The Rundown on Fasted Cardio: What It Is, Pros and Cons, and Whether or Not It's For You

Fasted cardio is a popular craze among gym-goers looking to improve their body composition and get rid of stubborn body fat. 

If you’re already an early morning person who likes to begin your day with a cardio session, you may be contemplating giving fasted cardio a try.

In short, fasted cardio refers to performing a heart-pumping workout on an empty stomach. The theory is that it maximizes the body’s ability to burn energy from fat cells, allowing people to lose fat faster than they would during fed workouts.

But is fasted cardio really a good idea?

Keep reading to learn more about fasted cardio, including its pros and cons and whether you should ditch eating breakfast before your next cardio session

KEY TAKEAWAYS

• Fasted cardio is performed on an empty stomach, usually eight to 12 hours after your last meal. 
• People trying to lose stubborn body fat may benefit from low to moderate intensity fasted cardio.
• Fasted cardio may hinder muscle building and may not be ideal for athletes focused on building muscle. 

WHAT IS FASTED CARDIO?

The fasted cardio theory was created by bodybuilder Bill Phillips in 1999. In his book titled "Body-for-LIFE,” Phillips suggests that our bodies maximize fat burn when working out in a fasted state.

Many fitness industry professionals recommend fasting for at least eight to 12 hours to reach your maximum fat-burning potential.

Most people who perform fasted cardio do so early in the morning after an overnight fast. However, it can also be done at other times of the day if you practice intermittent fasting.

WHAT DOES FASTED CARDIO DO?

Research suggests that fasted cardio may:

• Reduce Body Fat 
• Improve Metabolic Flexibility

Before we dive into the research, here’s the quick and dirty details on how fasted cardio works.

As we know, the body prefers to use glycogen (the stored form of glucose) when it needs energy. Glycogen is composed of multiple glucose molecules linked together. Glucose is the primary sugar found in your blood. It comes from foods rich in carbohydrates.

Although the body can use sugar and fat for energy, it uses sugar first because it is faster and easier. However, glycogen stores are limited. 

The theory behind fasted cardio is that after you’ve fasted for eight to 12 hours, your body has stopped breaking down food, your insulin levels have decreased, and you’ve used up most of your muscle glycogen stores. 

Because you have not eaten to replenish these stores, your body is forced to burn stored fat for energy.

Despite making logical sense, research remains conflicted on the effectiveness of fasted cardio.

REDUCE BODY FAT

If your main goal is to reduce your body fat percentage, fasted cardio can increase the amount of fat burned during a workout.

One small study published in the British Journal of Nutrition examined the impact of breakfast and exercise on metabolism, energy balance, and appetite in twelve physically active males.

Researchers found that morning exercise led to a reduction in appetite, which was greater when performed fasted.

Additionally, participants who ran on a treadmill in a fasted state burned 20% more body fat than those who had eaten breakfast. 

Other studies also suggest that fat oxidation, or the breakdown of fat to produce energy, ramps up when you exercise on an empty stomach.

Currently, evidence remains limited on long-term fat loss associated with fasted cardio.

Ultimately, you will lose weight and body fat as long as you eat in a calorie deficit.

For example, one study found that body composition changes associated with cardiovascular exercise paired with a hypocaloric diet are similar regardless of whether or not a person fasts before training.

IMPROVE METABOLIC FLEXIBILITY

Performing cardiovascular exercise with little to no glycogen stores may help improve metabolic flexibility.

Metabolic flexibility is your body’s ability to switch from using carbohydrates to fat as fuel.

Increased use of different fuel sources results in a more efficient fat-burning state and steady energy.

According to research, metabolic inflexibility is linked to:

• Obesity
• Increased Fat Storage 
• Diabetes
• Metabolic Syndrome (high blood pressure, elevated blood sugar, excess abdominal fat, abnormal cholesterol levels)

It’s important to note that adopting a well-balanced diet low in processed carbohydrates and refined sugar and regularly exercising can also help increase your metabolic flexibility.

CONS OF FASTED CARDIO

• Muscle Breakdown
• Decreased Energy Levels
• Decreased Performance

Despite the potential fat-burning benefits of fasted cardio, it does come with some risks. If you’re planning on performing high-intensity exercise for prolonged periods of time, you should not exercise in a fasted state.

Exercising on an empty stomach can decrease energy levels, leading to reduced physical performance. It also can cause protein and muscle tissue breakdown. 

MUSCLE BREAKDOWN

As we mentioned earlier, when glycogen stores are depleted, your body can switch to burning fat stores for fuel. However, your body may decide to eat away at your muscle mass instead. 

For instance, one 2011 review published in the Strength and Conditioning Journal suggests that there is no difference in fat loss between exercise performed in fed state versus a fasted state. It also found that training with depleted glycogen can lead to protein loss and, as a result, muscle loss.

Researchers concluded that fasted cardio has the potential to negatively impact those who are trying to gain strength and build muscle. 

Many athletes who perform fasted cardio workouts supplement with branched-chain amino acids (BCAAs) before or during training and a whey protein shake immediately after. This may help to maintain muscle mass.

DECREASED ENERGY LEVELS

If you are working out at 5 AM following a good night’s sleep in a fasted state, you may have enough energy to push through your workout. However, if you get up mid-morning or didn’t get adequate sleep the night before, fatigue may set in.

Additionally, high-intensity cardiovascular exercise requires carbohydrates for quick energy. Without them, you may begin to feel tired and fatigued, which may cause you to bail on your workout early.

In one review from 2018, researchers found that pre-exercise feeding before a workout enhanced performance and allowed participants to exercise for more extended periods of time. 

DECREASED PERFORMANCE

Fasted cardio has the potential to decrease endurance and athletic performance.

One 2018 study published in the Journal of Sports Medicine and Fitness found that exercising in a fasted state compromises exercise intensity and volume compared to consuming carbohydrates before exercise. 

A newer 2020 study published in the Journal of Sports Medicine suggests that endurance athletes should avoid high-intensity training while fasting. Researchers also suggest that fasting can decrease cognitive functions.

Moreover, some people report dizziness, lightheadedness, fatigue, and low blood sugar when performing fasted exercise. This can prevent you from reaching your peak potential during your workout and decrease physical performance.

ATHLETES THAT SHOULD AND SHOULDN'T BE TRAINING ON AN EMPTY STOMACH

Although fasted cardio may improve your ability to burn fat, it is not for everyone.

Those who should avoid fasted cardio include:

• Pregnant women
• Athletes looking to maximize muscle and strength gains
• People with diabetes
• Those with low blood pressure
• Athletes who are performing high-intensity exercise
• People who are new to exercise

Those who may benefit from training on an empty stomach include people who:

• Want to shed stubborn fat
• People who are not concerned with gaining muscle mass
• Athletes who are performing low to moderate-intensity exercise

IS FASTED CARDIO FOR YOU?

Scientific evidence remains mixed on the benefits and risks of fasted cardio, so it ultimately depends on what works best for you and your fitness goals.

If weight loss and fat loss is your ultimate fitness goal and you have been struggling with stubborn body fat, fasted cardio may aid in burning body fat.

However, if you are training to gain muscle mass and strength, it’s best to avoid fasted cardio to limit muscle protein breakdown.

Additionally, if you participate in high-intensity interval training or reach your maximum heart rate during exercise, you should not work out on an empty stomach.

Our pre-workout supplement PRE contains high-quality carbohydrates to allow for a constant energy source to fuel your high-intensity workout. It also improves oxygen utilization, fat oxidation, and increases time to fatigue. 

As always, if you have any medical conditions, you should speak with your healthcare provider before attempting fasted cardio.