Several/a lot of/many(1) studies have shown that physical and emotional health benefits of regular exercise. However, almost all of them are harmful when taken to an extreme.
Exercise addiction is a compulsive disorder that compels people to exercise excessively. It usually starts with a desire for physical fitness and sometimes is related with eating disorders.
A healthy athlete and an exercise addict may share similar levels of training volume - the difference is in the attitude. To an exercise addict, exercise has become the priority.
Exercise addiction is caused by the euphoric feelings resulting from the release of endorphins that occurs during intense sessions of exercise. An addict has to exercise more to trigger those chemical releases.
There are several similarities between exercise and drug addiction, including physiological and brain effects responses in the area of mood, tolerance and even withdrawal.
Increased fitness obsession and decreased social activity commonly indicate exercise addiction. More symptoms are likely, such as exercising more than once a day, feeling unable to skip a day, exposing yourself to dangerous weather or situations in order to exercise, and experiencing overwhelming feelings of guilt, anxiety, or irritability when you haven't exercised for 24 to 36 hours.
Exercise is beneficial only to a point. Excessive exercise without proper recovery stresses the body, weakens/damages the immune system and increases the chance of upper respiratory infections(2). It still may result in chronic damage to connective tissues. Addicts are considerably at higher risk of osteoarthritis and stress fractures. Furthermore, women (3) can become infertile when their body fat percentage is very low.
(1) various - to me, have a ring of "variety".
(2) infections- because there are several possible agents, hence several types of infections. My opinion, though.
(3)if it´s to be in academical form, you should refer to "women" in general, you are not talking about a given woman; if so, you should use "a woman''. But to keep it impersonal, it have to "women".
Other may polish the text more. Let´s wait for more.
Sofia, your text seems like the one of a writer that knows his stuff or that made a comprehensive search before making it.
So, it was a good piece, it shows your knowledge of the matter, the mastering of the vocabulary on the subject (even though you need some study on verbs and some grammar points, but not many.)
In general, you succeed in putting your thoughts in writing form, congrats.
I would give you a "D" mark. Keep up the good work.