The Benefits of Fasted Cardio and Three Reasons to Consider It

The internet could be considered the greatest resource of information ever created. However, it also could be considered one of the biggest sources of misinformation, especially when it comes to information about the types of fitness programs that actually deliver results.

From various online forums and blogs to the limitless stream of social media, it can be hard to determine which information is accurate and which is a bunch of nonsensical garbage best relegated to the recycling bin. A perfect example is information related to fasted cardio—doing cardiorespiratory exercise like walking, running or cycling while in a fasted state—specifically done first thing in the morning on an empty stomach.

The practice of working out in a “fasted state” has been promoted as a way to burn more calories from fat than from carbohydrates, making it an effective way to boost the fat-burning effects of working out.

First, please keep in mind that research does not “prove” anything. It merely shows that when certain variables are applied in a specific manner, there may be a predictable outcome. Research allows us to observe how the body responds to a certain hypothesis, but remember that each and every individual may respond in a slightly different manner, so it’s important to take the time to identify what may (or may not) work for you.

Fasted cardio: theories

When it comes to fasted cardio, there are a couple of theories on why it may be more effective for fat burning.

For example, cortisol is a hormone that helps promote the metabolism of fat for energy. One theory suggests that because cortisol levels are higher in the morning, working out on an empty stomach means that the release of cortisol, which normally happens to help produce energy as the body starts to move, will lead to greater amounts of fat burning (Schoenfeld, 2011).

A second theory suggests that because the body’s resting metabolism continues to work overnight while the body sleeps, lower levels of carbohydrates will be available for fuel before breakfast. As a result, the body will rely on fats as the primary source of fuel for a workout.

Here’s why any research needs to be viewed with a little skepticism: because research exists to support both these theories. (Notice the use of the word support as opposed to prove.) For example, studies have shown that fasted workouts can help endurance athletes burn higher levels of fat than carbohydrates. However, while there may be some benefit to working out on an empty stomach, here are three specific reasons why fasted fitness may not be a good choice over the long term.

Improper use of protein

Gluconeogenesis is the process of converting proteins to glucose for energy. Working out in a fasted state could burn more calories from fat, but it also could cause the body to use protein for energy instead of repairing and building muscle tissue. In short, if carbohydrates (glycogen in muscle and liver, glucose in the bloodstream) are not immediately available for energy, the body might start using cortisol to convert amino acids, normally used to repair muscle tissue, for fuel, which leaves fewer available to rebuild muscle after a workout.

Displaced fat

The body has plenty of free fatty acids (FFAs) available for exercise. The FFAs that are not immediately converted to energy to fuel muscle activity could be redeposited in adipose tissue in the abdominal region, leading to a gain in belly fat. (This is how “skinny fat” happens.) Working out when cortisol levels are higher could lead to more FFAs circulating in the blood than can be used. So rather than depleting levels of fat, movement first thing in the morning could actually shift body fat to the abdominal region.

Consider the whole day

If the goal is weight loss by metabolizing as much fat as possible, it is more important to consider energy expenditure over a 24-hour period and not just at one point during the day. Monitoring energy intake and expenditure throughout the day and identifying how to reduce excessive intake and increase opportunities to move can play a more significant role in long-term weight loss than trying to burn more fat by working out first thing in the morning.

Three reasons to consider fasted cardio

While it doesn’t offer a viable long-term solution, based on the evidence available, working out in a fasted state could be a SHORT-TERM technique for helping a client achieve a specific body-composition goal. Here are three things to consider about using fasted fitness.

1. For years, bodybuilders and figure competitors have used fasted movement to help reduce body fat before a competition. This is because low-intensity steady-state (LISS) movement below approximately 70 percent of your maximum heart rate will rely on FFAs as the primary fuel source. To be most effective for fat burning, working out in a fasted state should focus on LISS, in which the intensity is low enough that you are sweating but can still talk comfortably during. When you can talk while working out, it means fat—metabolized with oxygen—is the primary source of fuel. As movement intensity increases, the body will start using more carbohydrates, leading to a faster breathing rate (to expel carbon dioxide), which limits the ability to speak comfortably.
2. The body is an extremely adaptable organism. If the same workout is performed at the same intensity for an extended period, the body will become very efficient at producing energy to fuel the activity. Doing fasted workouts first thing in the morning could help you move past a plateau, but after a period of time, your body will adapt to this condition, which is the signal that it’s time to change the workout and challenge your body in a different way.
3. You may be one of those people who prefers to work out first thing in the morning on an empty stomach. If this is the case, you can maximize fat metabolism by focusing on LISS over higher-intensity workout protocols. However, if you do want to do high- intensity interval training (HIIT) in a fasted state, keep it short—less than 10 or 12 minutes—because too much HIIT could elevate cortisol and lead to gluconeogenesis, which should be avoided for optimal muscle growth.

It’s important to note that eating a meal or snack too soon before working out can raise the levels of insulin, which helps metabolize and store fat. If insulin is elevated before a workout, the result could be fewer FFAs available for energy. A low-glycemic snack more than 30 minutes before a workout can provide energy for a workout and reduce the risk of elevating insulin levels.

Finally, keep in mind that substrate utilization for producing the energy required to fuel workouts is regulated by a number of variables, including hormone levels, available nutrition substrates, enzyme activity and the intensity, duration and type of physical activity. Working out in a fasted state could provide some short-term benefits, but the downside is that it also could limit the amount of protein available for muscle building. Rather than focus on what should be a short-term approach to help with a specific goal, it’s more important to identify ways to be more active throughout the day as opposed to merely sweating at a specific time of day.

Schoenfeld, B. (2011), “Does Cardio After an Overnight Fast Maximize Fat Loss?” Strength and Conditioning Journal 33(11): 23-25.

Photo credit: Bojan, Adobe Stock