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Does Fasted Cardio Really Help You Burn Fat?

Lately, I’ve been hearing more about how doing cardio on an empty stomach can help you burn more fat! Fasted cardio is quickly becoming a hot topic in the fitness industry. Some people swear by skipping breakfast before their workout, but others prefer to eat prior to exercise.

So which is better, fasted or fed cardio? To understand why it’s believed fasted cardio burns more fat, it’s important to understand the way calories are burned during exercise.

Calorie Burn During Exercise

During exercise, your body burns the three macronutrients: carbs, fats, and proteins for energy.


Carbs are the preferred energy source because they are the quickest and easiest to burn! In fact, eating carbs before a workout can improve performance, and give you energy (1, 2). 


Fats take longer and require more oxygen to break down than carbs. But your body also has a way bigger storage of fats than carbs. Fat is used for fuel on a regular basis during exercise. They are burned during all exercise, but in a larger percentage during lower intensity exercise, or during prolonged endurance training. 


Finally, in cases where your body is running really low on carbs, it may resort to a combination of fats and protein for fuel. The use of protein for fuel is relatively inefficient and can be dangerous as this protein is typically derived from the breakdown of your own muscles (ketosis) to provide protein for fuel (3). If you are looking to maintain or build lean muscle mass, this is an obvious issue.

How When You Eat Can Impact What is Burned

The use of each of these macronutrients for energy is heavily dependent on what is consumed before your workout. If you are fed prior to exercise you will use readily available ingested carbs as fuel, and less of your fat and protein stores.

Conversely, if glycogen (stored carb) is depleted, your body will burn a bigger proportion of fat and protein for fuel. And this is the basis for fasted cardio!

Fasted Cardio

The ‘fasted’ in fasted cardio means a bit more than simply being hungry. It means that your food is digested and your insulin levels are low.

Many people like to perform fasted cardio when they first wake up in the morning because glycogen is often depleted after a night of sleep, which ensures their carbohydrate levels are low, and their body will depend on burning more fats during their workout.

If you have maintained a healthy diet the day previous, some experts say that you should have enough glycogen stores for an average cardio workout (up to 1-2.5 hours) before dipping into complete depletion.

If this is true, in theory, you wouldn’t be in danger of breaking down muscle to use protein as fuel, but you would have a lower than normal amount of glycogen to depend on during your workout. So your body would resort to burning more calories from stored fat. 

Some studies, have revealed that fasted cardio can result in increased lipolysis and fat oxidation (breakdown) rates than when compared with fed cardio(4, 5, 6). But the burning question is: does fat burn for energy equate to fat or weight loss?

Fat Loss all Comes Down to Calorie Burn

While in theory, it’s nice to think you can lose fat just by changing something as simple as what you eat before a workout, many studies have found no difference in body composition or overall calorie burn, whether exercise is done before or after a meal (4, 5, 6).

Ultimately, weight loss and fat loss come down to overall calorie balance. Whether you’re participating in fed cardio or fasted cardio- science says that you will not lose weight if you are consuming more calories than you burn.

At the end of the day to achieve weight loss, including fat loss, you must be in a caloric deficit. So if you’re interested in shedding some unwanted fat, make sure you are paying attention to your overall calorie balance! Are you consuming fewer calories throughout the day than you’re burning?

So, Should I Eat Before Cardio or Not?

Any exercise burns calories- and this will help you lose weight regardless of if it’s fasted or not. So, I say listen to your body and do what works best for you. If you don’t like the feeling of working out in a fasted state, you’re not alone. If you can handle fasted cardio, or prefer it then go for it!

I find myself somewhere in the middle. I tend to feel a little lightheaded and lackluster during fasted workouts. But I also don’t feel amazing when working out on a full belly of tacos. I feel I perform best, dig deeper, and work harder when I have a light snack in my system.

If I Eat: What and When is Best?

Determining what to consume prior to a workout can be confusing- because there’s so much mixed advice out there.

What You Shouldn’t Eat

Protein requires a large amount of energy to be digested and absorbed, and it’s breakdown facilitates dehydration during exercise. But high-glycemic carbs and simple sugars are not ideal either because they can cause an insulin spike. High insulin levels inhibit lipolysis. And when sugary carbs are uptaken by insulin, blood sugar levels drop, which can cause an energy crash mid-workout!

What You Should Eat

Before a cardio workout, the ideal snack should provide a source of glucose to maintain blood sugar and sustain muscle metabolism. But, at the same time not trigger an insulin spike.

To achieve this, eat a low-glycemic carbohydrate (starch with high amylose content or moderate-glycemic carbohydrate with dietary fiber) snack 45-60 minutes prior to exercise (7,8). What’s so ideal about a snack like this, is it eliminates an insulin surge and doesn’t tax your system. While also providing a steady supply of glucose as your workout progresses.

Good examples of this include an apple, peanuts, beans, lentils, chickpeas, whole wheat bread, oatmeal, sweet potatoes, most vegetables, and greener bananas. A few other things to note, glycemic index (GI) can vary on factors like ripeness and preparation. The riper a fruit is the higher the glycemic index (which is why greener bananas are ideal). And a smoothie will have a higher GI than whole fruit. Note, smoothies also involve degradation of fiber, which you want fiber for that slow sustained digestion.

In conclusion, make your decision based off of how you feel, as well as your goals! The better you feel, the better you will be able to focus and enjoy your workout!

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