Exercise is a stressor to the body meaning that exercise follows the inverted U-shaped affect; exercise is beneficial at the right dosage, however, too much exercise can become detrimental.
Over training leads to loss of libido and sex drive. This is likely a result of consistently elevated levels of cortisol and REDUCED levels of testosterone.
Among females, loss of period is a common side effect which can also result from constantly high levels of cortisol, but also due to excess inflammation from lack of recovery and inadequate nutrition.
“Tired but wired” is another common side effect of overtraining, which can also be traced to dysregulation of cortisol. Frequent wakeups throughout the night are likely to result from fluctuations in blood glucose which are characteristic of the overtrained state. Furthermore, disruption of Growth Hormone during sleep could also occur, as changes to GH release have been shown in studies of overtrained individuals.
Fat retention is likely to occur as T3 production and metabolic rate will go way down. This is an adaptive mechanism of the body in an attempt to hang onto extra calories and energy so that it can survive the constant stress and wear & tear from excessive exercise.
The excess inflammation created is likely to create gut dysbiosis and possible structural damage. For instance, some marathon runners experience gut permeability at the end of a race as a result of the extreme activity level. This would further inflammation in the body as bacteria in the gut escapes through the usual tight junctions of the gut. Furthermore, production of lipopolysaccharides (LPS) can occur during periods of intense training; this is the outer membrane of a specific type of bacteria, which is one of the most proinflammatory agents known to man.
This inflammation alongside some of the other mechanisms identified can also lead to anxiety, brain fog, and even symptoms of depression. In fact, excess cytokines produced by overtraining can get into the brain and trigger changes strikingly similar to (some cases) of depression. Which is the diversion of tryptophan (precursor of serotonin) down the NEUROTOXIC kynurenine pathway, leading to production of neurotoxic metabolites like quinolinic acid and disrupting other neurotransmitter systems.
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Savitz, Current Topics in Behavioural Neuroscience, 2017
Karhu et al., European Journal of Applied Physiology, 2017
Kreher and Shwartz, Sports Health, 2012