Working out or physical fitness training on an empty stomach has become a badge of honor for some fitness enthusiasts. Several influencers have also promoted it as an effective route to rapid fat loss. However, beneath the claims are critical conversations exploring whether fasted workouts or fasted training deliver real and measurable benefits.
The Fasted Training Myth: Working Out On An Empty Stomach Is Not Magic, Here Is What Actually Happens
Defining Fasted Training: Why So Many Swear By an Empty Stomach?
Fasted training refers to performing targeted physical activities without consuming any calories beforehand. The most common examples are morning workouts after an overnight fast or before eating breakfast, like fasted cardio or fasted high-intensity interval training. Some weightlifters also lift before eating because of the belief that it amplifies fat burning.
The purported benefits of fasted training come from a simple idea. Specifically, if the body has no consumed carbohydrates to burn, adherents believe it will be forced to tap into stored fat for fuel. Countless fitness influencers online promote this reasoning with before-and-after photographs and technical buzzwords. It is positioned as a route for accelerating progress.
Fuel Switching: How the Body Chooses Between Carbs and Fat?
Glucose from recently consumed carbohydrates is the most convenient and quickest to access fuel source. This makes it the preferred choice during most targeted or intentional physical activities and even in carrying out day-to-day activities. Only once these carbohydrate supplies are depleted does the body gradually begin drawing from stored fat.
However, in a fasted state, the carbohydrate supplies are already low, particularly after an overnight fast from nighttime sleep. This compels the body to shift toward oxidizing stored fat earlier in the workout. The result is a higher percentage of energy derived from fat rather than glucose. Remember that fats are the primary reserve energy stored by adipocytes.
This metabolic switch is consistently demonstrated in laboratory studies measuring fuel use during exercise. Participants who train without eating beforehand do indeed burn more fat during the session compared to those who consume carbohydrates first. The short-term advantage is real, measurable, and replicable in several scientific tests.
The Catch Behind: Why Burning Fat Does Not Always Mean Losing Fat?
It is still important to underscore the fact that burning fat during training is not always equal to losing fat overall. Burning a higher percentage of fat during a workout does not automatically translate to burning more body fat overall. Long-term fat loss is determined by total calorie balance across entire days and weeks. This also involves a proper diet.
Working out on an empty stomach or during a fasted state can also trigger invisible compensation. People often feel hungrier afterward and unknowingly eat more later, negating the temporary fat-burning advantage. Others move less throughout the day due to lower energy. In many cases, these adjustments cancel out whatever extra fat was burned in the morning.
Science Checks In: Do Fasted Workouts Really Deliver Results?
A systematic review and meta-analysis by D. Hackett and A. D. Hagstrom, published in 2017, compared groups performing fasted versus fed exercise over several weeks. Despite differences in fuel use during individual workouts, both groups achieved nearly identical reductions in body fat when measured across extended physical training periods.
It is worth noting that a survey published in 2020 revealed that recreational participants were more likely to perform fasted training than endurance athletes. The professional athletes, who prioritize long-term performance and body composition, seldom depend on fasted training, thus suggesting limited real-world confidence among elite competitors.
A 2025 paper by A. F. Vieira et al., which discussed the details of a 12-week randomized controlled trial aimed at determining the impact of fasted state versus fed state in adaptations to resistance training, concluded no difference in strength, power, or lean body mass. This showed that training to build muscles does not discriminate between fasted and fed states.
Pros and Cons: When Does Fasted Training Help and When Does It Hurt?
Fasted training can be surprisingly convenient for early risers who prefer ticking off working out to start their day. Note that there are people who feel sluggish after a meal due to a sugar crash. This can impact workouts and the rest of their morning routine. Skipping breakfast may help maintain a consistent routine. This works best for short and moderate workouts.
However, fasted workouts can be counterproductive. Many people overcompensate afterward with larger meals or poorer food choices. Those attempting long or high-intensity sessions almost always benefit from fueling beforehand. Doing intense physical activities on an empty stomach can affect performance by specifically decreasing endurance.
FURTHER READINGS AND REFERENCES
- Hackett, D. and Hagstrom, A. 2017. “Effect of Overnight Fasted Exercise on Weight Loss and Body Composition: A Systematic Review and Meta-Analysis.” Journal of Functional Morphology and Kinesiology. 2(4): 43. DOI: 3390/jfmk2040043
- Rothschild, J. A., Kilding, A. E., and Plews, D. J. 2020. “Prevalence and Determinants of Fasted Training in Endurance Athletes: A Survey Analysis.” International Journal of Sport Nutrition and Exercise Metabolism. 30(5): 345-356. DOI: 1123/ijsnem.2020-0109
- Vieira, A. F., Blanco-Rambo, E., Bandeira-Guimarães, M., Silva, R. T., Fergutz, A., Paz, I. de A., Munhoz, S. V., Colombelli, R., Vaz, M. A., Macedo, R. C. O., and Cadore, E. L. 2025. “Impact of Overnight Fasted State Versus Fed State on Adaptations to Resistance Training: A Randomized Clinical Trial.” International Journal of Sport Nutrition and Exercise Metabolism. 35(4): 291-302. DOI: 1123/ijsnem.2024-0215