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Fasting Cardio Complete Guide

Last modified on October 27th, 2022

Jennifer Lopez and former partner Alex Rodriguez swear fasting cardio is the best thing to do for your workouts and is the key to weight loss. What’s the reality?

In simple terms, fasting cardio means you do cardio workouts on an empty stomach. When you’re in a fasted state, you aren’t digesting food.

You would theoretically do fasting cardio first thing in the morning when your stomach is empty, but you don’t have to. If you frequently do intermittent fasting, you could find other times to do cardio on an empty stomach.

Some of the reasons you might consider fasting cardio include reducing body fat and using more fat for energy, weight loss, and getting more out of each of your training sessions.

While fasting cardio can be worth incorporating into your daily routines, it doesn’t work for all types of cardio. The duration of exercise is important as well if you want to see decreases in body weight.

Below we’ll explore why people do it and what the potential benefits of fasting cardio are, as well as cover any possible risks.

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What is Fasting Cardio?

Cardiovascular exercises are also called cardio. Cardio workouts are anything that increases your heart rate and keeps it raised for a prolonged period of time. When you do cardio, your respiratory system works harder. You breathe faster and also more deeply. During cardio workouts, your blood vessels expand, bringing oxygen to your muscles.

Your body releases endorphins during cardiovascular exercise.

Specific benefits and effects of exercise include:

  • When you do cardio exercise that’s moderate intensity, it can help you manage your weight or lose weight. According to exercise physiology concepts, there’s significant evidence showing that 150 minutes of moderate-intensity cardio exercise a week is ideal for weight maintenance or to reduce your body fat percentage. 
  • When you get your heart rate up regularly through a cardio session, it’s good for your cardiovascular health. Cardiovascular disease accounts for more than 30% of deaths worldwide. Cardio can help reduce your blood pressure and have many other benefits on the health of your heart. 
  • Cardio is a great way to improve your mood and feelings of happiness naturally. This type of heart rate-raising exercise produces endorphins that not only make you feel happy but are also natural pain killers.
  • When you do cardio, it can help you maintain an optimal circadian rhythm so you can fall asleep easily each night and get enough rest. 
  • The Mayo Clinic reports people who regularly do cardio live longer.

To truly be doing fasted cardio, based on scientific research, you need to have not eaten for periods of time of at least 10 to 14 hours.

Since your glycogen stores are depleted after this fasting period, your body is going to start searching for alternative fuel sources. Then, within a few minutes of beginning your fasted workout, the percentage of fat you’re going to burn will be at least a little higher.

You might have started burning your fat either way, but if you start fasted, you’re just going to that point nearly immediately, so you may be maximizing your workout time.

By doing fasting cardio, there could be more indirect benefits of exercise you notice. For example, if you plan to do your cardio after fasting, it can help you maintain a caloric deficit which is an integral part of losing weight and getting to a lower percent body fat. 

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Fasting Cardio Benefits

The idea of fasting cardio and the primary benefit is that theoretically if you do a workout on an empty stomach, it can help increase how rapidly you’re able to lose weight. If you fast overnight and then workout before you eat anything, your body’s glucose is depleted. Glucose is usually the primary source of energy stores for the human body. 

When you’re experiencing glucose depletion, you’ll use stored fat for fuel rather than glucose as your fuel source. 

There’s mixed evidence on the effectiveness of a fasted cardio approach.

  • In one review looking at several studies, exercising after a period of fasting led to higher metabolic performance and fat metabolism after the completion of the workout.
  • The same review, when comparing the fasting group, however, noted that for more prolonged aerobic activity, eating beforehand led to improvements in performance in healthy subjects. 
  • This could mean that if you were to eat before your workout, you might have a longer time to exhaustion and be able to push yourself harder in your physical training. Again, evidence is mixed, so you have to think about what you hope to achieve. 

