Blog Categories


Fasted Cardio: Pros and Cons of Working Out on an Empty Stomach


You might have heard about intermittent fasting and how it can help you reach and maintain a state of ketosis. But you may be new to the concept of fasted workouts or fasted cardio.

Even though fasting has become increasingly popular in the past several years, Traditional Chinese Medicine and Ayurveda have praised the health benefits of this practice for thousands of years.

The idea of working out on an empty stomach might sound a little strange. But once you learn about the potential health benefits and how it might help you on your weight loss journey, you may decide to give it a go.

What Is Fasted Cardio?

The basic idea behind fasted cardio is to increase your heart rate without eating anything beforehand.

This sounds simple enough, but there’s a major difference between being in a fasted state and being hungry.

You can have a snack or energy bar now and be hungry within an hour, but your body is still processing that food and absorbing nutrients. You’re still in a fed state.

Being in a fasted state means that your insulin levels are at their lowest, your body has completely processed all food, and your glycogen stores are depleted. This can take between 3 to 6 hours and depends on the amount of food you had during your last meal and on your digestive power.

The safest way to go for a fasted cardio workout is in the morning after you wake up. You’ve fasted overnight (between 7-12 hours), and your body has low blood sugar levels.

This means your metabolism is now ready to start using your fat stores for energy.

Fasted training can also be done at the end of the day if you skipped lunch or are intermittent fasting that day.

Read next: What is Fasted Training and Will it Help You Lose More Weight?

Benefits of Fasted Cardio

In general, fasting — and fasted cardio, in particular — can help you achieve your keto goals faster, especially if they include fat loss. Here are some of the primary benefits of fasted workouts.

#1. Can Promote Fat Burning

Fasted cardio is a popular practice among those who want to change their body composition. Once you’re in a fasted state, your body immediately taps into the most readily available and sustainable energy source in your body: stored body fat, i.e., your adipose tissue.

When you’re following a low-carb or keto diet, this is even more efficient since your body doesn’t have to work through any spare glycogen stores, and it can start to burn fat (fat oxidation) straight away.

Studies have shown that when you’re in a fasted state, the blood flow in your abdominal area increases, which can help burn fat cells (lipolysis) that are stored in that area[*].

#2: May Prevent Stomach Discomfort

Fasted cardio can be a great option for those who experience digestive discomfort or nausea after having an afternoon pre-workout snack or smoothie and then go for a cardio session.

Fasted workouts can help you avoid these unpleasant symptoms, letting you hit the treadmill without worrying about upsetting your stomach.

#3: Can Help You Get Into Ketosis

If you’re following a CKD (cyclical ketogenic diet), fasted cardio can be a good way to help you deplete your glycogen stores after your carb-loading days[*].

It can also be useful if you feel like you’ve gone over your daily carb limit and want to get back into a state of ketosis.

#4: May Increase Human Growth Hormone

Fasted cardio may help increase human growth hormone (HGH), a peptide hormone produced by your pituitary gland. HGH stimulates growth and cell regeneration[*].

HGH production declines after your 30s, but you can help your body continue producing this vital compound.

You can naturally support HGH production when you keep your insulin levels low, which happens when you practice intermittent fasting or go for a session of fasted cardio.

Potential Downsides of Fasted Cardio

Even though fasted workouts have a number of health benefits, there are two primary pitfalls you should know about.

#1: You Might Lose Muscle Mass

If you’re a bodybuilder or building muscle mass is one of your fitness goals, keep in mind that adopting fasted cardio might negatively affect your muscle tissue.

This happens because once your body is depleted of its glycogen stores, the search for an energy source may lead it to target your muscles instead of your stored fat.

However, one study conducted by a Finnish University discovered that muscle breakdown only starts to happen an hour and a half after a workout has been completed[*].

So, one way to avoid this muscle breakdown is to fuel your body with a good source of protein after your fasted cardio session.

#2: You Might Experience “Bonking”

As you start exercising in a fasted state, you may notice you don’t have the same energy and stamina as you did when you consumed your pre-workout smoothie.

This happens because the glycogen your body previously used to power your morning workout routine is no longer available. Therefore, you might feel an energy crash throughout your fasted cardio session, also commonly known as “bonking.”

If you find yourself bonking out on energy, consider adopting a TKD (targeted keto diet). This version of the keto diet might make all the difference as it allows for a higher carb intake one hour before and after your high-intensity training.

HIIT Training and Fasted Cardio

Close-up of ropes with fasted cardio

One of the best ways to reap the benefits of fasted cardio is opting for high-intensity interval training (HIIT).

HIIT is one of the most popular cardio workouts nowadays.

Many athletes and fitness lovers are huge fans of this type of training as it delivers the same benefits as a traditional cardio session in a much shorter period.

A typical HIIT session lasts between 20-40 minutes, and can burn up to 500 calories[*].

A well-rounded HIIT routine should combine cardio and resistance training for a complete full-body workout. Training should always be a fun, challenging, and rewarding activity that keeps you motivated and excited.

HIIT is versatile enough to practice in many different environments:

  • Do it in the comfort of your living room thanks to the many HIIT programs available online.
  • Join a HIIT class at your local gym.
  • Get a personal trainer who can create a tailor-made training program to suit your goals.
  • Look for HIIT outdoors training groups if gyms aren’t your thing. This is a fantastic option if you like to exercise outside, make like-minded friends, and breathe some fresh air.

Fasted Cardio: The Bottom Line

Like most things in life, fasted cardio has its pros and cons. It can be an additional method to help you get into ketosis and reach your health goals.

If you’ve tried and tested this type of cardio and your body doesn’t feel good, you feel faint, or you have limited energy, exercising in a fed state might be a better way to go.

As always, experiment and be aware of how your body responds to this type of training. Just because your friend feels good doing it doesn’t mean it will automatically work for you.

To learn more about exercise and keto, check out these informative articles:

Join the Internet's largest keto newsletter

We'll send you articles, product guides, and exclusive offers customized to your goals.