Some people swear by their pre-run protein bar or banana, while others prefer fasted cardio—exercising on an empty stomach. It may seem trivial, but whether you fuel before your workout or purposely skip may impact your performance, appetite, health, and more (1, 2, 3). So, what is fasted cardio, exactly? And most importantly should you do it?

What Is Fasted Cardio?

“Fasted cardio is doing cardio on an empty stomach, typically in the morning before eating breakfast,” says certified personal trainer, Marcel Dinkins, NASM-C.P.T. In theory, working out in a fasted state can lead to greater fat loss by increasing fat oxidation (the breakdown of fat for energy) (1, 2). Whether fasted cardio actually leads to superior fat loss is still up for debate. Fat loss comes down to overall calorie burn. 

While you might burn a higher percentage of fat during fasted, low-intensity cardio, you’ll burn more overall calories (and thus, fat) during fueled, high-intensity cardio—which has a greater overall impact on fat loss. “Some studies emphasize that there may be little to no significant difference in body composition outcomes between fasted and non-fasted exercise regimens at the same intensity (4, 5),” explains registered dietitian nutritionist Kelsey Costa, M.S., R.D.N. 

What the research does clearly show is that doing cardio on an empty stomach can decrease performance during high-intensity or long cardio sessions, because your body has less glycogen to fuel your workout. “Though the average exerciser won’t see a big difference, you may experience more muscle fatigue and difficulty pushing through intense cardio sessions when fasted,” says Costa.  

Low-intensity fasted cardio, on the other hand, might improve your endurance by helping your body get better at using fat for energy. One study suggests that well-trained runners are more fat-adapted—or able to burn fat more effectively, even during hard workouts—which lets them exercise more efficiently than casual runners (6). Other studies have linked fasted cardio—specifically, training on lower glycogen stores—to an increase in mitochondrial mass and density in muscle cells. This increase in mitochondria can boost aerobic capacity (7, 8).

About the Experts

Kelsey Costa, M.S., R.D.N. is a registered dietitian nutritionist and founder of Dietitian Insights.

Marcel Dinkins, NASM-C.P.T., is a NASM certified personal trainer, Peloton Tread instructor, and former Army Officer. 


Benefits of Fasted Cardio

If doing cardio after eating upsets your stomach, a fasted workout makes sense. But that’s not the only potential perk:

May increase ability to burn fat for energy

Perhaps the biggest benefit of fasted cardio is its potential to enhance fat oxidation—or your body’s ability to burn fat for energy. Several studies suggest fasted cardio may increase fat oxidation (1, 2). Experts think that since glycogen (your body’s preferred source of energy) is limited in a fasted state, your body is forced to turn to fat for energy.  

Some people may struggle to burn fat for energy—a problem called “metabolic inflexibility,” which has been linked to obesity, sarcopenia (muscle loss), insulin resistance, type 2 diabetes, and other metabolic conditions (9). Metabolic flexibility—being able to switch between using carbs or fats for energy—has been linked to greater overall health and exercise performance (9). 

Burning more fat for fuel may also help endurance athletes with sensitive stomachs adapt to using more fat for fuel (and needing less carbs) at a given pace. That said, if the goal is performance, you’ll constantly be pushing your pace and will likely need the fuel. 

May boost fat loss

Fasted cardio fans cling to the claim that it improves fat loss. It makes sense in theory: Increase fat oxidation, lose more fat. The truth is, fat loss comes down to a variety of factors including your metabolism, how hard and how long you exercise, and if you’re operating at a calorie deficit, explains Costa. 

“It’s largely a matter of personal preference and individual response,” says Costa. “If performing cardio on an empty stomach suits you and you observe greater reductions in body fat from doing so, then all power to you.”

May improve insulin function

In healthy people, fasted endurance exercise has been shown to enhance insulin sensitivity (10). “Improved insulin sensitivity can help your body better regulate blood sugar levels, potentially reducing the risk of insulin resistance and type 2 diabetes,” says Dinkins. 

More research is needed to confirm whether exercising in a fasted state is safe and effective for people with type 2 diabetes or insulin resistance. If you do have type 2 diabetes and choose to exercise fasted, check your blood sugar to make sure it doesn’t go too high or too low. 

