Abnormal heart rhythms
A long but gentle session on the treadmill can't hurt, right? Wrong. Those who regularly engage in endurance sports are at risk of causing permanent structural changes to heart muscles which scientists describe as "cardiotoxic".
Such changes are believed to predispose athletes to arrhythmia (abnormal heart rhythms), making them more prone to sudden cardiac death. For years, a handful of clean-living sports nuts have sat smug in the knowledge that tobacco, caffeine and recreational drugs are the main causes of an irregular heart beat. But studies released by the European Heart Journal in 2013 suggest that - especially for those with a family history of irregular heartbeats - overdoing the fat-burning workout can also contribute to poor cardio health.
The study, which measured the heart rhythms of over 52,000 cross-country skiiers during a ten year period, found that the risk of arrhythmia is increased with every race completed, and was up to 30 per cent higher for those who competed year-on-year for a period of five years. Exercise intensity also affected results: those who finished fastest were at higher risk for arrhythmia.
A shaky immune system
Cortisol - a hormone emitted by the adrenal gland during periods of physical stress - stimulates gluconeogenesis (the production of new glucose) in the liver and increases protein breakdown in the muscles.
It's essentially good. Keen to benefit from its inflammatory effects, professional athletes have been injecting their wearied muscles with the stuff for years (as have office workers who suffer from persistent RSI). But scientists recently came to the conclusion that the negative effects of cortisol can outweigh the benefits.
Whilst cortisol can decrease the swelling and reddening prompted by serious injuries, its immunosuppressive effects mean that those who endure high and consistent cortisol levels are at more risk of falling ill.
One way of understanding this is in terms of the 'fight-or-flight' instinct. Levels of cortisol increase dramatically during moments of intense stress - but these moments tend to be very fleeting. You fight, or take flight, and then the body's self-limiting response system returns to normal.
However, that doesn't happen so quickly when you over train. Essentially, your body doesn't have time to recover, so it stays in (or close to) fight-or-flight mode. Your immune system pays the price.
Weakened bones
Not only are those who over-exercise more at risk of illness but they're doubly as likely to end up bed-bound thanks to cortisol's interference with bone-building. When cortisol is in the bloodstream, more bone tissue is broken down than is deposited. This means that exercise addicts, whose bodies remains in a chronic state of stress, put themselves at higher risk of fractures and breakage.
The resultant loss in bone density can lead to serious conditions such as osteoporosis and arthritis, which can haunt excessive exercisers in later life.
Mental ill-health
Pumping iron on a daily basis might be a fast-track to the baywatch body you've always craved - but relentlessly hitting the weights has proven detrimental impacts on mental health.
Studies into what is known as 'Overtraining Syndrome' show that those who over train portray the same biochemical markers as those with clinical depression - which is to say that the emission of serotonin and tryptophan are altered by both disorders. Behaviourally too, the clinically depressed and the over trained were perceived to share lowered motivation, insomnia and irritability.
Last year the Technical University of Munich found that young athletes who don't leave enough time to recover from stress and injury are 20pc more likely to suffer from depression.
Struggling to find the motivation to lug your unwilling body to the gym? It might be time to ease off the weights.
