Too little, too much and the sweet spot in between.

Smoking is toxic. High blood pressure is the silent killer. The Mediterranean diet can add years to your life.

When it comes to heart health, there is often a black-and-white distinction between the good guys and the bad. In this cast of good and bad "actors," exercise has recently emerged as the sometimes-hero, sometimes-villain in cardiac health. Every few months, out comes a study showing that excessive exercise can take a toll on the heart muscle, spawning news headlines that proclaim runners are on a fast track to an early grave.

But what constitutes too much exercise, and is it a friend or a foe?

As preventive cardiologists and avid runners, we decided to outline some of the truths, myths and simple advice about exercise and heart health.

Exercise is good!

The evidence is unequivocal – a sedentary lifestyle has been linked to higher risk for heart attacks, strokes, heart failure and diabetes. Studies have shown again and again that moderate, regular exercise reduces cardiovascular risk and improves overall health.

The evidence of the beneficial effects of exercise is robust, and current guidelines from organizations such as the American Heart Association and American College of Cardiology recommend at least 150 minutes of aerobic activity at moderate intensity per week. More is better, and no upper threshold of benefit has been set.

Exercise can boost heart health by reducing body fat, improving blood pressure and glucose, lessening psychological stress and fueling the production of heart-protective HDL cholesterol while lowering artery-clogging triglycerides and bad LDL cholesterol.

But is more always better?

Runners will tell you they are often warned by concerned non-runners that too much running might be bad for them. It can ruin your knees! (Not really.) And, as of late, potentially damage your heart. After all, the warning goes, haven't you heard of Jim Fixx?

Fixx, the author of the 1977 best-seller "The Complete Book of Running," died while jogging at the age of 52. An autopsy showed he had severe coronary artery disease.

But such well-meant warnings often lack context. Fixx's father had severe heart disease, suffering his first attack at age 35 and a fatal one at age 43. Fixx outlived his father by nearly a decade, despite the fact that before taking up running at age 36, he was overweight and a heavy smoker with a stressful job and two divorces, all of which we now know fuel cardiac damage.

Fixx died in 1984, a time when many of today's life-saving and life-extending treatments like cholesterol-lowering statins were barely used.

Concerned non-runners also point to the ancient story of Pheidippides, the Greek messenger who ran 150 miles in two days to tell Spartans that Persians had landed in Marathon. He then ran 25 more miles from Marathon to Athens to announce the victory over Persia in the Battle of Marathon in 490 B.C. "We have won!" he proclaimed, then collapsed and died. Was Pheidippides the first marathon victim? Or was he another romantic invention of the ancient Greeks? Historians continue to debate the historical veracity of this account.

As powerful as those examples are, isolated anecdotes don't constitute scientific evidence.

But recently, running suffered another blow to its image when a study published a few months ago in the Journal of American College of Cardiology showed those who engage in excessive training have death rates similar to those who are physically inactive. The research generated outlandish news headlines like, "Fast running is as dangerous as sitting on the couch."

But let's take a closer look at the findings. The study compared mortality rates between non-joggers and joggers and further separated runners into light, moderate and strenuous groups, based on how fast, how far and how often they ran.

Light and moderate runners had lower death rates than the couch potatoes. Strenuous runners – those who ran at speeds higher than 7 miles per hour or more than four hours a week – tended to die at a higher rate than light and moderate joggers.

But the findings were far from conclusive. They merely showed a trend. As researchers themselves pointed out, the strenuous runners were too few, and there were only two deaths in this group to draw any definitive conclusions.

Yet absence of definitive proof that too much running is dangerous is not definitive proof that it isn't. There are valid concerns about excessive training. We know for a fact that regular running over time causes the heart muscle to enlarge. This is an adaptive change that allows it to sustain the increased workload. These changes seen in "athletes' hearts" can include mild enlargement, or hypertrophy, of the left lower part of the heart, the organ's main pumping chamber. We also know that all four chambers of the heart get somewhat bigger than normal to allow the heart to pump more blood. While these changes are generally benign and reversible, in a handful of individuals they may lead to abnormal heart enlargement and a weakened heart muscle, a condition known as dilated cardiomyopathy.

