Excess Duration, Not Just Intensity, Could Put You at CV Risl

Wed, 10 Oct. 2012 - 1:27 a.m. MT
Credit: ARA Staff - American Running Association

Regular exercise improves cardiovascular (CV) health, and now we’ve seen fairly dramatic numbers directly reflecting increased longevity from the Copenhagen City Heart Study (see Even 30 Minutes Twice a Week Can Lengthen Your Life, in this issue).  
Some of the well-established benefits of exercise training include: decreased blood viscosity, increased delivery of blood to tissues, aided breakdown of clot-causing fibrin in the circulatory system, increased HDL cholesterol blood levels, reduced LDL cholesterol levels and triglycerides in the blood, lowered blood pressure, improved insulin sensitivity and glucose levels, and reduced weight, body fat and stress.
Doctors, then, are increasingly prescribing exercise like they would a pharmacological regimen to help fight a multitude of diseases. However, long-term excessive endurance exercise may induce certain structural changes in the heart and large arteries. We’ve known this for some time, and medical professionals have given it a name: Athlete’s heart. 
New view of Athlete’s heart
This condition occurs when chronic exercise training—of a type requiring sustained elevations in cardiac work and at a level often associated with elite marathoners, triathletes, and cyclists—imposes increased demands that alter the loading conditions of the heart.
The cardiac adaptations these athletes typically develop, including enlarged left and right ventricles, wall thickness, and cardiac mass, are associated in the general population with “poor cardiac prognosis.” As long as good (indeed, superior) heart function was maintained and these athletes were able to continue on for years at their chosen sport, the medical consensus has been that Athlete’s heart is not of great concern.
Now, however, new analysis of the data suggests that Athlete’s heart might not always be benign. In June The Mayo Clinic Proceedings published a review from the Mid America Heart Institute of Saint Luke’s Hospital in Kansas City, MO, among others, that cites a good deal of accumulating information suggesting that some of the remodeling in elite athletes, e.g., change in cardiac dimensions, does not completely regress to normal levels even several years after the athlete has retired from competition and heavy endurance training. After years of repetitive injury, this process, in some people, may lead to patchy myocardial fibrosis, creating a fertile environment for atrial and ventricular arrhythmias. 
Additionally, it seems that chronic training for extreme endurance events such as marathons, long-distance bicycle races, ultra-marathons, and Ironman events, often causes more than just transient reductions in heart function (such as the amount of blood leaving your heart each time it contracts), and transient elevations of cardiac-event biomarkers. These were generally thought to return to normal within one week; they don’t always.
It’s important to note that lifelong vigorous exercisers generally have low mortality rates and excellent functional capacity. Despite this, the authors of the review rightly insist, “The hypothesis that long-term excessive endurance exercise may induce adverse CV remodeling warrants further investigation to identify at-risk individuals and formulate physical fitness regimens for conferring optimal CV health and longevity.”
Beyond rare sudden death
It’s a bit troubling that these cardiac issues are not intensity-related alone. Elite athletes do by many estimations run a fivefold increased risk of atrial fibrillation already, much of that by way of exertion, not all of it even prolonged. Duration in and of itself, of course, leads to increased intensity on top of that risk. But this is not really news about increased sudden-death risk—the data have begun to suggest we are talking about a complex physiological condition developing and sticking around even long after an athlete ceases prolonged intense activity.
The authors write, “As with any pharmacological agent, a safe upper-dose limit potentially exists [for exercise], beyond which the adverse effects (musculoskeletal trauma, metabolic derangements, CV stress, etc.) of physical [exercise training] may outweigh its benefits.” In sedentary individuals, for example, even a modest dose of physical activity (as little as 15 minutes per day) has been shown to confer substantial health benefits, which even accrue in a dose-dependent fashion. The upper limit for such benefits was found in one very large study to be an hour per day, beyond which more exercise did not seem to yield further benefits. See 30 Minutes Twice a Week for even more modest conclusions with regard to duration.
When you consider that professional endurance athletes exceed by up to tenfold the standard “recommended dose” of exercise for prevention of coronary heart disease, the data begins to make a great deal of common sense. The Consensus Guidelines for Physical Activity and
Public Health from the American Heart Association and American College of Sports Medicine call for at least two and a half hours per week of moderate exercise, or an hour and fifteen minutes per week of vigorous exercise, in the general adult population. The guidelines also caution that high-intensity exercise increases risk of musculoskeletal injuries and, as we’re reminded of anew, adverse cardiovascular events. 
Individual training load, medical history, genetics, and many other factors play roles in whether a person is at risk. There is also a big difference between the moderate exerciser and the elite competitor. Still, if you are a recreational veteran marathoner, triathlete, or ultra-runner, it’s worth having a serious conversation with your doctor to determine whether you should undergo in-depth cardiac testing for your ongoing safety.   
Mayo Clinic Proceedings, 2012, Vol. 87, No. 6, pp. 587-595, http://cardionutrition.files.wordpress.com/2012/06/too-much-exercise-can-be-a-bad-thing.pdf
(RUNNING & FITNEWS® May / June 2012 • Volume 30, Number 3)

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