It’s a no brainer that regular exercise is great for your mental and physical health. It adds years to your life and improves sleep, stress, strength, confidence, and dozens of other vital areas of life.

But everything good in life should be done in moderation, including exercise. Despite what some coaches, trainers, and self-proclaimed weight-loss gurus might tell you, there are plateaus to the benefits of running.

Chronic Training & Cardiac Trouble

In 2012, the Mayo Clinic published an exhaustive study showing that excessive endurance exercise, particularly from regular marathons and triathlons, may be worse for some people than a sedentary lifestyle. Long-term excessive endurance and chronic training were linked to structural changes in the heart and arteries, and volume overload of the atria.

All of this makes sense if you remember the immediate effect of exercise on the body: damage. Muscles increase in size and strength due to damage caused during exercise. After a workout or a run, muscles fibers (including those in the heart) are repaired and thickened. They become larger and stronger in preparation for having to undergo the same strenuous activity in the future. Your body is seeking the path of least resistance, with the ultimate goal of survival.

As such, it is constantly adapting to the environmental conditions. A body that is more efficient at running will waste less resources running. If you are regularly lifting heavy objects and running long distances, your body will seek to eliminate anything impeding the successful (and least taxing) completion of that task.

Wedding Day Diet Fallacy

This touches on another claim made in the running and weight-loss communities: that running is the fastest way to fitness.

It’s well established that excessive amounts of cardiovascular exercise are counterproductive to hypertrophy (increases in muscle size). Muscle is heavy, so your body will cut weight by shedding muscle and fat in order to adapt to regiments of continuous long-distance running. Again, the goal is ease, and the less weight you carry, the less energy your body has to expel to meet the physical demands placed on it.

But in practice, there are limits to this theory. Running too much may force your body to protect its fat reserves against depletion. It will use muscle as a source of fuel rather than fat, essentially storing fat at the expense of muscle. The consistently unsuccessful “wedding day diet” fails for the same reason: you cannot trick your body into cutting fat by short-term starvation or continuous cardio workouts.

Your heart is also a muscle. It responds just as a bicep would when placed under continued stress. It will increase in size, harden, and continually add fibers in order to meet the conditions within which it is forced to survive. But if the demands are unrelenting, you risk long-term damage.

The Stress Factor

Another factor at work in exercise is cortisol (a stress hormone). Physical trainers will recommend workouts lasting no more than 45 minutes in duration (without supplements) due to the spike in cortisol that results in strenuous activity lasting much longer than that. Regularly heightened cortisol is not only counter-productive to muscle growth, but it boosts stress.

Know Yourself

Pushing your body to extremes on a regular basis can cause long-lasting damage, especially depending on your age, experience, and overall health.

According to the researchers of the 2012 study, running 20 or fewer miles per week at six to seven miles per hour was linked to lower all-cause mortality. Consistently running more than that was not correlated with lowering all-cause mortality.

Every body is different, and excessive amounts of cardiovascular exercise won’t erase a lifetime of bad health choices. A poor diet, excessive alcohol use or drug abuse, environmental factors, and genetics all play a role in the health, durability, and susceptibility to repeated exercise-related damage of the body.

This is why it’s recommended that you see a doctor before entering a marathon, triathlon, or Ironman competition. It’s why you should get regular checkups and heed the advice of your doctors. And it’s why you should push yourself to the extreme in moderation, not daily.