There are several studies suggesting that endurance training may, over several decades, fuel other negative changes, such as scarring or fibrosis of the heart muscle, increased calcification or hardening of the arteries, and a higher propensity to developing abnormal heart rhythms. However, many of these studies were small and show a link but no definitive cause-effect relationship between excessive training and cardiac abnormalities.

Endurance Training's Bad Rap

Putting in the time to train for and compete in an endurance event, like a marathon, can provide tremendous psychological gain, foster a sense of accomplishment and encourage social support through a shared community of sport. After running the Marine Corps Marathon in 1994, Oprah Winfrey famously proclaimed: "Running is the greatest metaphor for life, because you get out of it what you put into it." In the United States, since 1990, there has been a 140 percent uptick in marathon finishers. The number of half-marathon finishers has quadrupled. However, running 13.1 or 26.2 miles for a half or full marathon is certainly a grueling event that places acute stress on the heart. In a small number of people with pre-existing and undiagnosed heart disease, this may turn out to be the proverbial straw breaking the camel's back.

Reassuringly, the dangers of cardiac arrest during an endurance event are very low. The 2012 RACER study published in The New England Journal of Medicine analyzed outcomes among nearly 11 million marathon and half-marathon runners and found that cardiac arrests occurred in less than 1 percent of every 100,000 runners. When these tragic events do occur in younger people – those under 40 – the most common cause is congenital or inherited forms of heart disease, mainly hypertrophic cardiomyopathy, an abnormal thickening of the heart muscle that can cause fatal rhythm disturbances. In those over 40, sudden cardiac death during an endurance race most often stems from garden-variety coronary artery disease, a condition in which the arteries get hardened and clogged by fatty calcified deposits – the most common cause of heart attacks and strokes.

So is too much exercise going to be bad for your heart in the long run?

It's important to point out that we don't know what exactly constitutes "too much" exercise, and that it likely varies from one person to the next. Some people may be more vulnerable to the long-term effects of endurance training, but for the vast majority of healthy runners, endurance training appears to be safe.

In fact, adding to the minimum recommended amount of exercise can have important benefits. A study published in JAMA Internal Medicine showed the highest survival and lowest death rates among people engaging in three to five times the recommended minimum of physical activity. Importantly, the researchers found no harm for those choosing to do 10 or more times the minimum amount.

A study published this year in the Journal of American College of Cardiology found no evidence that people with the highest exercise capacity were more likely to die. Quite the opposite – it showed that those at higher levels of fitness were the least likely to die.

The truth is that most Americans need to worry about exercising too little and not too much.

Less than half of U.S. adults meet the current recommendations for physical activity. The good news for those not interested in endurance sports is that they can reap health benefits at lower levels of exercise intensity and frequency.

So is endurance training right for you?

The right exercise regimen should be tailored to the individual. Talk to your doctor about cardiovascular risk factors, and get a health check-up before participating in an endurance event.

Currently, there are no recommendations in the United States to screen athletes for heart disease with electrocardiograms or echocardiograms.

But the American Heart Association does advise clinicians caring for athletes – professional or amateur – to perform a targeted personal history evaluation that includes the following:

Family history of premature heart disease
Symptoms that may indicate heart disease – chest pain or discomfort, shortness of breath
Physical exam that includes blood pressure check and listening to the heart with a stethoscope
And keep in mind that while running can improve cardiovascular health, it does not make one immune to underlying heart problems. Athletes should listen to their bodies and discuss with their doctors any concerning symptoms, like chest pain with exercise, any new excessive fatigue or shortness of breath that is brought on by exertion.

Regular physical activity confers many health benefits, including longer life expectancy, but as with everything else in life, moderation is key. Listen to your body, talk to your doctor and challenge yourself to exercise just a little more, and soon you'll find your own personal sweet spot.

And don't forget that exercise is just one component of a heart-healthy lifestyle. Running a few times a week is no panacea and not a license to overindulge elsewhere.

Disclosure: The authors would like to declare that they are exercise-holics and continue to train for endurance events. Dr. Michos is a marathon runner (albeit a relatively slow one), and has completed 24 marathons in 20 states and three ultramarathons. Dr. Konstantinidis has run marathons and completed duathlon races. He is currently preparing for an Olympic triathlon race.

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