Potential Adverse Cardiovascular Effects
from Excessive Endurance Exercise

by O'Keefe et al, Mayo Clin Proc, June 2012

People who exercise regularly have markedly lower rates
of disability and a mean life expectancy that is 7 years
longer than that of their physcially inactive contemporaries.

However, a safe upper-dose limit potentially exists,
beyond which the adverse effects of exercise may
outweigh its benefits.

Emerging data suggest that chronic training for and
competing in extreme endurance events such as marathons,
ultramarathons, ironman distance triathlons, and very long
distance bicycle races, can cause transient acute volume
overload of the atria and right ventricle, with transient
reductions in right ventricular ejection fraction and elevations
of cardiac biomakers, all of which return to normal within 1 week.

Over month to years of repetitive injury, this process, in some
individuals, may lead to patchy myocardial fibrosis, particularly
in the atria, interventricular septum, and right ventricle, creating
a substrate for atrial and ventricular arrhythmias.

Additionally, long-term excessive sustained exercise may be
associated with coronary artery calcification, diastolic dysfunction,
and large-artery wall stiffening.

Veteran endurance athletes have been noted to have a 5-fold
increase in the prevalence of atrial fibrillation.

Exercise training:
* increases HDL,
* reduces triglyceride, obesity, and blood pressure,
* improves insulin sensitivity, glucose levels, and endothelial function
* reduces stress,
* decreases hematocrit and blood viscosity,
* increases tissue perfusion and circulatory fibrinolytic activity,
* increases coronary flow reserve, coronary collateral circulation,
   and tolerance of ischemia, etc.