Guash and Nattel argue that a ‘safety threshold’ exists for endurance training, above which the risk of developing pathological cardiovascular conditions increases (Guash & Nattel, 2013). We make five points in rebuttal:
- 1Epidemiological data in more than 50,000 Swedes who completed a 90 km cross-country ski race showed a 52% decrease in overall mortality and a 57% reduction in cardiovascular mortality (Farahmand et al. 2003). More protection was seen in individuals who completed more than one race suggesting that exercise-related cardiovascular benefits extend to extreme endurance exercisers. The uniformity of the Swedish medical care and records system makes these data convincing.
- 2Intense endurance exercise is associated with increased risk of arrhythmias especially atrial fibrillation (AF), but the Swedish cohort suggests this risk is more modest than previously suspected (Andersen et al. 2013). Importantly, malignant ventricular arrhythmias were low in the skiers. Additionally, lone AF has limited health implications and veteran athletes with arrhythmias still have higher heart rate variability which is protective (Jensen-Urstad et al. 1998).
- 3It is unknown if extreme endurance exercise evokes increased coronary calcification. It is known that high cardiorespiratory fitness modulates the increased risk of coronary calcification (LaMonte et al. 2006). Additionally, the coronary vessels of ultramarathon runners are larger than those of controls and vasodilate more (Haskell et al. 1993).
- 4Reports on small numbers of master athletes demonstrating ‘cardiac fibrosis’ are countered by studies showing enhanced contractile responses (Seals et al. 1994). There is far too little data on this topic to make meaningful conclusions based on observations reported in small numbers of subjects.
- 5Caution is needed when extrapolating provocative data from animal studies to humans. Use of aversive stimulation (tail shock) in rats running at ∼90% of maximum heart rate for 1 h day-1, 5 days week-1 over 16 weeks, is equivalent to the same loads over ∼10 years when translated to humans (Ruiz et al. 2011). Such continuous intense exercise is unrealistic for most humans, including cyclists participating in the Tour de France, where mean cycling time spent at ‘rat intensities’ is below 1 h day-1 (Lucia et al. 2003). Participation in the Tour is in fact associated with increased lifespan (Sanchis-Gomar et al. 2011). Likewise, professional endurance athletes usually live longer and have lower cardiac risk that non-athletes, which is not consistent with the view that intense exercise above a ‘safety threshold leads’ to chronic cardiac damage that limits the longevity benefits of exercise.