Fasted training refers to exercising after a period without food, usually in the morning before breakfast. In this condition, the body has lower circulating glucose and insulin availability, which changes how energy is supplied during exercise.
The main change occurs in the balance between carbohydrate and fat utilization as energy sources. However, this does not mean that the body stops using carbohydrates or that fasted training is necessarily better for fat loss or performance.
What Happens to Energy Metabolism During Fasting?
During fasting, insulin levels tend to be lower and the availability of glucose from the previous meal is reduced. As a result, the body increases the mobilization of fatty acids from adipose tissue.
These fatty acids can be transported to the muscles and used to produce energy through fat oxidation.
At the same time, the liver helps maintain blood glucose mainly through glycogenolysis, which uses liver glycogen, and gluconeogenesis, which produces glucose from other substrates.
Therefore, even during fasting, the body continues to use carbohydrates. What changes is the proportion contributed by each energy source.
Does the Body Start Using More Fat?
Yes. In general, exercising after a period of fasting increases the relative contribution of fat to energy production, particularly during low- to moderate-intensity exercise.
This occurs because lower carbohydrate availability and reduced insulin levels favor the mobilization and utilization of fatty acids.
However, greater fat oxidation during exercise does not automatically mean greater body fat loss.
Changes in body composition depend primarily on energy balance over time, as well as factors such as food intake, training, recovery, and overall diet composition.
What Happens to Muscle Glycogen?
Muscle glycogen remains an important energy source during exercise, particularly as exercise intensity increases.
Depending on the duration of the fast, the previous training session, and food intake from the previous day, glycogen stores may be partially reduced.
As exercise intensity increases, the demand for ATP rises rapidly, making carbohydrate utilization increasingly important. Therefore, activities such as sprints, interval sessions, high-intensity workouts, and competitions tend to be less compatible with fasted training.
Does Fasted Training Improve Performance?
Not necessarily.
Although fasting increases fat utilization during exercise, this does not necessarily translate into better performance.
During prolonged, moderate-intensity exercise, some individuals may be able to train effectively in a fasted state. However, when a session requires high energy output, lower carbohydrate availability may impair the ability to maintain exercise intensity.
For endurance athletes, for example, adequate carbohydrate availability remains an important strategy for sustaining higher-intensity and higher-volume training sessions.
Can Fasted Training Promote Metabolic Adaptations?
There is scientific interest in the concept of “train low,” a strategy in which selected training sessions are performed with low carbohydrate availability.
The goal is not simply to train fasted all the time, but to strategically use periods of low carbohydrate availability to stimulate certain adaptations related to oxidative metabolism.
However, this approach needs to be individualized. Training with low carbohydrate availability can also reduce the ability to perform high-intensity sessions and, when used excessively, may negatively affect training quality and recovery.
For this reason, a more effective strategy for athletes may be to periodize carbohydrate availability according to the goal and demands of each training session, rather than simply skipping breakfast before every workout.
When Can Fasted Training Make Sense?
Fasted training can be an option for some individuals in specific situations, particularly for sessions that are:
- Low to moderate in intensity.
- Relatively short in duration.
- Associated with lower carbohydrate demands.
- Well tolerated without pre-exercise food intake.
- Part of an individualized nutrition strategy.
On the other hand, before key workouts, interval sessions, very long training sessions, or competitions, the priority should be to ensure adequate energy and carbohydrate availability to meet the demands of exercise.
Fasting Does Not Mean Skipping Hydration
Even when training fasted, hydration remains important.
During exercise, water and electrolytes are lost through sweat, particularly sodium, regardless of whether the individual has eaten beforehand.
Therefore, depending on exercise duration, intensity, environmental temperature, and sweat rate, it may be necessary to develop an appropriate hydration and electrolyte replacement strategy.
How Can Fasted Training Be Applied in Practice?
The best strategy depends on the goal of the workout.
Easy training: may be performed fasted by individuals who tolerate this condition well, particularly when there is no need to maximize exercise intensity.
High-intensity training: consuming carbohydrates before the session may be more appropriate to preserve training quality.
Long-duration training: the longer and more demanding the session, the more important it becomes to plan adequate carbohydrate availability and carbohydrate intake during exercise.
Key training sessions: important sessions designed to drive athletic progression should prioritize quality. In this context, having adequate energy availability before training may be more relevant than maximizing fat oxidation during that specific session.
Conclusion
Fasted training changes energy metabolism by increasing the relative contribution of fat as an energy source and altering carbohydrate availability.
However, using more fat during exercise does not necessarily mean greater fat loss, nor does it automatically lead to better performance.
In practice, fasting can be a tool, but it does not need to be a rule. For athletes and physically active individuals, the most effective approach is to consider the intensity, duration, and goal of each training session, adjusting energy and carbohydrate availability according to the demands of the workout.
