Fasted Cardio: Is It Better for Weight Loss?

As a personal trainer, you're the go-to expert for reliable advice on anything related to fitness and nutrition. How do I get ripped for beach season? Should I ? High-fat? Vegan? Gluten-free? Can you tell me ?

While it’s unrealistic to know everything under the sun, keeping up with the latest trends and scientific data does come with the territory of being a top fitness trainer. And one question that can be overheard across the training floors of just about every gym has to do with fasted training and weight loss.

Two Sides of Fasted Workout Advice

Trainers often clearly fall on one of two sides when it comes to exercising in a fasted state. Those who are for it might explain that when you wake up in the morning, glycogen and blood sugar levels are naturally low. This forces the body to burn fat for energy (referred to as fat oxidation). The result is greater weight loss.

Trainers who are against fasted exercise tell a much different story. Yes, the body may indeed turn to body fat if glycogen and fat stores aren’t enough to fuel cardio exercise. But the consequences of this approach have less to do with fat loss and more so with its other effects.

If the body doesn’t have enough energy to get through the cardio session, it will try to pull that energy from the muscle protein. This leads to muscle breakdown, which ultimately means muscle loss versus muscle gain. Plus, low blood sugar during a cardio workout increases a person’s risk of fainting. And even if they are able to stay upright, their athletic performance will likely decline. Neither is a desired effect of cardiovascular exercise.

Each trainer paints a completely different story when it comes to a fasted cardio workout. This can be confusing, especially for people following an intermittent fasting approach. But who’s right? Technically, both.

A Simple Explanation About Fasted Cardio Effects

The body does burn fat for energy when you exercise on an empty stomach. At the same time, if you don’t have enough stored fat to burn, your body will burn protein instead. Both are true statements. They just fail to consider the entire context.

Neither ideology fits into a comfy, cozy blanket statement that we can default to whenever a client asks us the question. So, what's the right way to address it? And more importantly, how do we decipher which strategy is most effective (and appropriate) in maximizing gains while decreasing risks for our clients?

Answering these questions requires us to look beyond the simple effects of a fasted cardio session. It involves doing a deeper dive to better understand whether aerobic exercise in a fasted state helps reduce body fat, as well as its other effects.

What Research Says About Fasted Aerobic Exercise

At best, research has provided mixed results for fasted exercise and weight loss. Some studies have found positive effects. Others have not. 

For example, in one study, half the subjects did fasted exercise. The other half had a meal replacement shake before training. Each group did 60 minutes of steady-state cardio three days a week. Each also followed a diet creating a caloric deficit. Participants in both groups lost a lot of weight and fat mass. There was no major difference between them. (1)

An article published in September of 2022 found different results. When participants exercised in a fasted state, their fat oxidation increased. This article also reported that exercising after a fast can help regulate energy balance. (2)

At the same time, not eating breakfast before exercise has some disadvantages. One is that it can lead to decreased exercise performance. There are also drawbacks to doing fasted cardio at night. In the 2022 study, subjects worked out after 7 hours of not eating. They reported less motivation and energy to work out. They also enjoyed the exercise less.

So, Does Fasted Cardio Lead to Weight Loss?

It may or may not. Research reinforces a lack of consistency in study findings. While some research suggests that fasted cardio can help lower our body fat percentage, other studies fail to support this effect.

The discrepancy may lie in part in how these studies were conducted. Not all research is valid or reliable. Biases, choosing subjects within a limited demographic, and an inability to control outside factors—such as how many calories participants take in when not fasting—can all impact the results.

This is further complicated by no two people being the same. Like with other exercise approaches, fasted cardio might not work for everyone. While one person may enjoy fat loss by working out on an empty stomach, another person may not experience a weight change. So, this isn’t an all-inclusive fat burning solution.

Helping clients understand this upfront enables them to keep their expectations in check. Let them know that fasted cardio is just one tool in the fat loss toolbox. Strength training can also increase fat burn by raising the number of calories the body uses in a day. Creating a calorie deficit (taking in fewer calories than we use) is also critical for losing weight.

Some Athletes May Want to Avoid Fasted Training

Trainers working with athletes may want to advise clients to avoid a fasted training approach. Based on research, exercising in a fasted state may do them more harm than good.

