Endurance athletes use various training methods to reach their goals — one of them is fasted training. This approach involves working out without eating beforehand, most commonly in the morning right after waking up and before breakfast. In this article, we explore why fasted training can be beneficial and what variations exist for applying it in practice.
Article published in Trimax Magazine, January–February 2023
By J.B. Wiroth, PhD in Sports Science and founder of the WTS coaching network (www.wts.fr)
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What Does Physiology Say?
Early in the morning — after a night’s sleep and without eating — your body is in a unique state:
- Blood sugar levels are lower than normal
- Cortisol (the stress hormone) is elevated
- Liver glycogen stores are reduced
- The stomach is empty
This physiological state is ideal for influencing energy metabolism. Through regular fasted training, the body adapts to draw energy from its nearly unlimited fat reserves.
The benefit: glycogen stores are preserved. Session by session, the body learns to use them more efficiently.
Already in 2011, research from Professor Peter Hespel’s team at the University of Leuven (Belgium) showed that after 6 weeks of fasted training (3 sessions per week, 1–2 hours at 75% of VO₂max):
- Muscle glycogen was better preserved
- Muscle oxidative capacity improved due to enzymatic adaptations
- Fat metabolism became more efficient, especially through better use of intramuscular triglycerides
In 2021, Tovar et al. from the University of California found that 4 weeks of training using the 16/8 method (16 hours fasting, 8-hour eating window) in 27 well-trained male runners reduced body fat — without affecting muscle mass.
Key Benefits of Fasted Training:
- Improves endurance by teaching the body to rely more on fat for energy
- Trains the body to stabilize blood sugar during low-carb efforts
- Promotes fat loss through increased fat utilization
The risks of fasted training
While fasted training can be beneficial, it is not without risks. The most immediate concern is a significantly increased risk of hypoglycemia (low blood sugar) when skipping breakfast. That’s why it’s smart to always carry a fast-acting source of sugar (e.g., an energy gel) during your session. After the workout, a noticeable energy dip throughout the morning is also common.
In the long term, excessive fasted training can negatively affect your cells:
- Essential fatty acids, which form the building blocks of cell membranes, are increasingly oxidized
- The amino acid pool is depleted, especially in the muscles, leading to the accumulation of nitrogen waste products (urea, ammonia, ketone bodies…)
Avoid These Mistakes:
- Doing long sessions (more than 90 minutes) as your first workout of the day
- Training at high intensity (above 80% of max heart rate)
- Doing more than two fasted sessions per week
- Staying fasted after the workout
The classic method
Protocol:
- Drink a large glass of water right after waking up
- Limit the workout to 20 minutes
- Increase intensity gradually, without exceeding 80% of your max heart rate
- Keep a gel handy in case of sudden hypoglycemia
- Drink another large glass of water after the session
- Eat a recovery breakfast immediately afterward
Progression should focus on increasing duration (but not beyond 2 hours) and incorporating technical drills (e.g., single-leg pedaling, cadence variations), while keeping intensity low to moderate (<80% HRmax).
After several months of consistent fasted training, short high-intensity efforts can be introduced. Many elite athletes train this way – especially top African runners.
Goal: Adapt the body to rely more on fat (lipolysis) and to produce energy via gluconeogenesis.
Fasted training variations
Variation 1: “Sleep Low” and Train Fasted
Do your workout in the evening, and eat a low-carb dinner. The next morning, you start your session fasted and with even lower glycogen stores than in standard fasted training. This increases the training effect.
Variation 2: Start Fasted, Fuel in the Second Half
For sessions longer than one hour, taking in carbs (e.g., sports drink) during the second half can reduce negative effects while still maintaining relevant intensity. It also trains the gut to absorb carbs under stress.
Variation 3: Train Twice a Day
The first session reduces glycogen stores. Afterward, eat a carb-free meal. The second session then takes place with very low muscle and liver glycogen. Studies show this can improve fat metabolism at the mitochondrial level.
Variation 4: “Recover Low”
Normally, it’s important to refuel with carbs and protein after long or intense sessions to replenish glycogen and support recovery. In this variation, carbs are intentionally withheld for 1–2 hours post-workout. While this slows recovery, it may boost gene-level adaptations.
Post-Fasted Training Meal
After a fasted session, runners have two options:
- If the goal is fat and weight loss:
A light snack is enough – recovery drink and a small sandwich.
Remaining fully fasted for hours afterward is not recommended. - If the goal is recovery and muscle preservation:
A full breakfast is ideal:
- Juice or fresh fruit
- Cereal or bread
- 20g butter + honey or jam
- 10 dried fruits or nuts (hazelnuts, almonds, walnuts)
- 2 slices of ham or 2 fried eggs
- Hot drink
Also ensure adequate intake of omega-3 fatty acids and branched-chain amino acids (leucine, isoleucine, valine).
Conclusion
With over 280 studies on the topic, fasted training is well-researched. However, questions remain — particularly around how to best combine training and nutrition to maximize performance gains in athletes.
References
- Beneficial metabolic adaptations due to endurance exercise training in the fasted state.
Van Proeyen K. et al. J Appl Physiol. 2011 Jan;110(1):236-45. - Four Weeks of 16/8 Time Restrictive Feeding in Endurance Trained Male Runners Decreases Fat Mass, without Affecting Exercise Performance
Tovar AP et al., Nutrients. 2021 Aug. 13(9), 2941. - Periodized Nutrition for Athletes
Jeukendrup AE. Sports Med 2017 Mar 47:51-63