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Research into the effects of exercise on mental health are numerous with many suggesting a positive effect on an individual’s mental health, including improvements in anxiety, depression and other psychological conditions (Mikkelsen, Stojanovska, Polenakovic, Bosevski, & Apostolopoulos, 2017; Skead & Rogers, 2016). Exercise can improve mental health via three main pathways; psychology, physiology and immunology (Mikkelsen, Stojanovska, Polenakovic, et al., 2017). The psychology aspect includes self-efficacy and distraction (Skead & Rogers, 2016). These refer to improvements in mental health due to the comradery associated with team sports, improved self-confidence and self-esteem as well as the distraction from daily stress brought about by exercise. The physiological pathway includes the action of endorphins, mitochondria and the hypothalamic-pituitary-adrenal axis (HPA-axis), whilst the immunology pathway includes inflammatory markers and vagal tone amongst others (Mikkelsen, Stojanovska, Polenakovic, et al., 2017). 

Chronic inflammation has been implicated in the development of anxiety, depression and other mental health disorders (Dantzer, O’Connor, Freund, Johnson, & Kelley, 2008) and there is increasing evidence demonstrating the link between the immune system and mental health. Immune dysfunction often occurs due to nutrient deficiency which alters neurotransmitter production. Furthermore, gut inflammation triggers an inflammatory response via the vagal/brain reflex which results in increased toxicity in the nervous system (Mikkelsen, Stojanovska, Prakash, & Apostolopoulos, 2017). Research highlights that the beneficial effects of exercise on mental health lie in its ability to reduce inflammation (Petersen & Pedersen, 2005). Specifically, exercise increases the level of anti-inflammatory actions amongst cytokines such as interleukin six (IL-6), a cytokine produced by muscle contractions (Pedersen & Fischer, 2007) and T-helper 2 resulting in an overall reduction in inflammation (Schindler et al., 1990). Moreover, the vagus nerve which facilitates the gut-brain connection, also regulates inflammatory responses through its interactions with the immune system (Mikkelsen, Stojanovska, Polenakovic, et al., 2017). A pro-inflammatory environment is brought about by a weak vagal tone, however high-intensity interval training can decrease the heart rate and increase vagal tone thereby reducing inflammation (Guiraud et al., 2013).

A physiological aspect called the endorphin hypothesis suggests that exercise creates high endorphin levels which assist the body to combat pain and stress. This is referred to as ‘runner’s high’ in athletes (Mikkelsen, Stojanovska, Polenakovic, et al., 2017). However, this difficult to prove since endorphin levels are measured via the blood which may vary from nervous system levels. Furthermore, it is an invasive procedure which may itself alter mood. Another physiological aspect involves the mitochondrial role of cellular homeostasis. Dysfunction of this system is said to be linked to multiple diseases including depression (Bansal & Kuhad, 2016). This occurs since mitochondria are important for brain neuroplasticity which is vital for modulating mental health. Aerobic exercise also increases the production of mitochondria thereby increasing oxygen capacity (Viña et al., 2009). The HPA-axis is a system that enables the body to adapt during stress by altering hormone release from the hypothalamus and anterior pituitary gland particularly during exercise (Droste et al., 2003). Overactivity of this system is implicated in anxiety and depression (Landgraf, Wigger, Holsboer, & Neumann, 1999).

Whilst the benefits of exercise on well-being are widely accepted, too much exercise may impact mental health negatively (Mikkelsen, Stojanovska, Polenakovic, et al., 2017). Exercising to manage weight may lead to compulsive and distressing outcomes particularly when associated with eating disorders (Dalle Grave, Calugi, & Marchesini, 2008). Individuals who become exercise-obsessed are at increased risk of exercise-induced injury, finding it difficult to curb their exercise routine due to withdrawal effects similar to those experienced in substance addiction (Li, Nie, & Ren, 2015). In conclusion, whilst exercise seems to be beneficial overall, Shakespeare’s words hold true, you can have too much of a good thing. 



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