As a personal trainer, you provide your clients with realistic, evidence-based health and fitness advice, and help them navigate through the countless fitness trends promoted online and on social media channels. 

One recent trend that has received a significant amount of attention, both in the online blogosphere and in traditional fitness media, is the idea of performing exercise first thing in the morning on an empty stomach. The practice of exercising 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 exercise. 

There are a couple of theories on why doing cardio in a fasted state first thing in the morning 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, exercising on an empty stomach will lead to greater amounts of fat burning. A second theory posits 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 have to rely on fats as the primary energy source for any physical activity. 

Research exists to support both theories. For example, studies have shown that fasted exercise can help endurance athletes burn higher levels of fat than carbohydrates. However, while there may be some benefit to exercising on an empty stomach, here are three specific reasons why fasted exercise may not a good choice over the long-term. 

1. Exercise in a fasted state could burn more calories from fat, but it also could cause the body to burn protein for fuel, which would reduce the amount that can be used to help repair and build muscle tissue. Gluconeogenesis is the process of converting proteins to glucose for energy. In short, if carbohydrates (glycogen in muscle and liver, glucose in the blood stream) are not immediately available for energy, the body can convert proteins for fuel, which leaves fewer proteins available to rebuild muscle post-exercise.
2. 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 can be redeposited in adipose tissue in the abdominal region. In other words, they will end up as belly fat. Exercising 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, exercise first thing in the morning could actually shift body fat to the abdominal region.
3. 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 exercising first thing in the morning. 

While it doesn’t offer a viable long-term solution, exercising 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 exercise: 

1. For years, bodybuilders and figure competitors have utilized fasted exercise to help reduce body fat prior to a competition. This is because low-intensity, steady-state (LISS) exercise below the first ventilatory threshold will rely on FFAs as the primary fuel source. To be most effective for fat burning, exercise in a fasted state should focus on LISS, where the intensity is low enough to allow the client to talk comfortably during exercise. Exercise at higher intensities will metabolize more carbohydrates and require more oxygen; therefore, the rate of breathing will increase to bring in more oxygen and exhale more carbon dioxide, which will make talking difficult.
2. The body is an extremely adaptable organism. If the same exercise is performed at the same intensity for an extended period of time, the body will become very efficient at producing energy to fuel the activity. Doing fasted exercise first thing in the morning could help a client move past a sticking point, but after a period of time the body will adjust to this condition. That’s the signal that it’s time to change the workout.
3. A client may like to exercise first thing in the morning and simply prefer doing so on an empty stomach. If this is the case, help them maximize energy metabolism by recommending LISS over higher-intensity exercise protocols. If a client wants to participate in high-intensity exercise first thing in the morning, it is important that he or she know how to properly fuel after the workout. 

It’s important to note that eating a meal or snack too soon before exercise can raise the levels of insulin, which helps metabolize and store fat. If insulin is elevated prior to exercise, the result could be fewer FFAs available for energy. A low-glycemic snack more than 30 minutes prior to exercise 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. Exercise in a fasted state could provide some short-term benefits, but the downside is that it could also limit the amount of protein available for muscle building. It’s more important to help clients identify ways to be more active throughout the day and guide them toward healthier nutrition choices than it is to recommend working out at a specific time of day.