What Happens When You Do Cardio on an Empty Stomach?


Is there an advantage to loosing weight when you do fasted cardio?

Is it more effective to do cardio on an empty stomach? We know that good nutrition fuels exercise. Macronutrients, particularly fat and carbohydrates, are the fuel that drives muscle contractions. What about protein? More commonly, your body uses protein as a building block for muscle tissue rather than as a fuel. Only when you’re in a carb-depleted state does your body turn to protein for fuel to make ATP. Your liver has the capacity to turn the amino acids in protein into glucose, which in turn cells can use to make ATP.

With nutrition being so important, why would you consider doing cardio on an empty stomach? Some experts believe that doing cardio in a low glycogen state, for example, first thing in the morning before breakfast leads to greater fat loss. This approach to cardio is sometimes referred to as “fasted cardio.” You’re doing a cardio workout in a fasted state if you haven’t eaten 8 to 12 hours beforehand. Many people plan it so they eat their last meal early in the evening and do cardio as soon as they wake up the next morning.

What Happens When You Do Fasted Cardio? 

The purpose of fasted cardio is to force your body to burn more fat. When you haven’t eaten in 8 to 12 hours, your body is forced to draw from glycogen, a storage form of glucose. You have glycogen stockpiled in your muscles and in your liver. Your body likes to use glucose or glycogen as long as there’s enough of it around. When you haven’t eaten in 8 to 12 hours, the low glycogen state causes your body to make fuel from fat stores. Another reason your body theoretically burns more fat when you haven’t eaten is that fasting lowers your insulin level. Insulin is important for nutrient absorption but it also puts a damper on fat breakdown.

Does Fasted Cardio Make a Difference?

In theory, fasted cardio sounds like a sound fat loss strategy – but what does science say? Results are mixed. One study found that subjects who did cardio while fasting burned 20% more body fat relative to those who fueled up beforehand. Yet, all studies aren’t so optimistic. In another study, participants who did cardio an hour daily three days a week in a fasted state and a non-fasted state lost equal amounts of weight after four weeks. Both groups ate a low-calorie diet. Changes in body composition were similar in both groups as well.

Still other research shows that exercising in a non-fasted state increases EPOC, also known as the after-burn. After-burn refers to the additional calories your body expends after a workout is over. One reason high-intensity workouts are so effective for fat loss is they produce a stronger after-burn. Greater intensity, in general, means a more sustained after-burn. Exercising at a high intensity with low glycogen stores can also limit your performance The more vigorously you work out, the more your body needs carbohydrates (in the form of glycogen) since fat breakdown is slower.

What about Muscle Loss? 

We mentioned that your body uses only a small percentage of protein as fuel, less than 5%, under normal circumstances. But if you work out in a carb depleted state, especially for a long time, your body uses a higher percentage of protein as a fuel source. That protein comes from the muscle tissue you work so hard to build and maintain. You may pay for your fasted cardio training with muscle loss. Remember, weight loss and fat loss aren’t the same things. You can lose weight and shed more muscle than fat. That’s probably not what you want. The goal is to lose body fat and hold onto muscle. Fasted cardio, especially if you work out at a high intensity, may interfere with this objective. Besides, how much fun is it to break a sweat when you haven’t eaten in 12 hours?

The only situation where fasted cardio makes real sense is if you’re doing a relatively low-intensity workout like walking or a light jog. For example, if you wake up in the morning and take an hour walk, you’ll burn more fat in a fasted state. You also won’t feel exhausted due to glycogen depletion since your body can easily mobilize fat as fuel at a lower intensity. It’s when you push the intensity higher that fasted cardio becomes less beneficial.

What does this mean? Fasting and doing a low-intensity workout is shortsighted if you’re trying to lose body fat. You’re not getting the after-burn or the metabolism boost that a more vigorous workout gives you. You’re also working slow-twitch fibers rather than a combination of slow and fast-twitch fibers as with higher intensity training. Plus, research suggests that more vigorous training is better for cardiovascular health and insulin sensitivity than low-intensity exercise.

If you prefer fasted, high-intensity cardio, there is some evidence, based on a study published in Journal of Applied Physiology, that your body adapts to fasted high-intensity training by becoming better at using fat stores, as opposed to using glycogen, as fuel. So, if you train at a high intensity in a fasted state for a while you may become a better fat burner

The Bottom Line

Save fasted cardio for light days when you’re keeping the intensity easy, like a recovery day. When doing high-intensity cardio workouts, eat a small meal so you can max out, reduce muscle breakdown, and, potentially, get a greater afterburn once you finish.

In conclusion, fasted cardio may be beneficial for lower intensity work but there’s less compelling evidence that it’s better for high-intensity training. In most cases, when you’re trying to lose body fat, high-intensity training is the superior choice and you’re best served eating something before you begin. Since each of us is a little different, you may want to experiment with both approaches, fasted and non-fasted, and see how you fee and adjust your workout accordingly.



British Journal of Nutrition, Volume 110, Issue 4. August 2013, pp. 721-732.

J Appl Physiol (1985). 2007 Apr;102(4):1439-47. Epub 2006 Dec 14.

Science Daily. “Lose Fat Faster Before Breakfast”

The Journal of Physiology 588.21 (2010): 4289-4302.


Related Articles By Cathe:

The Most Effective Way to Burn More Fat When You Exercise

Does Fasted Cardio Burn More Fat?

The Truth About Weight Loss, Body Fat Burning, and Exercise


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