Other potential benefits of fasting cardio include:

  • When you’re short on time, you can get to work out faster. Otherwise, if you plan to eat before you do a workout, you have to prepare the meal, eat it and then wait for it to digest for at least 30 minutes.
  • People who do intermittent fasting on a daily basis tend to like a routine with fasting cardio. You can get in your exercise before eating for the day.
  • Some people prefer working out on an empty stomach because they might feel more energetic.
  • Fasting cardio can help you get rid of stubborn fat more effectively or break through plateaus.
  • Some studies are showing fasted cardio can improve your body composition, such as lowering your waist circumference through what’s called adipose tissue lipolysis. 
  • Over time, with repeated fasting cardio workouts, your body will get more efficient at burning fat as its fuel rather than relying on glucose metabolism or carbohydrate metabolism. 
  • Fasted cardio can help prevent indigestion during your workouts.
  • Fasting cardio can support fat loss in less direct ways. For example, after you do a workout following a period of fasting, your body uses additional calories for recovery. Your metabolic rate will be slightly raised for around 24 hours. 

Fasting Cardio Workout Ideas

We go into this more below, but low-intensity, steady-state cardio is the best type of exercise to do while you’re still in your fasted state.

You do not want to do HIIT, weight-training, cross-fit, or anything very intense because your results will be counterproductive.

Walking on the treadmill is an excellent example of an appropriate fasted cardio workout.

Is a Fasting Cardio Morning Routine Best?

A fasting aerobic exercise morning routine can be good in a few situations.

If you want to burn fat and do low-intensity exercise, combining your workout with fasting can help speed up your results.

You might also want to go ahead and get your workout done without breaking your fast, in which case doing cardio on an empty stomach can help you maximize your time while maintaining intermittent calorie restriction. 

Otherwise, you’d have to find time to fast and then do cardio in a separate window, which can be difficult depending on your schedule. 

Who Shouldn’t Do Fasting Cardio?

Some people should potentially not do fasting cardio, and there are also downsides or negative effects to consider. Everyone is different, and so are their exercise needs. Some of the downsides of doing cardio in a fasted state include:

  • There’s a chance your body could end up breaking down your muscle tissue for fuel instead of your fat stores located in your adipose tissue. For example, in one study, working out in a fasted state led to twice the amount of protein breakdown in the muscles compared to non-fasting cardio. If your goal is to maintain or build muscle mass, fasting cardio might not be ideal for you.
  • If you’re going to do high-intensity training or high intensity interval training, boot camp, or strength training, don’t do your workout in a fasted state. If you do, you’re not maximizing the potential benefits of your workout, and you might even get injured. If you try to do high-intensity workouts without food on your stomach, you might feel tired or weaker.
  • When you’re weak, you might have less power output, and you can’t push yourself as hard as you’d like to during your workouts.

Some people wonder if it’s safe to do their cardio on an empty stomach, and generally, the answer is yes.

However, you have to be cautious about the potential for it to cause low blood sugar, with symptoms including lightheadedness and lethargy. If you have a metabolic disease, such as diabetes, you should talk to your doctor before doing fasted cardio to avoid low blood sugar.

Older people should avoid doing cardio after fasting as well because of the potential for a breakdown in muscles to occur. 

While it’s not inherently harmful to have an exercise bout without eating first, you have to weigh the pros and cons of the fat-burning benefits and general beneficial effects.

Even when you’re doing fasting cardio for the health benefits, you also have to make sure you’re watching your calorie intake later on so that you’re still maintaining a calorie deficit, or you won’t see results. 

How Long Should You Do Fasting Cardio For?

If you’re going to try fasted cardio, make sure you’re well-hydrated. Start slowly to see how your body responds. For example, begin with moderate-intensity cardio for around 10 minutes and see how that goes. You could run, bike, walk or use the elliptical as an example. If you feel okay at 10 minutes, you can work your way up to 30 minutes.

Following fasted cardio, make sure you have a balanced snack or meal with enough grams of protein and healthy carbs.

Don’t do anything too high-intensity, and don’t exercise on an empty stomach for more than an hour.

If you’re going to do low- or moderate-intensity fasted cardio, you can do it a few days a week, but give yourself one or two rest days from this type of workout.

If you ever feel lightheaded, stop doing cardio and get a snack with high amounts of protein, like a protein shake. 

Does Fasting Cardio Burn More Fat?

There are studies indicating exercise in conjunction with fasting can help you burn more fat. For example, in a study of people running on a treadmill in a fasted state, they burned 20% more fat than participants who ate breakfast.