May help you eat less

A small study found that men who performed fasted cardio in the evening decreased their overall calorie intake (2). Why? Fasting made them skip lunch. Another small study found that people who did cardio in the morning on an empty stomach ate about 10 percent more calories over the next day. Despite this, they burned more calories during their workout than the extra calories they ate, resulting in an overall calorie deficit (11). 

To lose weight, you need to consume fewer calories than you burn. If fasted cardio leaves you so hungry that you eat more later, it might not be worth it, notes Costa. 

Athletes who practice fasted cardio for the potential metabolic adaptations (not to lose weight) should also keep an eye on how much they eat. Recovery and performance both depend on getting enough fuel. A very low carb diet can deplete glycogen stores and impair high-intensity performance (12). If you plan to use fasted cardio as a training tool, getting an adequate amount of carbs the rest of the day can help ensure you have glycogen to tap into during fueled, high-intensity workouts.  

Downsides of Fasted Cardio

Fasted cardio does come with several downsides, which is why you likely won’t want to rely on it for every workout (or maybe even at all).

May decrease performance

Fasted cardio may increase your risk of experiencing the side effects of low blood sugar like lightheadedness, dizziness, shaking, or even passing out. Since blood glucose and glycogen are typically the main sources of energy during a workout, you may feel low energy during fasted cardio.

“Exercising on an empty stomach may lead to increased fatigue and reduced energy levels during a workout,” says Dinkins. These effects are usually more noticeable during high-intensity cardio, or workouts longer than an hour, since these workouts rely on more glycogen for fuel (1).

Very low-carb diets can also impair high-intensity performance, including an athlete’s “top gear” or sprint to the finish capacity (12). So, you’ll want to monitor your fitness goals, training schedule, and overall fueling strategy closely if you plan to use fasted cardio to enhance performance rather than tank it.

May increase muscle breakdown

The concern that fasted cardio kills gains persists, but research is sparse. In reality, whether you gain, maintain, or lose muscle mass comes down to getting adequate calories, and your whole-body net protein balance.

If you have more protein coming into your body than leaving it, muscle can be built. If it’s the opposite, you might lose muscle (13). Ultimately, your whole-body protein balance comes down to how much (protein and calories) you’re eating throughout the day. For example, if you aren’t eating enough overall calories, then your body will burn protein for energy instead of using that protein to maintain muscle.

One study suggests that low glycogen stores, like when you’re fasting, might lead to a lower  overall net protein balance and faster muscle protein breakdown during long cardio workouts (14). “It’s important to properly refuel post-workout to replenish glycogen stores and support muscle recovery,” says Dinkins.  


How to Do Fasted Cardio

There is no one clear right way to perform fasted cardio, but there is a wrong way. Here’s how Dinkins and Costa break it down: 

How long to fast before doing fasted cardio

You can do fasted cardio at any time as long as you’re in a ‘post-absorptive’ or early fasting state, says Costa. “About 3 to 4 hours after eating, your body starts to experience a decrease in blood sugar and insulin levels, initiating the shift to glycogen stores for energy. This transition marks the beginning of what is known as the early fasting state, which can extend up to 18 hours of not eating.”

During this state, your body relies less on glycogen, and starts breaking down fats for energy—the exact state you want to be in during fasted cardio. The easiest way to make sure you’re in the sweet spot is to do your cardio in the morning before breakfast, after an 8- to 12-hour overnight fast.

What to eat and drink before fasted cardio

Staying hydrated during fasted cardio is key, since dehydration can tank performance (15). “Drink water before and during your workout to stay hydrated, especially since you’re not consuming food beforehand,” says Dinkins.

Some people consume coffee or pre-workout before a fasted workout for an extra kick of energy. Whether this still counts as a fasted workout is debatable. Drinking low- or no-calorie beverages like tea or coffee before a fasted workout is unlikely to break a fast. However, pre-workout foods or supplements that contain sugar or carbohydrates likely will break a fast, since they can trigger an insulin response and provide fuel for your workout, Costa adds.

