Overtraining and the Endocrine System—Part 2: Review of the Scientific Studies

Based on the available literature, this meta-analysis will summarize the possible role of the previously examined hormones in connection with an overtraining syndrome. It will focus especially on the usefulness of single hormones as indicators for overtraining. Various sports will be considered separately in order to deal with the effects of different kinds of strain and load specifically. Studies only available as abstracts were not included. The majority of subjects examined in the course of the studies were competitive athletes with different performance levels, some studies included soldiers. Soldiers did not focus their training on certain sport competitions demanding highest performance levels in specific macrocycles and microcycles, though they exposed themselves to extreme physical strain nevertheless. Since not only the kind of sport might have different influences on the neuroendocrine system in connection with the overtraining syndrome, but also probably individual aspects of the athletes and the training performed, further explanations on the subjects as well as the respective study design including duration of the study, course of the training, and examination times have been included in the review tables. Part 1 of this publication on the terminology of overtraining revealed that there is no uniformity in the definition of the criteria for determining a case of overtraining. Therefore, the definitions used in the respective studies were also included in the tables in connection with the corresponding test procedures. The authors applied different criteria for determining an overtraining syndrome. Moreover, they conducted various load tests and further analyses such as blood and urine tests, stimulation tests, and so on, which are also listed in the respective tables. The individual studies are critically evaluated briefly in connection with the respective presentation, and a summary discussion is given separately for the various endocrine systems.