For example, a 2020 article talked about endurance exercise and fasting. The authors reported a lack of evidence that training in a fasted state increases fat oxidation. They further recommended that endurance athletes not engage in high-intensity training during fasting. (3)

A different review focused on training approaches for bodybuilders. One of the approaches discussed was fasted cardio. When bulking, these athletes often spend 45 to 60 minutes doing moderate-intensity cardio in a fasted state. During the cutting phase, this time typically increases to two hours. (4)

Authors reported that, for many bodybuilders, this is a common practice. However, previous studies indicate that this provides no fat loss benefits. It can also hurt these athletes’ energy levels and negatively impact their fat metabolism. Thus, fasted cardio is not recommended for them.

Help Clients Engage in Fasted Cardio Safely

One study found that 24% of respondents had tried intermittent fasting for weight loss. (5) With this type of dietary approach so common, trainers will likely have at least some clients who engage in fasted cardio. How can you help them do so safely? 

If they’re using a 5:2 intermittent fasting schedule, they can save their exercise for days when they aren’t fasting. This may be helpful if they feel energy lulls on fasting days. This may not be possible for people who follow a 14:10 or 16:8 fasting plan. If they want to fast and exercise, they may need to do their workouts in a fasted state.

Clients who engage in fasted cardio may want to limit their exercise sessions to help them keep their blood glucose levels stable. One study found that these levels even stayed level for people with type 1 diabetes. This was true whether they did 30 minutes of moderate-intensity fasted cardio or six one-minute intervals of high-intensity training. (6)

A different study looked at fasted cardio for people with type 2 diabetes. It reported long-term benefits such as insulin sensitivity. Fasted exercise for these individuals also supported blood glucose stability and lower insulin levels. (7)

Perhaps the most important point to make with clients is to listen to their bodies. During a fasted workout, how do they feel? Does doing morning cardio on an empty stomach make them feel better or worse? If they feel worse, it doesn’t matter if it has a positive impact on fat metabolism. The negatives may not outweigh the benefits.

It can also be helpful to provide clients dietary advice that can help them change their body composition whether they do fasted or fed cardio. You can offer this service as a certified nutritionist. ISSA has a Nutritionist Certification course that teaches you how to create personalized meal plans for weight loss, building muscle, improving performance, and more!

References

• Schöenfeld, B. J., Aragon, A. A., Wilborn, C., Krieger, J., & Sönmez, G. T. (2014). Body composition changes associated with fasted versus non-fasted aerobic exercise. Journal of the International Society of Sports Nutrition, 11(1). https://doi.org/10.1186/s12970-014-0054-7

• Slater, T., Mode, W. J. A., Pinkney, M. G., Hough, J., James, R., Sale, C., James, L. J., & Clayton, D. J. (2023). 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. https://doi.org/10.1123/ijsnem.2022-0132

• Hassane Zouhal, Ayoub Saeidi, Amal Salhi, Huige Li, M Faadiel Essop, Ismail Laher, Fatma Rhibi, Sadegh Amani-Shalamzari & Abderraouf Ben Abderrahman (2020) Exercise Training and Fasting: Current Insights, Open Access Journal of Sports Medicine, 11:, 1-28, DOI: 10.2147/OAJSM.S224919

• Gentil, P., De Lira, C. a. B., Paoli, A., Santos, J. a. B. D., Da Silva, R. D. T., Pereira, J. R., Silva, E., & Magosso, R. F. (2017). Nutrition, pharmacological and training strategies adopted by six bodybuilders: case report and critical review. European Journal of Translational Myology, 27(1). https://doi.org/10.4081/ejtm.2017.6247

• Ballard, J. (2020, February 24). Americans say this popular diet is effective and inexpensive. YouGov.  https://today.yougov.com/consumer/articles/28065-most-effective-diet-intermittent-fasting-poll

• Scott, S. N., Cocks, M., Andrews, R. C., Narendran, P., Purewal, T., Cuthbertson, D. J., Wagenmakers, A. J. M., & Shepherd, S. (2018). Fasted High-Intensity interval and Moderate-Intensity exercise do not lead to detrimental 24-Hour blood glucose profiles. The Journal of Clinical Endocrinology & Metabolism, 104(1), 111–117. https://doi.org/10.1210/jc.2018-01308

• Hansen, D., De Strijcker, D., & Calders, P. (2016). Impact of Endurance Exercise Training in the Fasted State on Muscle Biochemistry and Metabolism in Healthy Subjects: Can These Effects be of Particular Clinical Benefit to Type 2 Diabetes Mellitus and Insulin-Resistant Patients? Sports Medicine, 47(3), 415–428. https://doi.org/10.1007/s40279-016-0594-x