The 2008 U.S. exercise guidelines recommend a minimum of 2.5 hours of moderate intensity exercise per week or 1.25 hours of vigorous exercise per week. This can be expressed as 500 METS Min per week or 8.3 MET hours per week. This minimum value is widely quoted but the panel also suggested that further gains could be made by increasing those exercise levels to twice those values, i.e 5 hours of moderate or 2.5 hours of vigorous exercise. Moderate intensity was defined as between 3 and 5.9 METS and vigorous as over 6 METS.For example running at a 15 min per mile pace is about 7 METS which would be the lower end of what the panel meant by vigorous.This does not address the problem of relating exercise levels to a person's exercise capacity. For example, to a person with a 02 max of 60 a fifteen minute mile is very mild exercise while the same pace for someone with a 02 max of 25 the term vigorous is appropriate.
Cardiovascular disease (CVD) risk reduction has been shown in a number of epidemiological studies for people exercising at those minimum levels and arguably at even lower levelz. Lee et al (1) showed that running at little as 5 to 10 minutes per day would significantly reduce CVD mortality.This would make a little running seem very effective while others (Pandy et al see below) did not show that degree of risk reduction with that low level of exercise but the minimal recommended level will likely reduce CVD risk according to several epidemiologic studies.
Eijsvogels (2) reported that the maximal risk reduction was found at a volume of 41 MET-hrs/week, which is 3-4 times the minimal recommended level.However, only 3.5% of the subjects exercised at that level or above and therefor the confidence interval for hazard ratio (HR) estimation was wide and not statistically significant. The authors emphasized that they found no evidence of harm or adverse cardiovascular outcomes at this level.
While rather low levels of exercise seem to decease CVD risk, heart failure risk may require higher levels of exercise. Pandy et al (3) and coworkers from Southwestern Medical School reported that heart failure (HF) risk reduction occurred only at levels higher than the minimum guideline recommended values. "We observed a linear dose response for HF risk with a marked reduction in risk at very high doses of PA (physical activity ) ( 35% risk reduction in HF risk at 2000 MET-min per week)".. This would be 4 times the minimal recommended value or 10 hours of moderate or 5 hours of vigorous exercise per week.
Schnohr et al (4) reported that their analysis of a data set from the Copenhagen City Heart Study demonstrated a U shaped association between all cause mortality and dose of jogging. A number of other articles cited by Schnohr actually report a inverse relationship showing no U or J shape. Accepting the thesis of the existence of a U shape curve, he goes to comment on results from several large studies and speculates where the curve might ascend.The studies found that running about 35 miles per week was the upper limit of incremental health benefits and "these studies found that a weekly cumulative dose of approximately 30 miles of running per week or 46 miles of walking is approximately the "safe" ( my quotes) upper limit for optimizing long term CV health and life expectancy", So if the curve curves up it does not seem to do so at running volumes in the 30 miles per week range. This volume of running would satisfy the running volume "requirement" sufficient to decrease HF risk. suggested by the work of Pandy.
So the epidemiological data would tend to confirm that the 2008 minimum guidelines would decrease CVD and all cause mortality but not the risk of heart failure which requires at least twice that level of exercise.
Several publications from a group in Dallas (5) provide useful insights regarding the mechanism by which a higher level of exercise would lessen heart failure risk, that is diastolic heart failure ( i.e heart failure with preserved resting ejection fraction frequently abbreviated as HFpEF). Dr Paul Bhella and his associates did extensive physiological studies on four groups of healthy volunteers over the age of 64. Four groups were designated on the basis of their exercise history for the preceding 25 years. (not a typo) 1) sedentary-no more than one exercise session per week 2) casual exerciser-2-3 session per week 3) "committed" exercisers-4-5 session per week and 4)competitive master level athletes -6-7 session per week and competed regularly. All had normal systolic function ( as defined by a normal resting ejection fraction) but groups one and two has decreased left ventricular compliance while the committed and competitive groups had left ventricular pressure volume curves and left ventricular masses similar to young healthy controls. ( see here for my further comments and a few caveats regarding this paper including reference to Tanaka's work that challenges the notion that long term endurance exercise can preserve ventricular compliance)
Quoting Bhella ". . at least 30 minutes of dynamic exercise per session for 4-5 days per week over a lifetime can sufficiently prevent most of the decreases in LV compliance and distensibility observed with sedentary aging" So the idea is that sedentary aging leads to stiff left ventricle and life long running may mitigate that process.
In other words 2.5 hours of vigorous exercise per week which was the higher level of exercise ( i.e. twice the minimum recommenced by the 2008 panel) might beneficially reduce the age related increase in ventricular stiffness. It does not take that much exercise per week but you have to put in a lot of weeks- remember Bhella's groups activity levels were for the 25 years before the testing. The authors speak of Lifelong exercise.
This exercise level is far below the typical exercise histories that one typically finds in the cases of athletes with atrial fibrillation or the athletes in whom an abnormal gadolinium uptake was reported and whose cases are sometimes emphasized by various writers warning the public about the dangers of excessive exercise. 2 to 3 times the 2008 Panel's minimal recommendation should not be considered excessive.
1)Lee, D Leisure-time Running Reduces All-cause and cardiovascular mortality risk. JACC 64. 472-481.2014
2) Eijsvogels, T, Exercise at the extremes-The amount of exercise to reduce cardiovascular event
JACC, 67, 316-329, 2016
3)Pandey,A Dose-Response Relationship between Physical Activit and Risk of Heart Failure.A meta-Analysis, Circulation 2015, 132 1786-1794
4) Schnohr,P Dose of Jogging and Long term Mortality. JACC, 65, 311-410,2015
5)Bhella, P Impact of Lifelong Exercise "dose" on left ventricular compliance and Distensibility.
JACC 64, 1257-1267 2014
John VonNeumann " There's no sense in being precise when you don't even know what your're talking about"
addendum: 5/22/17 Several editorial flourishes made and spelling errors corrected.