While there’s a rule of thumb that consuming less than 50 calories might still keep you in a fasted state, there’s no scientific evidence to define this calorie limit exactly, and it might differ from person to person, Costa notes.

What type of cardio to do during fasted cardio

Stick to low-intensity (zone 2 and below, or less than ~70% effort) cardio or base training when fasted. Zone 2 is the sweet spot where your body burns the maximum amount of fat for energy (16). Plus, studies suggest the more you train in zone 2, the better your body gets at burning fat for energy (17, 18).

One review found that eating before exercise is helpful for cardio workouts lasting more than an hour; though fasting before shorter workouts wasn’t detrimental (1). Cap your workouts at 60 minutes, or fuel with carbs and/or protein at the 60 minute mark if you plan to go longer. When your carb stores are low, eating protein in the middle of your workout can help improve performance, reduce muscle damage, and boost glycogen synthesis (19).

Generally, skip fasted cardio during high-intensity cardio sessions or a race, where you need to perform at a high level. The higher the intensity (zone 3, 4, and 5), the more glycogen your body needs—which is in short supply when you’re fasting.

How to Perform Fasted Cardio Safely

When it comes to maximizing the safety and effectiveness of fasted cardio, details matter. Here’s what Dinkins and Costa want you to know.

Start slow

Substitute one 30-minute low-intensity fasted cardio session for a regular cardio session to start. “As you get more comfortable working out on an empty stomach, slowly work up to longer fasted workouts,” says Dinkins. Cap fasted workouts at one hour, and aim for a blend of fasted and fed cardio workouts throughout the week—fueling for more intense sessions. 

Listen to your body

Pay attention to how your body responds before and after fasted cardio. “ If you experience excessive fatigue, dizziness, or discomfort, stop immediately and consider adjusting your approach or reverting to non-fasted workouts,” says Dinkins.

Fuel the night before

If you’re doing fasted cardio in the morning, prioritize proper nutrition the night before. “Eat a balanced meal that includes a good source of lean protein, complex carbohydrates, and healthy fats,” says Costa.

Eat strategically post-workout

After a workout, eating a meal containing protein and carbs will help replenish glycogen stores and support muscle recovery, says Dinkins. Aim for a macro split of 2-4:1 carbs to protein (and at least 20 to 40 grams of high-quality protein) within two hours of wrapping up your workout—but the sooner the better (18).

Hydrate before, during, and after

Dehydration can cause fatigue, which can reduce performance. A review published in the American College of Sports Medicine Health Fitness Journal suggests that even mild dehydration can impair physical and cognitive performance (14).

Keep your goals in mind

If burning more calories is your goal, you might be able to burn more by fueling before your workout, and increasing the intensity, says Dinkins. Same goes for performance—you’ll perform better with fuel in the tank. If fasted cardio isn’t a match for your goals, it might be worth a skip.

Who Should (and Shouldn’t) Do Fasted Cardio

A review in the Proceedings of the Nutrition Society concluded that more research, particularly long-term research, is needed to determine if certain people may benefit more from faster cardio than others (20).

If you prefer working out in the morning, or are interested in the metabolic benefits of fasted cardio, it might be worth a shot. However, Costa warns fasted cardio may not be suitable for people with diabetes or hypoglycemia. If that’s you, talk to your doctor first.

For athletes like runners, fasted cardio isn’t recommended for higher-intensity training sessions. “You can do fasted cardio for base mileage runs, but avoid it for any runs at tempo intensity (about 70% effort) or higher,” Dinkins explains.

Dinkins also points out that if you don’t like how fasted cardio feels, regular cardio offers similar benefits in terms of overall cardiovascular fitness and weight management. “Consistent engagement in cardio exercises, whether fasted or not, can still contribute to improved health and fitness outcomes,” she says.

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  2. Slater, T. et al. (2022) Fasting Before Evening Exercise Reduces Net Energy Intake and Increases Fat Oxidation, But Impairs Performance in Healthy Males and Females. 
  3. Liu, X. et al. (2023) The Effects of Six Weeks of Fasted Aerobic Exercise on Body Shape and Blood Biochemical Index in Overweight and Obese Young Adult Males. 
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