Do you exercise on an empty stomach to optimise fat loss, but fear failure, fatigue or the risk of hypoglycaemia? Did you know that this practice, although promising for experienced athletes, requires a targeted approach? From night-time preparation to the mistakes to avoid, this guide reveals the secrets to burning energy reserves while protecting your body. Perfect for endurance enthusiasts or “health sport” practitioners, provided you respect best practices…
What exactly is fasted training?
Definition and physiological mechanisms
Fasted training refers to physical activity carried out after a period of food deprivation, usually in the morning before breakfast. When the body is in a fasting state, its hepatic glycogen reserves (stored sugars) are reduced after several hours without food.
In response to this decrease, the body activates a process called lipolysis: it converts fat reserves into energy. This mechanism is facilitated by a drop in insulin and a rise in cortisol, promoting fat mobilisation. However, this adaptation has a downside: the body can also break down muscle proteins to produce energy, increasing the risk of muscle fatigue.
The different types of fasting for sport
Several forms of fasting exist, but some are better suited to physical activity than others. Here are the main ones:
- Intermittent fasting (16/8): The most popular. It involves fasting for 16 hours (including overnight) and eating within an 8-hour window. Exercise is often performed at the end of the fast, when glycogen reserves are depleted. Ideal for endurance sports.
- Wet fasting: Allows the consumption of water, herbal teas or non-caloric broths. Recommended for fasted training as it prevents dehydration, which is a greater risk when exercising without food intake.
- Dry fasting: Prohibits any intake, solid or liquid. Absolutely to be avoided in sport due to the risks of dehydration, fainting or muscle loss. Reserved for supervised practices under medical monitoring.
Choosing the right type of fasting depends on the sporting goal and experience. Beginners should favour the 16/8 intermittent fast to avoid unwanted side effects.
The real benefits of fasted training: myths and realities
Drawing on fat: an advantage for weight loss?
Fasted training stimulates fat oxidation during exercise, a mechanism often highlighted for weight loss. In the absence of recent carbohydrates, the body activates fat stores to produce energy. A study on cyclists showed a 20% increase in fat burning during moderate exercise, confirming its relevance for burning fat reserves during activity.
However, overall fat loss depends on the 24-hour calorie deficit, not just fasted activity. The body often compensates by slowing metabolism or mobilising less fat outside the session. Thus, while fasted training may help, it does not replace a balanced diet and overall energy deficit. The key lies in combining this practice with a comprehensive nutritional strategy.
Other benefits for the body and performance
Fans of morning fasted training report a feeling of lightness, avoiding digestive discomfort such as side stitches. Less energy dedicated to digestion frees up resources for the muscles, optimising recovery. For endurance athletes, this practice refines energy efficiency, as shown by a 5.5% improvement in functional threshold power (FTP) in trained cyclists over 3 weeks of monitoring.
The “Sleep Low” strategy illustrates this potential: by combining intense training in the evening with overnight carbohydrate fasting, it boosts mitochondrial-related cell signalling, improving long-distance performance. A study noted a reduction of 1.1 kg in body mass among participants, thanks to increased mobilisation of reserves. This method, detailed in the article on ultra-endurance sports, is reserved for experienced athletes.
These benefits are not without limits, however. Intense effort on an empty stomach can cause hypoglycaemia (dizziness, fatigue) or excessive use of muscle proteins. Maximum performance (e.g. sprints) does not always improve, and sometimes worsens, according to studies. Balance is therefore crucial: favour short (30–40 minute) and moderate sessions to maximise gains without harming health. Metabolic adaptations, such as increased fat-burning enzymes, also strengthen the interest of this practice for those engaged in prolonged effort.
Risks and contraindications: when fasted training is a bad idea
Hypoglycaemia and reduced performance
Training on an empty stomach exposes you to hypoglycaemia, a drop in blood sugar levels. The brain, dependent on glucose, reacts with symptoms such as:
- Hot flushes
- Dizziness or light-headedness
- Sudden fatigue
- Rapid heartbeat
- Blurred vision
- Trembling
For intense sports (HIIT, cross-training), the lack of glycogen limits explosiveness. A study noted a 30% drop in performance among 78% of beginners. Hepatic reserves (depleted overnight) and muscle reserves (300–500 mg) are insufficient for long or intense efforts. The critical threshold of 80–100 mg/dl upon waking makes those training beyond 45 minutes more vulnerable. In untrained athletes, this energy deficiency can even slow muscle recovery, as glycogen reserves, essential for repair, are not replenished.
At-risk profiles and contraindications to respect
Certain groups should avoid fasted training:
- Beginners, not yet adapted to lipolysis, whose bodies draw on muscle proteins, leading to unnecessary catabolism. Their energy system, untrained to use fat, does not produce enough ATP for effective effort.
- People with diabetes or heart disorders, for whom hypoglycaemia can be fatal. In type 1 diabetics, lack of sugar can trigger ketoacidosis, a metabolic acidosis state that may cause organ damage.
- Pregnant women, already in greater need of glucose for the foetus. Hypoglycaemia can cause dizziness or falls, but also affect foetal brain development, which is sensitive to maternal energy fluctuations.
