaten before training [1].

The theory behind fasted cardio is that as you sleep and fast overnight your body conserves its stores of carbohydrate and switches to using the body’s fat resources for fuel.

People who particularly want to lose fat (as opposed to muscle building, for example) should consider incorporating fasted cardio into their training regimen and supporters of fasted cardio also claim that it is especially useful for those with ‘stubborn’ areas of fat that are hard to shift such as hips and thighs for women, and lower back and abs for men.

Caffeine, which has been found in many studies to be helpful to fat loss in conjunction with a calorie deficit diet and exercise, is also more easily absorbed before a workout if the person is in a fasted state [2].

There is also the psychological motivation of exercising hard at the gym in order to finish a workout and eat, which may help people who find it difficult to motivate themselves to get up and go in the morning. Linked with these psychological effects, doing exercise in the morning tends to put people in a more positive and motivated mindset to make healthier choices throughout the day as they don’t want to spoil their good work, leading to faster weight loss.

Any nutrition that you take in after completing fasted cardio will be better absorbed than if you had eaten pre-exercise, which means it is also less likely to be laid down as fat stores.

“No.”

While fasted cardio may help with a small increase in calories burned during exercise, that is only one small factor in the bigger picture of weight loss.

What is more important for achieving and maintaining a healthy weight long term is creating lean muscle mass, which increases metabolism around the clock leading to a higher volume of calories burned every day.

Your body breaks down amino acids (proteins) into glucose overnight and so exercising while in a fasted state mobilizes proteins as well as fats for fuel, which obstructs muscle building and can easily lead to muscle loss. You can temper some of these effects by consuming a combination of fast and slow digesting protein after your exercise but it is not ideal.

Fasted cardio works better for slow, easy workouts such as a two-mile walk, but these are not the kind of workouts that will kick your metabolism up a notch and create a long term weight loss pattern.

For those on lower carbohydrate diets, doing fasted cardio could create an extra problem- without sufficient glycogen stores readily available for your body to use, you are likely to ‘hit the wall’ during exercise and run out of energy [3].

A lack of weight bearing exercise can contribute to muscle wastage, leading to a decreased rate of metabolism.

People who took part in a trial of fasted cardio also found that they were much hungrier throughout the day than when they exercised after eating- this could create a cycle of self-sabotage, as the few extra calories that were burned during exercise are replaced by taking in more food throughout the day.

Conclusion

Some people prefer to exercise first thing in the morning, and there is plenty of evidence to suggest that this is the most efficient and guaranteed way of ensuring that you get in your daily workout before life gets in the way. Whether you exercise before or after eating is a personal choice that has both benefits and things to consider before making it a long term part of your workout regimen.

If you do decide to partake in fasted cardio be aware that you may feel hungrier throughout the day, so have low calorie, low sugar snacks such as vegetable crudites to snack on so that you do not overeat. You should also ensure that you incorporate resistance exercise into your workout plan and consume both fast and slow digesting protein after each session.

Hannah Canavan
Hannah Canavan
Researcher at DietProbe
Hannah is a health and lifestyle journalist with a passion for veganism and nutrition.
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