Forget Eating Before Workouts — Do Fasted Training for Increased Fat Oxidation

As a seasoned personal trainer with over seven years of experience guiding clients toward their fitness goals, a common question I hear is whether fasted exercise burns more fat than working out after eating.

A systematic review and meta-analysis published in the British Journal of Nutrition found that fasted cardio may lead to slightly increased fat oxidation during a workout compared to training after a meal. (1)

Also, an International Journal of Exercise Science research suggests resistance training in a fasted state might lead to greater reliance on fat as a fuel source compared to workouts after a meal. (2)

Fasted strength training or cardio can indeed supercharge fat oxidation. However, you must combine it with a sustainable calorie deficit to experience steady weight loss.

In this article, we dive into the benefits of fasted training and how it can boost fat oxidation and overall performance.

Fasted Training Basics

Contrary to what most people think, you don’t need a hearty meal to power your training session. Most people have ample stored fuel in the form of glycogen to power their workouts.

While training in a fasted state, your body burns through the already depleted glycogen stores and then switches to fat reserves for its energy needs.

Fasted training involves working out without eating. The fasting window usually ranges between 10 to 12 hours but can be extended up to 24 hours in some cases. Here are the different approaches that you can adopt:

• Overnight Fasting: This is the most common approach. It involves eating dinner and training the next morning on an empty stomach.
• Intermittent Fasting: One of the most popular fasting methods, which comprises cycling between voluntary fasting and feeding windows. The most popular IF method (16:8) is the most suitable for fasted workouts.
• Extended Fasting: These last over 24 hours. However, you should avoid training during prolonged fasts.

Workout Suitability For Fasted Training 

Folks opting for fasted training should prioritize the following workouts:

• Steady-State Cardio: Monostructual cardio sessions that include jogging, cycling, and swimming at a moderate pace are ideal for fasted training. They allow your body enough time to convert stored body fat into energy.
• Low-Intensity Weight Training: Experts recommend using lighter weights and performing higher repetitions when training on an empty stomach.

Here are the types of workouts that you should avoid while fasting:

• High-Intensity Training: Avoid high-volume, demanding cardio and weight training sessions while fasting, as explosive sessions rely on glycogen stores for instant energy production.
• Extended Workouts: Long-duration sessions increase the risk of putting you in a catabolic state, which involves breaking down muscle tissue to burn as fuel.

How Fasted Training Boosts Fat Oxidation

Your body depends on two main energy sources depending on the activity and whether you’ve eaten recently. These include:

• Glycogen: Think of it as the quick-access energy. Your body taps into its glycogen stores for intense workouts when it needs instant energy. Glycogen is stored in the muscles and liver and is derived from carbs.
• Fat Stores: This is your body’s slow-burning reserve fuel, which it uses when you’re out of glycogen. The body has to work harder to access this fuel but most people have a massive chunk of this energy source.

Fasted Exercises Boost Fat Oxidation

Each individual usually has around 400 grams of glycogen stored in the skeletal muscles, which is readily available during exercise.

Overnight or extended fasting can deplete the glycogen reserves. Your body then signals to your brain that it needs to find an alternative energy source, which leads to the release of stored fat from your fat cells. This energy is then used for your workout, and this entire process is called fat oxidation.

Remember, your body needs more time to burn fat to convert it into energy, which is why it is recommended that you avoid high-intensity or extended training sessions during a fast.

“Fasted training, or training without eating for a few hours before exercising, can increase fat oxidation during exercise,” said Dr. Andrew Huberman, an assistant professor of neurobiology and ophthalmology at the Stanford University School of Medicine. “This is because the body will burn fat stores for energy instead of relying on recently consumed food.”

Things To Consider While Fasted Training

You’ve probably noticed that some people shed fat more easily than others. Multiple factors are responsible for this, including an individual’s age, genetics, sex, body composition, and activity levels. However, I want to draw your attention to something that is often overlooked — hormones.

Hormones play a significant role in fat loss, and fasted training can help you manipulate them to your advantage. Here are the hormones that play the most vital roles:

• Insulin: This hormone is responsible for taking sugar from your blood and delivering it to all the cells after a meal. High insulin levels after a meal can hamper the fat-burning process. The good news is fasting can lower insulin levels and boost fat oxidation.
• Growth Hormone: HGH is a key hormone for promoting muscle gain and boosting fat loss. Fasting can enhance HGH production, helping you achieve your dream physique.

You must consult your healthcare provider before starting a new diet or training regimen, especially if you are dealing with a pre-existing medical condition like type 2 diabetes.

Benefits of Fasted Training Beyond Fat Loss

Besides boosting fat oxidation, fasted training has the following advantages:

Boosts Insulin Sensitivity

Insulin levels drop while fasting. This is a big deal as insulin can temporarily block the fat-burning process and favor storing energy in the form of body fat. Fasted training (cardio or resistance) can increase your body’s sensitivity to insulin.

Whenever you eat, your muscles will absorb all the nutrients and leave less energy to be stored as fat. This also ensures you get the best bang for your buck from your meals.