- Athletes aiming for muscle mass gain, as catabolism is triggered without carbohydrate intake. Amino acids (such as BCAAs) are diverted towards glucose production, reducing muscle synthesis by 15–20%.
Medical advice is crucial. For example, type 1 diabetics or pregnant women risk severe complications. A light snack (dried fruit, black tea) stabilises blood sugar. According to a survey, 65% of accidents occur in practitioners who ignored these precautions. Metabolic disorders (anaemia, unbalanced thyroid) also aggravate hypoglycaemia, making medical follow-up essential. People with anaemia, for instance, already have compromised cellular oxygenation, which, combined with low glucose, increases the risk of fainting.
Which sports to choose (and avoid) for a successful fasted session?
Moderate-intensity activities to prioritise
| Type of Sport | Recommended Intensity | Suitable for fasting? | Advice |
|---|---|---|---|
| Running / Cycling / Swimming | Low to moderate (basic endurance) | Yes | Aim for 60–70% of your max heart rate. Don’t chase speed. |
| Brisk walking / Hiking | Low to moderate | Yes, ideal to start | Excellent to begin and accustom the body gently. |
| Yoga / Pilates / Stretching | Low | Yes | Choose gentle styles (Hatha, Yin) rather than dynamic ones (Vinyasa, Ashtanga). |
| Strength training | Light (body weight) | With caution | Avoid heavy loads. Focus on technique and long sets. |
| HIIT / Cross-training / Sprint | High | No, not recommended | These sports require glycogen for explosiveness and increase the risk of hypoglycaemia. |
| Team sports / Combat sports | Variable / High | No, not recommended | Intensity is unpredictable and often high. Risk of reduced performance and fainting. |
For a successful fasted session, opt for moderate endurance sports. Brisk walking, light jogging or controlled-pace cycling activate fat reserves without drawing on muscles. These activities help the body prioritise fat as energy.
Avoid intense efforts such as HIIT or team sports. Their high energy demand, combined with lack of glycogen, increases the risks of muscle catabolism and hypoglycaemia. Drink water before and after exercise to stay hydrated.
A 30–40 minute session, adapted to your level, is ideal. If in doubt, consult a professional to tailor your approach. Fasted training requires self-awareness and gradual progression.
Your practical guide to starting fasted training safely
Preparation: what to do the day before and just before?
To optimise your fasted session, the meal the night before is crucial. It should be balanced, rich in complex carbohydrates (brown rice, wholemeal pasta, legumes) to replenish glycogen reserves, and in lean proteins (fish, poultry, tofu).
Avoid fatty or overly heavy dishes that slow digestion. White rice or white pasta are preferable before exercise, as they are easier to digest. Also include healthy fats (avocado, olive oil) and limit fibre to avoid digestive issues.
In the morning, prioritise hydration: drink a large glass of water upon waking and another before exercise. Herbal teas are allowed, but black coffee or unsweetened black tea do not replace water. Good hydration prevents dizziness and improves performance.
During the session: duration, hydration and warning signs
Start with short sessions of 30–40 minutes to give your body time to adapt. Opt for gentle disciplines such as brisk walking, light yoga, easy swimming or light strength work.
Remember to drink regularly, a few sips every 15–20 minutes. In the absence of sufficient reserves, fainting can occur: be alert to signs such as dizziness, sudden fatigue or nausea.
Always carry a small snack (dried fruit, energy gel) to react in case of a drop in sugar. Finally, never push yourself: this practice is above all about listening carefully to your sensations to progress safely.
After the session: the crucial importance of recovery
Post-exercise recovery is key. Eat a complete meal within 30–60 minutes after training to replenish reserves and repair muscles. Your plate should include:
- Proteins: Eggs, Greek yoghurt, chicken, tofu for muscle repair.
- Carbohydrates: Oats, fruit, wholemeal bread to restore glycogen.
- Healthy fats: Avocado, nuts for hormonal functions.
- Good hydration: Keep drinking water to make up for losses.
To go further, explore complementary recovery techniques. In case of post-exercise cravings, choose nutritious foods (almonds and an apple) rather than empty-calorie snacks.
Finally, monitor your overall condition: persistent fatigue or malaise after several sessions means fasting may not be suited to your physiology. Consult a professional to adjust your approach.
Is fasted training for you?
Assessment: for whom and for what goals?
This method mainly concerns experienced practitioners seeking to improve energy reserve utilisation. It can help reduce fat mass, but the result depends above all on overall calorie balance and a balanced diet.
It is not suited to beginners, very intense disciplines or goals of muscle mass gain. It may cause fatigue or energy dips. Better to reserve it for moderate exercises such as walking, easy cycling or yoga, progressing step by step.
The importance of self-awareness and medical advice
Fatigue, dizziness, excessive sweating: these signals should not be ignored. Hydrate regularly and keep a snack handy to avoid sugar crashes.
Fasted training can help burn fat and suit moderate endurance efforts, but it is not for everyone. Its effectiveness relies on personalised practice, good hydration and the advice of a health professional.
