Sick but still training? A risky game with your health

Are you feeling ‘a bit under the weather’ but don’t want to lose your rhythm? Be careful: Even light exercise whilst you have an infection can weaken your immune system – and, in the worst case, put strain on your heart. In this article, you’ll find out what really happens in your body, how to recognise the warning signs, and why taking a break is sometimes the best way to make progress.

Infections – whether a flu-like illness, a gastrointestinal infection or COVID – are nothing more than the body’s way of sounding the alarm. The immune system works flat out to fight off intruders such as viruses or bacteria. In the process, pro-inflammatory messengers are released, a fever may develop, and energy is mobilised – but not for performance, rather for defence. Anyone who trains during this phase is subjecting their body to additional strain, which can be dangerous.

Particularly treacherous: You might feel only slightly run down or a bit under the weather – yet internally, the body is in full crisis mode. And that is not compatible with exercise that raises the heart rate or demands muscular effort.

So-called myocarditis, i.e. inflammation of the heart muscle, is one of the most feared complications following viral infections. Viruses such as Coxsackie viruses or influenza viruses can attack the heart directly – particularly if you do not take it easy. Untreated myocarditis often goes undetected, but can lead in the long term to cardiac arrhythmias, reduced cardiac function or even sudden cardiac death.

Typical symptoms of myocarditis:

• • Persistent tiredness and a feeling of weakness
• • Heart palpitations or a racing heart
• • A feeling of pressure in the chest
• • Shortness of breath with minimal exertion

A sports medicine screening is essential if myocarditis is suspected.

Gastrointestinal infections such as stomach flu are particularly insidious. Diarrhoea, vomiting and fluid loss can quickly lead to an electrolyte imbalance, which can impair the functioning of the heart, kidneys and muscles. Anyone who resumes exercise too soon risks circulatory problems, muscle cramps or even collapse.

The same rule applies here: you should only consider training once your digestive system is fully functional again and your body and energy levels have stabilised. You should ease back into training gradually – ideally whilst paying close attention to how your body is responding.

There are a few simple but important rules for assessing whether your body is ready for exercise.

Symptoms that mean you shouldn’t exercise:

• • Fever
• • Aching limbs
• • Sore throat with difficulty swallowing
• • Exhaustion
• • Sore throat with difficulty swallowing
• • Diarrhoea and/or nausea, vomiting

Also exercise caution if you have:

• • A dry cough or shortness of breath
• • A resting heart rate significantly above normal
• • Unusually high resting blood pressure

If, on the other hand, you only have a mild cold without any general symptoms, a short walk or some light stretching may be acceptable – but avoid exercise that raises your heart rate.

During an infection, the immune system works at full capacity. This leads to a rise in inflammatory markers, altered cytokine release and, often, a temporary decline in physical performance. The body prioritises defence – not muscle building, fat burning or improving VO2max.

After the infection, the body is initially weakened, as energy reserves are depleted and repair processes are underway. During this time, it is crucial to get enough sleep, eat a high-quality diet (particularly protein, vitamin C, zinc and omega-3 fatty acids) and reduce stress.

Even after the symptoms have subsided, the body is still in recovery mode. The rule of thumb from sports medicine: 48 hours completely symptom-free before resuming exercise – and only at a moderate level:

• • Walks in the fresh air
• • Light mobility exercises
• • Short yoga or breathing sessions

Only once these are well tolerated can endurance training be slowly resumed. High-intensity strength training should be avoided for up to a week after full recovery.

Many ambitious athletes worry about losing muscle mass or endurance if they take a break. However, this concern is usually unfounded. Studies show that after a 7–14-day break, the loss is minimal – and is usually made up for after just a few training sessions. It is more important not to risk any long-term damage that could lead to months of inactivity.

Tip: Make the most of this period for recovery, sleep, mindful eating and mental relaxation. If you return with a clear head and a strengthened body, you’ll train more effectively and sustainably.

Exercise is only healthy if it is in tune with your body’s signals. Anyone who trains whilst suffering from an infection risks inflammation, heart damage and long periods of inactivity. Taking a proper break, making a sensible return to training and trusting your own body are signs of maturity – not weakness.

So the most important rule is: Get well first. Then train.