Physical exercise and sport are key to preventing and treating many so-called "diseases of civilization". However, the upper limit of health-promoting exercise has not yet been clearly defined.  Scientists from the Swedish School of Sport & Health Science (GIH) and the Karolinska Institute investigated the effects of increasing training intensity on test subjects over a period of four weeks.

Following the week of highest training load, there was a significant decrease in mitochondrial function and metabolic capacity. The results suggest that "overtraining" can reverse the positive effects of exercise stress. 

The study results were published in the journal Cell Metabolism:

Published: March 18, 2021DOI:  Excessive exercise training causes mitochondria functional impairment and decreases glucose tolerance in healthy volunteers  Nilsson, LC, Tais, S., Ekblom, B., Apro, W. Larsen, FJ

You can access the article here