Working out on an Empty Stomach – Is it Safe?


Tammy George

Young female exercising outdoors on an empty stomach

You’ve probably heard training on an empty stomach might increase the amount of fat burned during your workout. It makes sense that your body would need to find fuel from its fat stores, but what are the downsides and is it a safe strategy? While not all sports and medical experts agree, the research available shows fasted cardio has its pros and cons. 

What is Fasted Cardio?

Fasted cardio is exercising when the body is fasting, meaning over three hours have passed since last eating, so the body isn’t processing food. When you have an empty stomach, levels of insulin are low or baseline. As a result, there are plenty of fatty acids floating around the bloodstream which the muscles can use as energy. 

Pros and Cons of Working out on an Empty Stomach

Exercising in a fasted state is a double-edged sword. There are pros and cons to each, but you should consider whether the potential pros outweigh the cons. Factors like your age, lifestyle, gender and metabolism all contribute to how your body will react.    

Pros of Exercising on an Empty Stomach

The two main reasons people exercise on an empty stomach are for comfort and to burn more fat. 

Avoid Indigestion While Training

For some people, working out after a meal or a drink isn’t an option because of their level of discomfort. Having food or liquid other than water in their stomach while exercising can cause them to cough, experience acid reflux or feel sluggish resulting in a poor workout. When exercising on an empty stomach, they don’t have a full feeling or the discomfort of food repeating on them.     

Young woman suffering from indigestion while running

Burn More Fat

The majority of people who exercise on an empty stomach do so because they want to burn more fat. If the body doesn’t have any fuel to burn while exercising, it uses the body’s fat stores. 

In a study published in the British Journal of Nutrition, a group of physically active men went for a run in the morning after eating breakfast while the other half ran in a fasted state with no breakfast. The study found that those running on an empty stomach burned on average 20 percent more fat

A Japanese study concurred and found fasted exercise increases fat burning throughout the day. Over the 24 hours after exercise, fasted cardio leads to almost twice as much fat burned than if the same exercise is done after lunch. 

One study of 10 overweight men found part of the success in burning fat is the activation of fat burning genes. When the genes are activated the body finds it easier to break down stored fat and burn it off compared to exercising in a fed state.

Researchers have also examined the type of fat lost during exercise in fed and fasted states.  It’s thought exercise after fasting overnight, causes an increase in the release of abdominal fat stored under the skin because of improved blood flow to the area. Reducing abdominal fat can decrease bad cholesterol, life threatening cancers, heart disease and improve mental health.  

Exercise Before Which Meal For the Greatest Benefit?

If you leave at least three hours between meals, it’s possible to exercise in a fasted state at any time of the day, not just early in the morning. You might eat breakfast but skip lunch and exercise in the afternoon in a fasted state, for example. 

But a Japanese study found 24 hour fat oxidation increases if the exercise induced transient energy deficit is significant. This was achieved when participants exercised before breakfast after their longest period of fasting during the night.  

Cons of Exercising on an Empty Stomach

Before you commit to working out while you’re starving hungry, consider some of the potential pitfalls. 

Burn Muscle Not Fat

Some critics of working out on an empty stomach believe the body doesn’t burn fat, it burns muscle which defeats the purpose of training for most people. Breaking down too many muscle cells during a workout, means the body can’t keep up with the repair resulting in less muscle growth and even muscle loss.

Elderly exercisers are vulnerable because by the age of 75-80, the average body weight comprises  25% muscle mass compared to 50% in young adults. Senior trainers need to preserve the muscle they have so they may be recommended to complete fed not fasted training sessions. 

Poor Stamina

During a workout the body needs energy. When exercising on an empty stomach, there’s less energy and less stamina. There’s also the possibility that a low blood sugar level can leave you feeling lightheaded, nauseous and shaky.  

Young woman needing more energy while exercising on an empty stomach

Burning stored fat (rather than the carbohydrates stored in muscles and liver) takes longer to convert into energy. A study in British Journal of Sports Medicine was conducted using moderately-trained men. Half of the group fasted, and the other half ate before a 60-minute run. During the second half of the run, the men who fasted couldn’t run as far or as fast as those not running on an empty stomach. 

