Fasted Cardio: Training on an Empty Stomach

Fasted Cardio: Training on an Empty Stomach

Staff Writer
5 minute read


A piece of common and trendy advice for people trying to lose weight is that of going for a run early in the morning before having breakfast. This kind of fasted training is supposed to help you lose fat, ignite your metabolism for the rest of the day to achieve your weight and health goals, be more focused and productive the rest of the day, and have tons of other health benefits.

In this article, we will review what research says about this practice if it has something real going on and is safe to practice, or if it is just PR and potentially harmful to your goals and overall health.


There is a bunch of possible bright sides to doing some physical activity in a fasted state early in the morning before your breakfast:

Greater fat oxidation: There might be some truth here as some studies show that you can potentially burn up to 20% more calories if training on an empty stomach, it largely depends on what kind of activity you’re doing. Most of the positive results come from doing some form of lower intensity cardio compared to high intensity or heavy training. This can be explained by the fact that physical activity in a fasted state resorts more on calories from fatty acids rather than carbohydrates that are at the moment not easily available to the body

Better endurance:  We all know that frequent physical activity will help your body adapt better and get stronger and more resilient, for example increasing VO2Max which is a way to measure cardiovascular resistance and endurance. Research has found that training in a fasted state can diminish the rate of perceived exertion and the power-to-weight ratio up to a 40%. Basically means you might feel less tired during your morning training session if done on an empty stomach.

Satiation: There seems to be a good relationship between skipping breakfast before your morning exercise session and the amount of calories consumed during the day in comparison to doing your training session after breakfast, which leads to a negative energy balance and to weight and fat loss. Again, doing lower-intensity exercise 

Insulin sensitivity: Training in a fasted state can have a beneficial effect on glucose tolerance and insulin sensitivity. It is more potent than fed training to facilitate adaptations in muscle and improve glucose tolerance in the whole body.

Keep in mind all of these benefits seem to be more related to lower to moderate intensity exercise and forms of cardio than heavy weightlifting or more intense training sessions. That might be the key to reaping these benefits.


There are however some findings that might not be so promising and that should be weighed against the benefits before grabbing your running shoes first thing in the morning.

Muscle loss: A study found that exercising in with your muscles depleted of glycogen (fasted) could lead to a nitrogen loss of almost double in comparison to muscles that are filled with glycogen, this results in a loss of protein of around 10% of the total caloric cost of exercise. This basically means that for your early training session on an empty stomach your body might resort to breaking down your muscles and use that as fuel. 

Low energy: When training without food in your belly energy levels can be low if engaging in a high-intensity exercise like heavy weightlifting, which can also be translated into lower performance and increased injury risk.

Evidence conflict: Unfortunately there is a lot of conflict in research findings and mixed results on performance, maintaining muscle mass, and weight loss. However, there seems to be a common trait that if the exercise modality is of low to moderate intensity the effect might be positive, and on the other side, high intensity or heavy exercise will suffer and go against you. 


As always implementing some of this advice depends entirely on your goals and your current status, schedule, preferences, and what you are capable of doing. However, based on the research, there are some pointers you can take into account to give it a try safely and hopefully get some great benefits:

  1. Remember that the results are not consistent for every group and every exercise modality, so it depends a lot on your unique body and habits. Give it a try gradually and see how it affects you. After a couple of weeks if you’re feeling the benefits keep going, if not be ready to change course.

  2. Most research agrees on opting for lower to moderate intensity over high-intensity heavy training. So it might be smarter to go for light cardio activities and stay away from the weight room in your early morning session. 

  3. If you’re not sure how to start or are worried about overdoing it, maybe just a 10-20 minute walk early in the morning, before breakfast, and see how it feels. It might be more than enough to get the benefits of fasted cardio and those of a daily walk.

  4. Remember fasting still needs water. Stay hydrated before, during, and after your morning fasted training, Especially if you go closer to the moderate intensity range and it is hot outside.

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