Understanding Fasted Training Correctly: Why It's Not About Skipping Breakfast

“Just go for a run without breakfast” – that’s how most people picture fasted training. The misconception behind it: whether you’ve eaten anything since waking up is almost secondary to the actual training effect. What matters instead is something else entirely – namely the state of the glycogen stores in your target muscles, not your general “fasted state” since breakfast.

A “normal” fasted training session without prior targeted depletion of the stores still provides some benefit – but different physiological mechanisms come into play here, and the training effect on fat metabolism is noticeably smaller than with a targeted pre-exhaustion. To understand why this is the case, it’s worth looking at what actually happens in your body overnight.

The Physiology: Liver Glycogen vs. Muscle Glycogen

The body stores carbohydrates (glycogen) in two different places with different tasks:

• Liver glycogen primarily serves to maintain a stable blood sugar level for the brain and organs. Overnight, during the multi-hour fasting period of sleep, the body relies almost exclusively on this store to keep blood sugar constant [1].
• Muscle glycogen is locally bound in the muscle and primarily serves as an energy reserve for muscle contractions. It can only be depleted through actual muscular work (movement, training) – not through fasting alone, since the necessary transport pathway out of the muscle is largely absent [2].

If you eat normally the evening before and do fasted training the next morning without any prior activity, you start with a depleted liver but with largely full muscle stores.

The Benefit of “Normal” Fasted Training

Even with full muscle stores, moderate training on an empty stomach has real physiological benefits:

1. Low insulin level. After several hours without food intake, the insulin level is at a very low point. Insulin inhibits hormone-sensitive lipase and thus lipolysis (release of fatty acids from fat tissue) [3]. With little insulin in the blood, the body is more willing to release free fatty acids for energy production – provided the intensity stays strictly in the aerobic range (zone 2), since rising intensity shifts the body toward carbohydrate burning anyway [4].

2. Mild mitochondrial adaptation. Training without immediately supplied external energy sends a mild stress signal to the cell to stimulate mitochondrial biogenesis (the formation of new mitochondria, the “powerhouses of the cell”) [5]. This signal is real but significantly weaker than with more strongly glycogen-depleted training protocols like the “Sleep Low” method described below.

3. Practicality and digestion. A 40- to 60-minute fasted run in the morning is time-efficient and trains the digestive tract to perform even without prior stomach contents – a side effect that can be particularly relevant for race situations with an early start.

The Problem with Full Muscle Stores

Since the muscle still has enough carbohydrates locally available, the body takes the path of least resistance as soon as intensity rises even slightly: it increasingly draws on the available muscle glycogen instead of fatty acids [4]. Actually simulating a “race situation with empty stores” doesn’t happen with normal fasted training, because the body isn’t forced to maintain fat oxidation under higher load.

The “Sleep Low” Method: The Targeted Alternative

The so-called “Sleep Low” strategy goes a step further: the evening before, a targeted training session actively depletes the muscle glycogen in the target muscles. Afterward, you deliberately eat a carbohydrate-reduced meal (or at least don’t fully replenish), so the muscle stores remain partially empty overnight. The training the next morning then actually takes place with reduced local glycogen stores.

This method was studied, among others, in a controlled study by Marquet et al. (2016) in Medicine & Science in Sports & Exercise on cyclists: the “Sleep Low” protocol led to a significantly stronger improvement in endurance performance over ten days compared to identical training with normally replenished stores [6]. Other work on training with reduced carbohydrate availability (“train low”) consistently shows enhanced signaling for mitochondrial adaptations and a shift in the maximal fat oxidation rate (FatMax) [7].

The catch: this protocol is significantly more hormonally and metabolically demanding than normal fasted training. Cortisol levels rise more sharply, recovery time is extended, and with too frequent use, the risk of overtraining and a weakened immune system increases [7]. This method should therefore be used in a targeted and dosed manner – not as a daily routine.

Conclusion

Both variants have their place in a structured training week:

• Normal fasted training is excellent for relaxed, regenerative zone 2 sessions in the morning. It moderately activates fat metabolism without heavily burdening the central nervous system – a good building block for base endurance.
• The “Sleep Low” method is the heavy artillery: it forces the body into stronger metabolic adaptations and shifts fat oxidation more significantly. Because it’s hormonally demanding and extends recovery, it belongs in the training plan in a targeted, dosed way – not too often.

Practical Example: A “Sleep Low” Day in Practice

Evening before (around 6 PM): Moderate to intense training session in the target muscles (e.g. a tempo run or an intense bike session of 45–60 minutes) that specifically depletes muscle glycogen in the muscle groups used.

Dinner afterward: A deliberately carbohydrate-reduced meal – the focus is on protein and vegetables, e.g. chicken breast with plenty of vegetables and a small portion of legumes, instead of the usual large portion of pasta or rice. The goal isn’t “zero carbs,” but deliberately not fully replenishing the stores.

Small breakfast before the session: Unlike “complete” fasted training, I recommend a small, deliberately chosen meal before the morning session – for example some natural yogurt with a dollop of nut butter. This combination provides some protein and healthy fats, but hardly any relevant amounts of quickly available carbohydrates. This keeps the insulin level low enough to not slow down fat oxidation, while the stomach isn’t completely empty going into the session – which improves tolerability and training quality for many people without diluting the deliberately induced training stimulus.

Fasted training in the morning: Easy, purely aerobic zone 2 session (e.g. 45–60 minutes of easy running or cycling) based on the reduced muscle glycogen stores from the evening before.

Immediately afterward – refuel quickly: After completing the session, unlike the evening before, you should now quickly consume both protein and carbohydrates to speed up recovery and kickstart muscle protein synthesis – for example a protein-rich breakfast with oats, fruit and milk or skyr. The targeted training stimulus has already been set at this point; now what counts is fast, complete recovery instead of further carbohydrate restraint.

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Sources:

[1] Jensen, J. et al. (2011). “The role of skeletal muscle glycogen breakdown for regulation of insulin sensitivity by exercise.” Frontiers in Physiology.

[2] Hultman, E., & Nilsson, L. H. (1971). “Liver glycogen in man: effect of different diets and muscular exercise.” Advances in Experimental Medicine and Biology.

[3] Jensen, M. D. (2003). “Fatty acid oxidation in human skeletal muscle.” Journal of Clinical Investigation.

[4] Romijn, J. A. et al. (1993). “Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration.” American Journal of Physiology.

[5] Hansen, A. K. et al. (2005). “Skeletal muscle adaptation: training twice every second day vs. training once daily.” Journal of Applied Physiology.

[6] Marquet, L.-A. et al. (2016). “Enhanced Endurance Performance by Periodization of Carbohydrate Intake: ‘Sleep Low’ Strategy.” Medicine & Science in Sports & Exercise, 48(4), 663–672.

[7] Impey, S. G. et al. (2018). “Fuel for the Work Required: A Theoretical Framework for Carbohydrate Periodization and the Glycogen Threshold Hypothesis.” Sports Medicine, 48, 1031–1048.

Note: This article is for general information purposes only and does not replace individual sports or nutritional medical advice. Anyone wishing to try targeted training methods like “Sleep Low” should do so considering their own health status and training level.