One of the main reasons that people do cardio (besides the health benefits) is to look better. The idea is that by burning more calories with increased activity levels — it will help to tighten the tummy and those flabby arms.
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But, does this mean that we should skip meals before exercising to maximize fat loss?
Of course, the body needs carbs and fats to fuel those cardio sessions, so logically, people shouldn’t do cardio on an empty stomach. But, sometimes biology isn’t logical. Understand the potential benefits, and drawbacks, of exercising in a fasted state.
Read more: How Much Cardio to Lose 30 Pounds?
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 by doing cardio on an empty stomach, the body will be forced to burn more of its stored fat.
How Fasted Cardio Can Help Burn 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.
In a small study published in the March 2005 Journal of Physiology and Pharmacology, six healthy men were tested to determine the fat burning effect of cardio on an empty stomach. After only a half hour of cardio, those who fasted burned more fat than the test subjects who had a sugary drink before exercising.
Research published in the March 2017 American Journal of Physiology, Endocrinology and Metabolism showed further 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.
Is it Good to do Cardio on an Empty Stomach to Lose Weight?
A study in the September 2014 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; healthy men and women increased their V02 max significantly when they exercised after fasting overnight compared to when they ate carbs before training, according to a study in the July 2010 Journal of Science and Medicine in Sports00073-3/fulltext). 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 amount of calories burned during a workout session and improve performance.
Read more: The Best Cardio Exercise to Lose Belly Fat.