We touched on this above, but the reason fasting cardio can help you burn more fat is that you don’t have fuel stores readily available from food to meet your energy demands. Your body, as a result, has to look somewhere else to get the energy it needs to fuel your aerobic workouts. 

If you’re someone who is dealing with stubborn fat that’s hard for you to get rid of, or you’ve hit a plateau with your workout results, fasting cardio could help you make changes to your body composition. This could be appealing to females with body fat around the hips and midsection, for example. 

According to fitness experts, the duration of your workout will have an impact on whether your body burns fat during fasted cardio or breaks down muscle. If you stay within 50 to 60% of your target heart rate, your body is more likely to use fat. It sounds counterintuitive, but the easier your workout, the more likely you’ll burn fat.

You have to make sure that you don’t push your body to the point where it feels like it’s in “survival mode” because that means you’re going to burn fewer calories than you would otherwise. Your body will try to protect itself by preventing too much fat from being burned and maintain energy balance, and that will end up being counterproductive and making it harder to lose weight.

What are Examples of Fasted Cardio?

The best way to do fasting cardio is to use low-intensity workouts. For example, you might do a light jog, go biking or take a walk.

Fasting cardio is an excellent time to use low-intensity steady-state cardio or LISS. Things to know about LISS include:

  • This is a method of cardio where you do aerobic activity at a low-to-moderate intensity for a continuous period. In some cases, you might do it for an extended period of time, at least compared to HIIT.
  • The goal of LISS is to keep your heart rate around 50 to 65% of your max heart rate during longer duration exercise. 
  • LISS is the opposite of HIIT, where you’re doing alternating short, intense exercise and then having low-intensity recovery periods. During HIIT, you should aim for a heart rate that’s 80 to 95% of your max for your high-intensity intervals and 40 to 50% for low-intensity intervals.
  • LISS cardio benefits include that it helps your body use fat as fuel for your energy levels instead of using glycogen stores for energy consumption. 
  • Using continuous aerobic exercises when you work out can be more effective than HIIT to improve your body’s fat distribution.
  • LISS is appropriate for all fitness levels, beginner to advanced, and from sedentary subjects to elite athletes. 
  • You can recover more quickly when you do LISS since you’re putting less strain on your body and heart.
  • This is a good way of training for endurance.
  • You can also use LISS as a way to recover after a high-intensity worko
fasting cardio, fasting cardio benefits, what is fasting cardio, fasting cardio workout, fasting cardio results
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What Happens When You Do Fasted Cardio?

What happens when you do fasted cardio?

  • First, you’re doing exercise when you’re in a fasted state.
  • Your body’s not processing or digesting food when you’re in this state.
  • Benefits include lower insulin levels, more fat oxidation, and increases in lipolysis.
  • Lipolysis is when your body breaks down fat cells to use them as energy.
  • Fat oxidation is when your body is burning energy from fat cells when your glycogen is low.

Fasting Cardio Results

Fasting cardio results can be good if you follow guidelines which include:

  • Don’t do it for more than an hour
  • Stop exercising if you feel weak or lightheaded
  • Don’t do weight or resistance training, HIIT, or any other high-intensity workouts without eating
  • Keep your intensity low to moderate to make sure you’re burning fat rather than muscle, which would be the case during an intense workout. 

A question people tend to have is whether or not fasted cardio is worth it. The answer is possibly, but the evidence is mixed. Whether or not you do your cardio this way depends on your preferences and your goals.

For example, some people simply don’t like how they feel if they eat before cardio, especially if they’re best at exercising in the morning.

If you have blood sugar issues or worry hunger will affect your exercise performance, then you might avoid fasted cardio.

While there’s not anything that’s usually unsafe about doing fasted cardio, you should talk to your health care provider or an exercise physiologist if you have any concerns.

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Ashley Sutphin Watkins
Ashley Sutphin Watkins is a graduate of the University of North Carolina at Chapel Hill. She's a medical content writer, journalist and an avid researcher of all things related to health and wellness. Ashley lives near the Smoky Mountains in East Tennessee with her family.
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