​Exercising on an empty stomach: A boost to fat metabolism or a risk?

Sports nutritionist Katja Mangold explains what ‘training on an empty stomach’ really means. Training on an empty stomach is a buzzword in the world of fitness. It is often equated with ‘setting off without breakfast’. But that is an oversimplification that is misleading. The crucial factor here is not whether your stomach is empty, but how full your muscles’ glycogen stores are.

By way of explanation: Muscles store energy from our food within their cells. This enters the digestive tract in the form of carbohydrates and is distributed throughout the body as glucose, for example to fuel muscle activity. In the liver and muscles, it can in turn be stored as glycogen (the name for stored sugar).

To distinguish between them:

Liver glycogen: This store acts as a buffer for blood sugar levels and primarily supplies the brain. Overnight, this store is partially depleted in order to keep blood sugar levels stable.

Muscle glycogen: This store is reserved exclusively for the working muscles. It cannot be released back into the bloodstream. The muscles can utilise the stored energy as and when required. Fasting training involves tapping into and releasing this store. True ‘fasted training’, as defined by sports science, only takes place once the energy stores within the muscles have also been depleted.

In sports science, the concept behind this is known as ‘Train Low, Compete High’ – train with low carbohydrate stores, but go into competition with your stores fully replenished. How low your stores are in the morning depends on several factors:

• Training session the day before: If you did an intense or long workout the night before, this will deplete your glycogen stores
• Diet the day before: If you eat a low-carbohydrate meal in the evening, you won’t replenish these stores in your muscles and liver to the same extent
• Breakfast: If you want to ‘train on an empty stomach’, a low-carbohydrate breakfast is often a good idea – e.g. scrambled eggs, avocado, quark

So, in this context, ‘training on an empty stomach’ actually means: low glycogen stores – not an empty stomach.

What happens in the body?

When training with depleted glycogen stores, the body has to draw on its fat reserves to generate energy and make it available to the muscles. In a widely cited study involving well-trained cyclists, fat burning increased significantly in the ‘low’ group. The control group, which trained with full glycogen stores, did not show this adaptation. However, it is always important to note that performance and output – for example, in watts – decrease during such training sessions. This has also been scientifically proven. This type of energy management is therefore not suitable for high-intensity training. In short: this is primarily about a metabolic adaptation at relatively low intensity. When it comes to improving performance, scientific research suggests that the ‘Train Low’ approach is actually counterproductive.

The theory behind the concept: a metabolism that can switch flexibly between fat and carbohydrates offers advantages during long-duration endurance exercise. However, recognised scientific facts are being used here to draw a conclusion that, in turn, has not been proven. The hypothesis: those who burn more fat during moderate exercise conserve their limited glycogen reserves – and are less likely to hit the ‘hitting the wall’ stage during long rides or marathons. However, particularly when it comes to recreational athletes, there is no clear evidence to support this. It is also important to note that this is not about weight loss. Burning more fat during training does not automatically mean greater weight loss over a 24-hour period, as the body compensates for this during the rest of the day.

Risks: When it does more harm than good

Lower training quality: If you train with depleted glycogen stores, you’ll be unable to perform at your best. This isn’t a problem during light base training sessions – but if you don’t consistently keep the intensity low, you’ll find yourself training in a no-man’s-land: too hard for recovery, too light to provide a genuine training stimulus. We have highlighted this ‘pointless’ training on several occasions, for example in the context of training polarisation.

Increased stress response: Training with low glycogen stores increases cortisol release. Cortisol is a stress hormone – normal in moderation, but problematic if released in excess over the long term. For example, there is a risk of losing the important recovery effect. Anyone who already experiences a lot of everyday stress (and what working amateur cyclist doesn’t?) should be cautious about adding further stressors.

Immune system: If you regularly exercise when your energy levels are low, this can make your immune system more susceptible to infections. However, an occasional lack of energy is not automatically harmful to the immune system.

Particular caution is advised in the case of women

Practitioners in the field of sport particularly emphasise this link, which remains an underestimated risk: women are more sensitive to low energy availability than men. Research shows that even short-term periods of insufficient energy intake in female athletes can trigger hormonal cascades: elevated cortisol levels, impaired thyroid function and menstrual cycle disturbances. This does not mean that women should not train on an empty stomach.

However:

The sessions should be even shorter and more relaxed

The frequency should be very low

If you experience irregular periods, stop immediately and seek medical advice

If in doubt: it’s better to train after a low-carbohydrate breakfast than on a completely empty stomach

Who might sometimes benefit from training on an empty stomach

Training on an empty stomach is not a one-size-fits-all solution; there is a lack of universally applicable findings. It may be suitable for, and is therefore currently used for:

• Experienced endurance athletes, incorporated in a measured way into the training plan
• Cyclists who cover long distances (races, bikepacking, brevets)
• Athletes in the foundational phase who wish to boost their metabolism as part of a training programme

It is not suitable for:

• Beginners with little training experience – they lack the foundation needed to assess the workload correctly
• High-intensity sessions – intervals, threshold training or sprints – require carbohydrates as fuel
• Long rides – anyone who cycles for several hours with depleted energy stores risks a drop in performance and an increased stress response
• People with medical conditions, breastfeeding women, pregnant women

Is training on an empty stomach right for you? Here’s how to go about it

Our practice consultant, Katja Mangold, has put together a typical ‘Train Low’ programme:

Preparation (the evening before)

A normal evening training session (e.g. 60–90 minutes at an easy or moderate pace)

Low-carbohydrate dinner: vegetables, fish or meat, salad, pulses – no pasta, no rice, no bread

In the morning

Optional – low-carb breakfast (scrambled eggs, avocado, quark, nuts). Coffee is allowed and even provides an extra boost to fat burning. Alternatively: fast completely, if you can tolerate it

The Unit

Take it easy! Zone 1–2, base pace

In short: 45–60 minutes is more than enough

Moderate cadence, no pressure, no climbs at full throttle

After that

Eat promptly – within 30 to 60 minutes

Now, choose your carbohydrates wisely: muesli with fruit, bread with honey, porridge

Plus protein: yoghurt, eggs, protein shake

Drink plenty of fluids

Frequency: No more than once a week, not on consecutive days.

Conclusion

Training on an empty stomach is not a miracle cure, nor is it a substitute for regular training with an adequate energy supply. In the short term, it may lead to reduced performance and cause blood sugar levels to drop. Caution is advised due to the risk of energy or nutrient deficiency. Those who use it during light, basic training may hope to see some progress – and are free to give it a go. However, anyone who undertakes long rides without carbohydrates, cuts out this energy source unexpectedly, or does so very frequently, is actually putting themselves at risk.