Tomatoes are good for you, right? They are filled with antioxidants, good for the heart, and associated with decreased risk of cancer. Likewise, exercise is good for you. It improves circulatory, respiratory, and psychological health. But you wouldn’t be in good health if all you ate were tomatoes, and the same goes for exercise. As with everything in life, moderation is key.

We are bombarded by statistics about how children today do not get enough exercise, and that everyone is becoming increasingly obese. It is fair to say that the majority of us probably don’t get enough exercise. However, these staggering statistics can cloud the problems occurring at the other end of the spectrum: over-exercise.

Although an odd concept in our lazy society, over-exercising is a real problem with devastating effects. As thousands prepare for the 31st London marathon this Sunday, it is something to bear in mind.

Of course, it’s nearly impossible to clearly draw a line between a healthy amount of exercise and too much. More than the government guidelines of 30 minutes 5 days a week is not necessarily dangerous. Nevertheless, if you find your exercise is taking over the way in which you lead your life, then it is possible you are at risk of developing into an over-exerciser.

Compulsive exercise (also called anorexia athletica) is when an individual no longer chooses to exercise, but feels compelled to do so. They struggle with guilt and anxiety if they cannot exercise and will go to extreme lengths to ensure they do. Exercise takes priority over everything else, and can result in social withdrawal.

There are many reasons people choose to exercise in the first place. But it can get out of hand and eventually result in compulsive exercise. In anorexia nervosa, individuals excessively workout as a means to control their weight, and this can gradually reach extremes levels.  Distorted body image, body dysmorphia, is another common component of an exercising disorder and a growing problem in both men and women. Whether it is aspiring to burn off calories to look like a size 0 model, or to attain an overly muscular physique, society today is increasingly obsessed with “physical perfection.”

We envy our ultra fit colleague, who jogs everywhere, spends lunch in the gym and attends every fitness class going. However, rarely do we consider the possibility of their potential to over-exercise. Are they in fact suffering from a disorder which is compelling them to exercise, whatever the costs?

With adequate nutrition and rest, overtraining is avoided and the body adapts to exercising by increasing muscle strength, endurance, bone density and toughening connective tissue. However, taking exercise too far and ignoring your body, can lead to damaged cartilage, ligaments and bones and can cause women’s periods to stop.

The coalition government recently announced it had dropped the Labour government’s commitment to increasing physical exercise and sports participation among young people. When London won the Olympic bid, Labour pledged to use the power of the Games to inspire one million adults to play more sport (three or more times a week). This causes us to ask, would such a target really encourage the right people to exercise? It is likely that those at risk of over-exercise will be inspired by the Olympics, rather than those most in need of physical activity.

The demanding training schedules of student athletes may, in fact, cause over-exercise. Pressure from peers, coaches, and parents, as well as the athlete’s motivation, can compel them to go too far in a quest to be the best. They begin to believe that just one more training session will make the difference between first and second place, and this accelerates to dangerous amounts.

On Sunday, as you sit down to watch in awe as the runners complete the 26.2 miles (42.2km) round the streets of London, think about the bodies of these athletes. Not only are the super-runners physically and mentally fit, they have trained an incredible amount for this day. They prepare everything from the previous months’ training regimes, to the clothes they wear, and the foods they eat. If you are running a marathon, you need to take it seriously.

The world record holder, Ethiopian Haile Gebrselassie, completed the 2008 Berlin Marathon in 2:03:59. He was running an average of 12.7 miles per hour, a speed most would struggle to keep up for 100m! During an average week, Gebrselassie’s clocks up 160 miles in training, in order to produce a body capable to win a marathon.

Despite the dangers of compulsive exercise, it is by no means an excuse to avoid it. Be inspired and put on your running shoes- The key is in the balance.

Sarah Welsh is a Clegg scholar, BMJ