The concept of “fasted cardio” involves performing exercise, typically low-to-moderate intensity aerobic activity, after an overnight fast and before consuming any food. Proponents believe that exercising on an empty stomach forces the body to burn stored body fat for energy instead of relying on recently consumed carbohydrates. This approach is thought to accelerate overall fat loss. This common belief requires a closer look at the science of how the body manages fuel during exercise and whether an empty stomach provides a long-term advantage for body composition goals.
How the Body Fuels Exercise
The human body relies on two primary fuel sources to power muscle contraction during physical activity: carbohydrates and fats. Carbohydrates are stored in the muscles and liver as glycogen, which is the body’s most readily available energy source. Fat is stored extensively throughout the body in adipose tissue, offering a vast, though slower-to-access, energy reserve.
The intensity of the exercise dictates which fuel source is prioritized, a concept known as the “crossover” effect. During low-intensity exercise, such as a brisk walk or a slow jog, oxygen supply is plentiful. The body can efficiently use fat oxidation to meet its energy needs, and fat can account for up to 50% or more of the fuel at this intensity level.
As the intensity increases to moderate or high levels, the demand for energy outpaces the slow rate of fat metabolism. The body shifts its preference to carbohydrates, which can be broken down much more rapidly to produce the necessary energy. Consequently, high-intensity efforts, like sprinting or heavy weight training, rely almost exclusively on stored muscle glycogen.
Metabolic Shift During Fasted Exercise
Training in a fasted state fundamentally alters the hormonal environment, which then influences fuel selection. When a person has not eaten for 10 to 12 hours, the levels of the hormone insulin are significantly lowered. Low insulin levels reduce its suppressive effect on lipolysis, the process of breaking down stored triglycerides in fat cells into free fatty acids (FFAs).
Exercise stimulates the release of catecholamines, such as epinephrine and norepinephrine, which act as powerful signals to ramp up lipolysis. This combination of low insulin and high catecholamines creates an ideal environment for liberating stored fat into the bloodstream. These circulating FFAs are then readily available for the working muscles to use as fuel.
Research confirms that during an exercise session of the same intensity and duration, a person who is fasted will burn a greater proportion and absolute amount of fat compared to a person who has recently eaten. This acute increase in fat oxidation during the workout is the scientific basis for the popularity of fasted training.
Distinguishing Acute Fat Burning from Long-Term Fat Loss
While the acute increase in fat burned during a fasted workout is scientifically sound, it does not automatically translate into superior long-term body fat loss. The body maintains metabolic equilibrium over a 24-hour period, meaning a temporary increase in fat burning during exercise is often compensated for later in the day.
When the body burns more fat during a fasted morning session, it tends to reduce its fat oxidation and increase its carbohydrate oxidation during the subsequent fed hours. Conversely, a person who eats before training might burn more carbohydrates during the workout but will then increase their fat burning throughout the rest of the day. Over the full 24-hour cycle, the difference in total fat oxidized between the two conditions is negligible when the total daily calorie and nutrient intake are matched.
The ultimate determinant of long-term body fat loss is achieving a sustained calorie deficit. The Total Daily Energy Expenditure (TDEE) must be consistently higher than the calorie intake for the body to tap into stored fat for fuel. A study comparing a fasted and a fed group performing cardio while maintaining an equal calorie deficit found no statistically significant difference in overall body fat reduction after four weeks.
Practical Considerations for Fasted Training
The decision to train in a fasted state should be based more on personal preference and the type of exercise than on the expectation of superior fat loss. While low-to-moderate intensity activities like walking are well-tolerated in a fasted state, performance for higher-intensity exercise is often compromised.
Glycogen is the preferred fuel for intense efforts, and its relative depletion in a fasted state can lead to reduced power output and a lower quality of the workout. A decreased training intensity means that the total calories burned during the session may be lower, potentially undermining the overall energy deficit for the day.
Some individuals may experience side effects such as lightheadedness, nausea, or a sense of increased perceived effort when exercising without food. These negative sensations can make adherence to a regular exercise routine more challenging.
For most people, the best time to exercise is the time they can consistently stick to and that allows for the highest quality of training. If an individual prefers morning workouts and feels good training before breakfast, low-intensity fasted exercise is an acceptable option. However, for those aiming for high-intensity or prolonged performance, consuming a small, easily digestible carbohydrate and protein meal beforehand is likely a better strategy to maximize training output.