Considerable data exist that allow reasonable estimates of the volume of exercise required to 1) decrease cardiovascular disease (CVD) risk 2) decrease the risk of heart failure.
There are considerable coarse grain epidemiologic data that support the notion that regular aerobic exercise will decrease heart attack risk,reduce heart failure risk and reduce all-cause mortality.
The 2018 Physical activity guidleines for Americans recommended at a minimum of approximately 8 metabolic-equivalent hours per week ( 8 met-hours or 500 met minutes per week.)
The recommendations translate to 150 minutes of moderate exercise (less than 7 Mets) or 75 minutes of vigorous exercise per week.
Arem et al ( 1) pooled data from 6 studies (661,137 men and women) and demonstrated a 20% reduction in mortality among those who exercised at the recommended level with an HR of 0.8 (CI 0.78-0.82) and a 37% lower risk for those who exercised at 2-3 times the minimum level and the maximal benefit at 3-5 times the minimum ( 0.61, CI 0.59-0,62)
Further no excess risk was evident even at exercise levels of 10 times the minimum.
Wen et al (2) published all cause mortality data similar to Arem and also found that the maximum CVD mortality risk reduction (45%) ( HR 0.55 CI 0.46-0.66) at an exercise level of 523 minutes per week, a level roughly 3.5 times the minimum recommendation.
These 2 articles are part of the evidence regarding the safety of exercise levels higher than the 2018 recommendation. Drs Carl Lavie and James O'keefe had earlier spoken ( TED) and warned about the purported hazards of long bouts or too many years of aerobic exercise. In 2015, perhaps in part reaction to the Arem article and commentary by Dr. Ben Levine, Lavie and O'Keefe toned down their emphasis on the risk of long distant running.See here for a Runner's World article concerning that issue.
The conclusion to the Arem publication is refreshingly non-ambiguous and prescriptive ,quoting
"In regard to mortality health care professionals should encourage inactive adults to perform leisure time physical time activity and do not need to discourage adults who already participate in high-activity levels."
Coarse grain epidemiological evidence strongly support the generally accepted notion that people who exercise none or very little have the highest risk and those who exercise the most have the lowest risk.
In a nutshell the large coarse grain epidemiologic studies demonstrated that relativity low levels of exercise are better than none and further these studies were not able to demonstrate a upper level of exercise that is more risky than no exercise though some data indicted that risk reduction at the highest level studied was perhaps slightly less protective than the next highest level .
It is possible that there could be some harmful or potentially harmful cardiac effects in a small numbers of long time and/or high volume endurance athletes that would not be detected by large data analysis such as that of Arem and Wen.
That appears to be the case.
There are three conditions that appear to be more common in high level exercisers. These are 1) atrial fibrillation 2) myocardial fibrosis and 3)coronary calcification.
While the current consensus view is that the relationship between exercise level and all-cause and cardiovascular mortality is curvilinear and not U shaped, the same cannot be said in regard to atrial fibrillation (AF) .
In a 2018 review (4) of the "extreme exercise hypothesis", which is basically that there is some level of exercise achieved by some endurance athletes that results in a harmful effect. Three conditions have been considered in that regard: 1) atrial fibrillation (AF), 2)myocardial fibrosis (detected by MR gadolenium scaning) and 3) acceleration of coronary artery calcification (CAC).
Eijsvogels,Thompson and Franklin (4) concluded in regard to AF :
"that the relationship between physical activity and incident AF is best summarized by a reverse J shaped curve.Light to moderate amounts of exercise decrease but large volumes of exercise potentially increasing the risk of AF."
Two of the studies influencing their conclusion are the Henry Ford study and Anderson's cross country ski study .
Even though a study from the Henry Ford Exercise testing project (5) demonstrated that higher cardiorespiratory fitness was associated with a graded reduction in AF (the higher the fitness level the lower the risk of AF),a large cohort study of cross-country skiers showed that those who finished more races and those who raced faster had higher risk of AF than those who did only one race and those who raced at a slower pace. In that study by Anderson (6) of 52,755 long distance skiers those who finished five or more races versus those who finished only one race had a Hazard ratio for AF of 1.29 95% CI 1.04-1.61.
The Henry Ford study looked at the relationship between fitness and incident AF and the Anderson study looked at volume of exercise and intensity of exercise and while fitness and exercise level are correlated they are not the same. After the entry into the study when fitness level was determined there is no data on the exercise history of the participants at Henry Ford.The Anderson study used number of races and speed of racing to give some broad measure of amount exercise which relates to the questions of exercise volume versus AF risk which is not directly addressed by the Henry Ford study.
The large mega data studies,such as Arem and Wen have published, lack the statistical power to detect any mortality effect that might occur from AF,CAC and myocardial fibrosis in endurance athletes whose exercise volumes fall at the right end of the volume axis.
Even though the relationship between exercise volume and/or intensity and CVD and all cause mortality is not U shaped, the relationship between exercise volume and AF is.
1) Arem H et al. Leisure time activity and mortality: a detailed pooled analysis of the dose-response relationship. JAMA Int Med 2015 Jun 175 (6) 959 (full text is on line)
2)Wen,CP Minimum amount of physical activity for reduced mortality and extended life expectancy .A prospective cohort study. Lancet 2011,378 , 144.
3)Franklin, BA Exercise related acute cardiovascular events and potentially deleterious adaptations
following long term exercise training. Placing the risks into perspective-An update A scientific statement from the American Heart Association. Circ 2020 Feb 26 PMID 32100573
4) Eijsvogels, TMH et al The "extreme exercise hypothesis":Recent findings and cardiovascular health implications." Curr Treat Opions cardio med 2018 20 84
5)Querishi,WT Cardiorespiratory fitness and risk of incident atrial fibrillation:results from the Henry Ford Exercise (FIT) project Circ 2015: 131 ;1827
6) Anderson K et al Risk of arrhythmias in 52, 755 long distance cross country skiers: a cohort study
Eur Heart J 2013 Dec 34 (47)36
addendum 7/27/2020 Several changes made in the et to clarify meaning .