In recent times, fasted exercise - doing aerobic exercise without eating beforehand - has been hailed by those looking for new, quicker ways to lose fat. Also referred to as fasted cardio, some swear by the workout technique whilst others think it has little to no effect.


Reviews are mixed, and different studies conflict, but here at The Recommended we’ve assembled a handy guide to fasted exercise. We take a look at the benefits, drawbacks, and tips and tricks, so you can decide if fasted exercise is right for you. So, should you work out on an empty stomach?

What is fasted exercise?

Fasted exercise is the term for exercising without eating. It is doing any aerobic exercise that raises the heart rate when you haven’t eaten for a ’prolonged’ period of time. To be counted as fasted exercise, the time you haven’t eaten for has to be above four hours.

Many people choose to do fasted exercise straight after waking up so they could be doing exercise without eating for 8-16 hours prior. Running, cycling, and swimming are often the most performed exercises that people choose to fast prior.

Does fasted exercise work?

This question has been debated recently because of growing claims that fasted exercise helps people burn fat and lose weight more effectively than regular exercise. The experts at Runners World have stated that when you exercise after fasting, your body uses up its glucose reserves more quickly than when you have eaten.

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As you work out, your body has to burn fat and use it as an alternative energy source because it has used up its glucose. But, during this low glucose period, the body also resorts to burning muscle proteins for fuel.

Why do people choose to do fasted exercise?

The main reason that people choose to do fasted exercise is the belief that it burns fat at a faster rate than usual. This belief stems from the fact that after not eating the body is low on glucose, its natural energy source. When exercise is performed in a fasted state the body has to find an alternative source of energy because glucose reserves become depleted quickly. Fat becomes one of the energy sources the body opts for burning up after glucose.

The belief is that this burning of fat will help you to lose fat more quickly than doing exercise on a full belly. But, when glucose is low the proteins in muscle are also chosen as an alternative energy source and burnt for fuel as well. Over time, this could lead to muscle breakdown if you don’t implement more protein into your diet.

Whether fasted exercise helps you lose fat at a faster rate is contested. A study by American scientists in 2014 found that those performing fasted exercise did not lose weight or fat at a higher rate than those who ate before working out. The study was published in the Journal of the International Society of Sports Nutrition.

On the other hand, a review of 27 studies published in the British Journal of Nutrition in 2016 concluded that doing fasted exercise did help with fat loss, Healthline reports. Although research is murky as to whether it makes a massive difference to workout after eating or after a fasting period, there are still other reasons people perform decide not to eat before exercising.

Some of these are time constraints, stomach issues, digestive problems, convenience, and those committed to intermittent fasting.

Four benefits of fasted exercise

The main reason that people decide to complete fasted exercise is the belief that it helps with weight and fat loss. Research still isn’t clear as to whether it does aid those wanting to shake some weight but there are still other reasons to workout after an extended fasting period.

1. It could help with fat loss

The research is mixed but some studies do support the motion that fasted exercise helps with fat loss. There is also research that suggests that exercising whilst fasting could make you less hungry throughout the day- also helping with fat loss (see above).

2. It can save time

For people in a rush, it can save time to practise fasted exercise. If you want to wake up and get a workout in straight away it can be inconvenient to eat and then wait for the food to digest before starting to exercise.

3. Personal preference

Some people prefer to work out on an empty stomach. Often people report feeling lighter, or more agile if they haven’t eaten anything for some hours beforehand. Some people also feel heavy and sluggish if they have recently eaten prior to exercise.

4. Help with health problems

People who have digestive problems, IBS, or any other issues that can be agitated with food, may feel more comfortable when exercising on an empty stomach.

Three drawbacks of fasted exercise

There are some negatives of fasted exercise and it’s more than just feeling hungry whilst you work out. Fasted exercise is not for everyone and if it doesn’t feel right for you then you should stop.

1. Decreased performance

Some people suffer from decreased performance as a result of fasted exercise. If you are not used to fasted exercise then you might find that you don’t have the same level of energy to work out- this could affect your run time, strength, etc. If you work out for an extended period of time this is especially the case as you have to rely solely on your reserves of fat and muscle.

2. Muscle breakdown

Experts from Runner World explained that when you practise fasted exercise the glucose (energy) that you rely on comes from both your fat reserves and your muscle. The glucose in your muscle becomes depleted quickly which means the body then starts to break down protein (muscle) to continue energising itself.

Unless you continue to train your muscles and eat an abundance of protein then you could see a reduction in muscle size and strength through fasted exercise.

3. Uncomfortable side effects

If you practise fasted exercise you might find that you feel some uncomfortable side effects. Many of these come as a result of poor preparation and include dizziness, fatigue, light-headedness, confusion and weakness. To prevent these from happening make sure you are properly hydrated and listen to your body. If you feel unwell or strange, stop.

Five tips for doing fasted exercise

If you do chose to start fasted exercise then here are some helpful tips to follow in order to make the transition as safe and enjoyable as possible. Again, the most important thing is to make sure you keep tabs on yourself and if you feel uneasy then stop.

1. Stay hydrated

Dr. Niket Sonpal spoke to Healthline and explained that staying hydrated while fasting will mean that your body doesn’t start to feel the effects of dehydration. She said to remember that a lot of water in our diets comes from food so you should actually drink more water when fasting. Other benefits include avoiding feelings of tiredness, and headaches, and an overall energy boost.

2. Eat the right food post-workout

A report from Medical News Today explained that eating the right food after your workout is important because your body needs to replace the protein and energy it has burnt through whilst fasting. They suggest eating a mix of carbohydrates and protein as these will help the recovery process and are a valuable energy source before and after working out.

3. Don’t over train

Try only to moderately exercise and keep the length shorter than your normal exercise periods. Especially if you are just starting to explore fasted exercising you should take it easy and slow to ease yourself into the process.

4. Listen to your body

Your body knows best and if you are feeling dizzy or fatigued then stop and have a rest. If you are feeling any unwanted side effects from exercising whilst fasting you should quickly get some liquids on board as well as a high-calorie snack, followed up by a balanced meal.

5. Don’t risk it

If you are pregnant, have a pre-existing medical condition, or are wary about fasted exercise, it's probably best not to do it. If you have any questions or concerns about fasted exercising, you should speak to your doctor or healthcare provider.


Want more guidance on fitness and exercise? Check out our wellness and fitness sections, with guides including yoga vs pilates, best posture tips and the best multi-gym equipment for working out at home.


Finn ByrneEcommerce Writer

Finn Byrne is a Digital Writer for Immediate Media. He works across several brands including The Recommended,, MadeforMums and BBC Gardeners’ World. Finn has previously written for publications including MyLondon, The Mirror, The Express, and The Star. When not writing Finn enjoys spending time on the football pitch and getting stuck into a book.