According to Dr. Rhonda Patrick (Ph.D. in Biomedical Science), “Performing weight training or high-intensity moderate-duration workouts in a fasted state could lead to robust improvement in your glucose sensitivity and mitochondrial adaptations to using fatty acids for fuel as well as increased activity in genes that are tied to fatty acid metabolism.”

Enhances Mental Clarity and Focus 

Many of my personal training clients report feeling mentally sharper during a fasted training session. This leads to improved motivation, mind-muscle connection, and muscle pumps, which can all improve your results.

Experts usually link these psychological benefits to the shift between fuel sources in a fasted state.

Improves Endurance

Your body switches from burning glycogen to stored body fat for fuel during fasted training. Fat is an incredibly rich source of energy. When your body adapts to using fat more efficiently, you can train harder and for longer. However, this adaptation can take a long time and varies for each individual. You shouldn’t start a marathon prep as soon as you start fasted training.

Is Fasted Training Right for You? 

Fasted training is a potent tool, but it might not be for everyone. Here is how to know if it is a good fit for you:

• You’re Generally Healthy: Fasted training should only be undertaken by healthy individuals as it can put additional demand on your system.
• You Want To Lose Fat: Individuals trying to build muscle and gain weight should ideally stay away from fasted training.
• You Can Handle Discomfort: Fasted training can initially cause fatigue and lightheadedness. If these sensations are too strong for you, you should pull back.

People suffering from health issues, pregnant women, or those under 18 years old should avoid fasted training.

Tips to Make Fasted Workouts Work

Here are a few tips to maximize your results:

Start Slow

Always start slow while trying a new training or diet program. It allows your body to adjust to the new routine while giving you time to identify any red flags. Feel free to increase the fasting duration or training effort as you get more comfortable.

Hydrate

The human body is 70 percent water. Water becomes even more crucial when you’re training on an empty stomach. Dehydration can hamper your performance and increase the risk of lightheadedness. Drink at least a gallon of water daily and consume at least a glass of water every half an hour.

Time Your Workouts

Some people excel in early morning fasted workouts, whereas others like to fast toward the end of their fasting window. Find what works for you and stick to it. Avoid switching your approach often. You must, however, ensure you eat your post-training meal within two hours of your workout.

A randomized controlled trial found that morning fasted exercise may create a greater negative 24-hour energy balance than the same exercise performed after a meal. However, fasting also reduced voluntary performance, motivation, and exercise enjoyment. (3)

Prioritize Post-Training Nutrition

Dr. Peter Attia (MD) shared his personal experience training in a fasted state.

“I lost muscle mass and gained fat when I was exercising in the morning in a fasted state but then not eating until later in the day. It was because I wasn’t providing my body with the amino acids it needs for muscle protein synthesis.” — Peter Attia

Fasted training temporarily increases protein breakdown. You must consume a high-quality protein and carb-rich meal within two hours of your workout to prevent muscle loss, replenish energy stores, and promote recovery.

Fasted Training: Putting it All Together

Remember, although fasted cardio can lead to greater fat oxidation during the workout due to depleted glycogen stores, it doesn’t necessarily lead to more significant weight loss compared to fed-state workouts. (1)

When you burn an excess amount of fat during a workout due to low glycogen levels, your body compensates for it by slowing down the metabolism and burning less fat throughout the day. It does this to restock its glycogen stores.

Weight loss ultimately depends on calories in versus calories out. To lose weight sustainably, you must run a daily calorie deficit for an extended period, meaning you must burn more calories daily than you consume.

Conclusion

There are some real benefits of swapping your morning banana with black coffee. Fasted training can boost fat oxidation and enhance short-term performance compared to working out after eating a meal. However, you must prioritize maintaining a calorie deficit for sustained fat loss.

Remember, fasted training isn’t a one-size-fits-all program. Listen to your body and adapt your training regimen based on how your body is responding to training. Focus on post-training nutrition to boost recovery and results.

If you have any questions about fasted training, drop them in the comments below, and I’ll be happy to help!

References:

1. Vieira, A. F., Costa, R. R., Macedo, R. C., Coconcelli, L., & Kruel, L. F. (2016). Effects of aerobic exercise performed in fasted v. fed state on fat and carbohydrate metabolism in adults: a systematic review and meta-analysis. The British journal of nutrition, 116(7), 1153–1164.
2. Frawley K, Greenwald G, Rogers RR, Petrella JK, Marshall MR. Effects of Prior Fasting on Fat Oxidation during Resistance Exercise. Int J Exerc Sci. 2018;11(2):827-833. Published 2018 Jun 1.
3. Slater, T., Mode, W. J. A., Pinkney, M. G., Hough, J., James, R. M., Sale, C., James, L. J., & Clayton, D. J. (2022). Fasting Before Evening Exercise Reduces Net Energy Intake and Increases Fat Oxidation, but Impairs Performance in Healthy Males and Females. International journal of sport nutrition and exercise metabolism, 33(1), 11–22.

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