Should You Try Fasted Cardio to Burn More Calories?
Key takeaways:
Fasted cardio involves working out on an empty stomach, typically first thing in the morning.
Some experts and fitness enthusiasts claim that this allows you to burn more fat and calories than working out after you've eaten.
Currently, there is not enough scientific evidence to support these claims.
Fasted cardio involves exercising on an empty stomach. This approach has been trending in the fitness world for several years. And some claim it's the key to burning fat — though, others say it doesn't make a difference. And the current research on fasted cardio is mixed.
So, is there any way to settle the debate? Here's what you should know about fasted cardio –– including details about its efficacy and safety –– before you decide to try it.
What is fasted cardio?
Fasted cardio refers to performing aerobic exercise on an empty stomach. This often means working out first thing in the morning before breakfast, when you haven't eaten for hours. But it could also be done later in the day if you practice intermittent fasting. This approach involves alternating between long periods of fasting and short periods of eating.
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Proponents of fasted cardio say it increases the amount of fat you burn during exercise. There are several theories about how this may work, including:
Your body uses stored fat and carbohydrates from food to fuel exercise. But when you haven't eaten for hours, there are fewer carbs available. So your body may rely on fat stores as a primary energy source, allowing you to burn more fat.
The hormone cortisol stimulates fat metabolism. Cortisol levels are highest in a fasted state — typically, in the morning, for most people. Therefore, fasting before exercise could help you burn more fat.
Eating alters gene expression in fat tissue. These changes could affect how your body uses fat during exercise. For example, there is a decline in the gene PDK4 after you eat. This may make it harder to use fat for energy during a subsequent bout of exercise.
Does fasted cardio help you burn fat?
There are no large-scale studies that show that fasted cardio helps burn more fat than non-fasted cardio. But some small studies suggest that fasted cardio may improve body composition.
In a 2015 study, researchers compared fasted and non-fasted cardio in 10 young men. They found that fasted cardio was more effective than fed cardio for burning fat up to 24 hours after exercise. This benefit only applied to fasted cardio completed before breakfast. Researchers noted that the findings might be due to a carbohydrate and calorie deficit during a morning workout.
However, other research refutes these findings. Several studies suggest that fasted cardio makes little difference in helping you burn fat or lose weight. In a small study, 20 young female volunteers were assigned to two groups to train in either a fasted or fed state. Both groups did 1-hour aerobic workouts three times a week. They also ate the same calorie-controlled diet. After 4 weeks, both groups lost weight and body fat. But there was no significant difference in the amount lost between the two groups.
Another small-scale study found that eight male participants burned more calories working out in a fed state. Their caloric expenditure was also higher for up to 24 hours after their workout.
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What are the benefits of fasted cardio?
Minimal research has been done on the benefits of fasted cardio, and the results have been mixed. So while there may be benefits for certain individuals, that could have more to do with factors like workout preferences and overall health.
For example, some people might find that it's more convenient to exercise when they first wake up. So fasted cardio in the morning can help them stick to their fitness plan. Others may feel like they have more energy when working out on an empty stomach. This may be because the body isn't devoting energy to digestion. Still, others might choose fasted cardio to help prevent stomach upset –– such as nausea, bloating, or diarrhea –– during or after a workout.
Is fasted cardio safe?
Fasted cardio is generally safe for most healthy adults. But there are potential risks. In some cases, exercising –– especially at a high intensity –– without proper nutrition could cause muscle breakdown.
Protein is the body's last choice as an energy source. But if your body can't get enough fuel from carbs and fat, it will burn protein instead. This could hinder muscle growth and repair after a workout.
Free fatty acids (FFAs) circulate in the bloodstream to help power your workouts. If they aren't burned during a fasted cardio session, those FFAs may be redistributed to the abdominal region. Increased cortisol –– which occurs during fasted exercise –– may increase FFAs. That could lead to higher amounts of accumulated belly fat.
Doing fasted cardio could also change your eating habits. A study found that people ate more the night before a planned fasted workout. But, according to another study, fasted cardio may help people eat fewer calories throughout the day. The researchers also noted that working out on an empty stomach in the evening could hinder performance and reduce motivation. And additional evidence suggests that fasted cardio at any time of the day may hurt performance, especially for endurance athletes who exercise vigorously for long periods.
So, in general, it's a good idea to start with a short, light workout if you're considering fasted cardio. Pay attention to how you feel during your workout. If you feel lightheaded or nauseated, this is a sign that your body might not be suited to working out before eating.
Just because fasted cardio works for one person doesn't mean it will work for you. And that's OK. Everyone's body is different.
Who should not try fasted cardio?
People with certain underlying health conditions, such as diabetes, should not try fasted cardio without talking to their healthcare provider first.
Diabetes can cause dangerous fluctuations in blood sugar. Hypoglycemia, or low blood sugar, can occur without adequate nutrition, including the proper mix of carbohydrates, protein, and fat. This could lead to dizziness, increased heart rate, and other symptoms that increase the risks of fasted exercise.
There is some evidence that fasted cardio may enhance insulin sensitivity and blood sugar control in people with diabetes. But there's a need for more research to confirm these findings.
The bottom line
Despite its popularity, fasted cardio has not been proven to help burn more calories or fat. But some people may prefer the approach for a variety of reasons.
For example, if exercising in the morning before breakfast works better for your schedule and doesn't cause discomfort, you might make a habit of it. Otherwise, eating a healthy diet and getting proper nutrition to support your exercise — at any time of day — is a safe and proven way to stay healthy and manage your weight.
If you have an underlying condition, to your healthcare provider before trying fasted cardio.
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References
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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. International Journal of Sports Nutrition and Exercise Metabolism.