Unsustained Fat Burn

Some experts believe studies between exercising in the fed and fasted state might show differences on a single day, but the results aren’t sustainable over the long-term. The McMaster University in Canada did a fasted versus fed six week trial with overweight and obese women. The fasted group went to the gym before breakfast while the other group went 60 minutes following eating. After six weeks there was no difference in fat loss between the two groups, both had lost an average of 1.3 pounds of fat. 

Our bodies are good at adapting to changing conditions. So, there’s the possibility that your body will store more fat because it’s using fat reserves for energy. Known as ‘survival mode’, your body burns fewer calories to prevent fat loss, making it difficult to shift the unwanted kilos.    

High Cortisol Levels

The body stress hormone, cortisol, increases when there is a threat to the body. Fasting is considered a threat and causes cortisol levels to rise. Our bodies also perceive exercise as a threat and release cortisol to counter it. With the combination of the two, someone doing a prolonged fasted workout can have lengthy periods of elevated cortisol. These periods of high cortisol levels promote fat storage and reduced fat burning.   

Best Foods to Eat Before Exercise

If you decide exercising while fasting isn’t for you, you may need to experiment with a range of foods to find what foods or liquids work best for you. Everyone is different and what’s best for one person may not be right for the next. 

Meals Before Training

If you’re going to eat a meal rather than a snack before exercise, choose one that’s easily digestible and contains carbohydrates, proteins and fats. Eating a meal two to three hours before your workout will reduce the chance of feeling nauseous or sluggish.   

Array of healthy food like lean meats, eggs and low fat dairy to eat before working out

Eating protein such as lean meats, eggs and low-fat dairy will assist as your muscle cells break down and rebuild after exercise. Protein from animal sources also contains amino acids. Complex carbohydrates like whole grains and legumes provide exercise fuel, fibre and nutrients. 

Snacks Before Training

If you’re avoiding eating a meal before you train or you only have time to grab a snack, try a good quality one that will give you the energy you need.   

Protein Shakes provides the body with fuel without spiking insulin levels and helps with repairing muscles. Energy bars are a good source of protein and fibre and lower in sugar and saturated fat than many other snack foods.  

Fresh or dried fruit will provide the energy you need and a hit of nutrients. 

Post Workout Options

After training it’s important you eat to aid your body’s recovery. Foods containing carbohydrates, protein, or fibre help with muscle recovery. Try eating between 30 minutes and two hours after completing a workout.  

Foods to Avoid Eating Before Exercise

Most people will know from experience which foods don’t agree with them before exercise. Some of the more common ones to avoid include:

Bloating Foods 

Cruciferous vegetables such as cauliflower, beans, raw broccoli, and cabbage contain a carbohydrate that’s difficult to digest and can cause bloating and excess gas. 

Refined Sugar 

While it seems like a good idea to indulge in sweet treats just before a workout because you’ll work them off, it’s not ideal. The quick energy source is burned off quickly leaving you feeling sluggish. Try a healthier option like fruit which still offers sugar with some nutrients. 

Spicy Foods

One of the worst foods for feeling uncomfortable while you train is spicy food. Hot or spicy foods can give you indigestion or heartburn if you start any kind of exercise soon after eating. 

Salty Foods

Snack food or meals high in salt should be avoided before training because they can cause dehydration which causes headaches and cramps. 

Flat lay of salty foods that should be avoided before training such as hot chips, popcorn and pizza

High Fat Foods

Avoid foods high in fat before working out because they slow your digestion. Even some nutrient rich high fat foods should be avoided pre-workout. Nuts and avocados contain good monounsaturated fat which makes them slow to digest. Give your body two hours to digest these foods to stop stomach cramps.      

The Importance of Good Hydration 

Whether you exercise in a fed or fasted state, staying well hydrated is the most important factor. Drink water or sports drinks before, during and after training. Good hydration helps regulate body temperature, keeps joints lubricated and helps transport nutrients to give you energy.  

To Eat or Not to Eat Before Working Out?

The jury is still out on whether there are any benefits to exercising on an empty stomach. With a lack of long-term, large scale studies, it’s difficult to know what you should do. It's always best to talk to your local GP to discuss any new exercise regimes or programs.

Tammy George

Please note: Tammy's blog is general advice only. For further information on this topic please consult your healthcare professional.

Category:Nutrition, Fitness

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