© Andrea Piacquadio
March 12, 2026
Nils Behrens
Fatigue, lack of motivation, stagnant performance: Health expert Nils Behrens explains when training becomes too much – and what a Tour de France study reveals
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"Overtraining" is one of those words that sounds like there is a clear metric for it. Like a fever. From 38.5 degrees, it's serious. The reality is less romantic: No one knows exactly, when training becomes too much. There are no reliable blood markers, no threshold, no red line on the lab report. Ask two coaches – you'll get three definitions.
What we have are symptoms: fatigue, lack of motivation, chronic heaviness in the legs. And above all one thing: You don't get better despite training.
That's the point where performance doesn't stagnate but quietly runs backwards.
Why is it so hard to grasp? Because we don't have a clean model for systematically "overtraining" people. The crucial question remains unanswered:
Is it really too much training – or simply too little recovery?
If you want to know how the human body reacts to extreme training, you need a scenario that breaks every comfort zone. The answer is not in the gym, but on French Alpine passes: the Tour de France.
Three weeks.
Over 3,400 kilometers.
More than 56,000 meters of altitude.
Daily energy consumption: 8,000 to 10,000 calories.
This is not a training camp. This is a metabolic stress test.
Researchers accompanied seven professional cyclists throughout the entire tour and took fasting blood samples from them at four time points - from start to finish. It wasn't a single value that was analyzed, but the big picture: Metabolites, that is, small molecules that show what our body is currently consuming, sacrificing, or no longer providing.
Nils Behrens is the host of the health podcast HEALTHWISE and Strategic Brand Partner at Sunday Natural. Previously, he was the face of the Lanserhof Group as Chief Marketing Officer for over 12 years and host of the successful Forever Young podcast. In more than 350 expert interviews, he explored ways to achieve a longer and fitter life.
Almost half of all measured metabolites changed - predominantly downwards.
Especially drastically in the first seven to ten days. The body first falls out of balance before it stabilizes provisionally.
Two systems stood out:
1. Fat metabolism
The riders burned fat under high load – so much so that central transport molecules like carnitine measurably decreased. Not because fat burning became worse, but because demand outpaced supply.
Interesting: Especially saturated fatty acids were depleted. Apparently the 'cheaper fuel' when efficiency is everything.
2. Antioxidant system
The strongest drop was seen in Cysteine, a key amino acid for glutathione production. Glutathione is the most important shield against oxidative stress. If cysteine decreases, it means: The body can hardly keep up with neutralizing free radicals. This is not damage – this is constant operation at the limit.
The drivers rated their exhaustion themselves on a scale of 0 to 10.
From 0.5 at the beginning to 8 at the finish. And now it gets exciting: 84 blood markers correlated significantly with this perceived fatigue – most of them negatively. The more exhausted the athletes were, the emptier their metabolic reserves became.
The strongest correlations were not found where one might expect them (lactate, cortisol), but with molecules of the Cell membrane. This suggests that fatigue is less of a muscle problem—and more an issue of cellular stability.
In other words: It's not the engine that fails first, but the wires.
We're probably not going to have a metabolomic analysis done. And that's a good thing. The most important insight from this study is not a new blood value, but an old truth:
Your subjective feeling is not a soft factor. It is a highly precise sensor.
When performance stagnates.
When motivation is lacking.
When training feels hard, even though it should be getting easier.
Then it's not a mental problem.
Then it's our body saying: I'm using more than I can replace.
Three concrete recommendations
Overtraining is not a clearly defined condition. But exhaustion is measurable – every morning when you get up. And sometimes the smartest training stimulus is the one you consciously don't set.
You can find all columns by Nils Behrens here!
