Does doing cardio on an empty stomach bring any increased benefits? If you're stepping up your cardio routine to speed up weight loss, you might think that skipping a meal before your workout or getting in your cardio before breakfast will make a big difference.
Exercising in a fasted state can help burn fat quickly, but most people should eat before a workout to properly fuel their body.
Doing cardio on an empty stomach might burn more fat and increase your VO2 max.
What Is Fasted Cardio?
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.
Fasted Cardio Burns More 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 March 2017 issue of the American Journal of Physiology, Endocrinology and Metabolism showed evidence of the potential benefits of fasted cardio. When overweight men walked for an hour on an empty stomach, it activated specific genes that increased their rate of burning body fat (instead of glycogen), compared to others who exercised after eating a breakfast with a high carbohydrate content.
It's not just overweight people who can burn more fat by doing cardio on an empty stomach. Lean and otherwise healthy individuals can burn almost 20 percent more fat when doing cardio on an empty stomach compared to training after eating a carb-rich breakfast, according to a study in the August 2013 British Journal of Nutrition.
Cardio Before Breakfast
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. Unfortunately, despite the research that suggests you burn more fat at a workout done on an empty stomach, science doesn't show that you lose more fat in the long run with such a strategy.
But, there's still an important benefit to doing cardio on an empty stomach; fasted exercise can help increase your V02 max significantly when they exercised after fasting overnight compared to when they ate 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.
- UC Davis Health Staff: "Oxygen Consumption - VO2"
- American Journal of Physiology, Endocrinology and Metabolism: "Feeding Influences Adipose Tissue Responses to Exercise in Overweight Men"
- British Journal of Nutrition: "Breakfast and Exercise Contingently Affect Postprandial Metabolism and Energy Balance in Physically Active Males"
- Journal of the International Society of Sports Nutrition: "Body Composition Changes Associated With Fasted Versus Non-Fasted Aerobic Exercise"