Is It Good to Do Cardio on an Empty Stomach?

Is working out on an empty stomach really better than fueling up before you hit the elliptical? It depends on your goals and when you're exercising. If you're stepping up your cardio game to speed up fat loss, you might want to consider waiting to eat breakfast until after your morning run.

But if you're training for a marathon and your run is in the afternoon or evening, you might want at least a quick snack to properly fuel your body before you lace up your sneakers.

Fasted cardio is just what it sounds like: Doing cardio training without eating beforehand. Typically, fasted cardio is done first thing in the morning, so the stomach has had all night to empty out. The theory is that doing cardio before breakfast will force the body to burn more of its stored fat.

Carbs and fats are the two fuel sources that are used to supply the energy needed for muscle contraction during physical activity. For cardio training done at a moderate intensity, the body obtains between 50 and 60 percent of the required energy from glycogen (which comes principally from carbs) and the rest from fat.

When glycogen stores are depleted while sleeping through the night, or even by going several hours without eating when awake, fat (in the form of fatty acids) is broken down by the mitochondria to be utilized as a backup energy source.

Research published in the October 2019 issue of the ​Journal of Clinical Endocrinology and Metabolism​ showed evidence of the potential benefits of fasted cardio. Men who have obesity or overweight who exercised on an empty stomach burned twice as much fat than those who exercised after breakfast.

Lean and otherwise healthy individuals can also burn more calories when exercising on an empty stomach compared to training after eating breakfast, according to an August 2019 study in ​The Journal of Nutrition​.

Unfortunately, despite the research that suggests you burn more fat at a workout done on an empty stomach, science doesn't necessarily show that's it's any more effective for weight loss in the long term. A study in the September 2014 issue of the ​Journal of the International Society of Sports Nutrition​ found that women lost almost the same amount of body weight with an hour of moderate-intensity cardio regardless if they had fasted or not.

​Read more:​ What to Eat Before and After Workouts to Lose Weight

Another potential benefit to doing cardio on an empty stomach: Fasted cardio may help increase your V02 max significantly when exercising after fasting overnight compared to eating carbs before training.

An article published on the UC Davis Health website states that a higher V02 max could allow you to produce more energy during exercise — which may help increase the number of calories burned during a workout session and improve performance.