The Cooper Institute

Founded in 1970 by the "Father of Aerobics"
Kenneth H. Cooper MD, MPH


How Much of a Good Thing is Too Much?

Posted in
Live well

Thursday, Jun 25, 2015

It’s pretty much accepted far and wide that regular aerobic exercise is good for you. From the earliest studies by Drs. Jeremy Morris and Ralph Paffenbarger in the 1950s up to the present time, there have been thousands of publications documenting the beneficial effects of regular aerobic exercise on health and well-being. A partial list of these benefits (from our Personal Training education course) can be found in Table 1.

Current public health guidelines for aerobic activity recommend a minimum of 150 minutes per week at a moderate intensity, or a minimum of 75 minutes per week at a vigorous intensity. Heart rate during exercise is used to differentiate moderate versus vigorous intensity. There is an established dose:response relationship between the amount of aerobic exercise and potential health benefits. Therefore, these same guidelines tell us that up to 300 minutes per week at a moderate intensity, or 150 minutes per week at a vigorous intensity is likely to yield greater health benefits than just meeting the minimal guidelines. While very few people would disagree with these guidelines, some important questions arise: 1) if doing 150 minutes per week is much better than doing nothing at all; and doing 300 minutes is better than doing 150 minutes, does this relationship continue beyond 300 minutes per week?   2) What about ultra-endurance athletes who are spending ~900 minutes (15 hours) or more per week doing aerobic training; with some of it being quite vigorous?

To help answer these important questions, O’Keefe and colleagues published a review article on the potential adverse cardiac effects of excessive endurance training and competition.1 First, they note that sudden cardiac death (SCD) during marathon running is very rare, with 1 event per 100,000 participants. SCD during triathlons is twice that of marathons (2 events per 100,000 participants), with most events taking place during the swim portion. In younger people who experience SCD during endurance events, the most common cause is a genetic defect in heart structure or function (example: cardiomyopathy). In older people who experience SCD during endurance events, the most common cause is coronary heart disease (blockages in the arteries of the heart), resulting in heart attack.

The next portion of the O’Keefe paper focused on possible mechanisms by which excessive endurance training might be potentially harmful to the heart. They noted that various markers of cardiac damage appear in the blood of ~50% of participants during and following a marathon race. However, these changes are temporary; with values typically reverting back to normal within a few days. The clinical significance of the temporary increase in these markers of cardiac damage is unknown, and some argue that this might even be a normal response to long-term endurance training. Indeed, following a strength training session, markers of skeletal muscle damage appear in the blood temporarily; with values typically reverting back to normal within 48-72 hours. As long as the skeletal muscle damage is not extensive and does not cause severe muscle soreness, this is recognized as a normal, benign, and beneficial response to strength training.

O’Keefe also cites studies where adverse structural changes in the heart have occurred with excessive endurance training, noting that ‘some individuals may be prone to the development of chronic structural changes…that might predispose to serious arrhythmias such as atrial fibrillation and/or ventricular arrhythmias…long-term training for and competing in extreme endurance exercise may lead to myocardial fibrosis and remodeling in a small subgroup.’

It is actually very well-documented that atrial fibrillation (a type of irregular heartbeat) is ~5 times more common in endurance athletes than in the general population. I can personally attest to this. During my competitive running days (1972-1986) I put in nearly 35,000 miles, and have stationary cycled well over 125,000 miles on a Schwinn Air-Dyne since that time. I’ve had brief bouts of atrial fibrillation a few times a year for the past 20 years. While it is annoying, my condition is benign. I cannot prove cause and effect between my many years of endurance training/competing and my atrial fibrillation, but I am reasonably sure that the former contributed in some way to the latter.

Putting Things in Perspective

In closing, a few points need to be strongly emphasized.

1.  There is strong agreement among health professionals that for the overwhelming majority of the population, the benefits of regular aerobic exercise greatly outweigh any potential risk. 2.  The number of Americans who do not meet the minimal public health guidelines for aerobic exercise is exponentially greater than the number of Americans who might be doing too much aerobic exercise. While significantly exceeding the upper end of the public health guidelines is obviously necessary to compete at a high level in endurance events, it does not seem to result in further health benefits as compared to staying within the guidelines. As I have heard Dr. Kenneth Cooper state many times over the past two decades, “If you’re running more than 15-20 miles a week, you’re doing it for reasons other than to achieve good health.” 3.  For those who are training for and competing in extreme endurance exercise, there appears to be a subgroup that may experience undesirable changes in cardiac structure and function as a result. O’Keefe concluded that ‘More than an hour of daily vigorous aerobic exercise produces diminishing returns, and may even cause adverse cardiovascular effects in some individuals.’ 4.  “If we could give every individual the right amount of nourishment and exercise, not too little and not too much, we would have found the safest way to health.”  Hippocrates ~2500 B.C.

Table 1.  Benefits of Aerobic Training.

  • Increased blood HDL cholesterol level
  • Conversion of small-dense blood LDL particles to larger-buoyant LDL particles (the small dense LDLs are much more harmful than the large buoyant LDLs)
  • Decreased blood triglyceride level
  • Decreased resting blood pressure
  • Improved blood glucose and insulin levels
  • Improved ability to neutralize potentially harmful free radicals
  • Decreased percent body fat
  • Decreased levels of stress/anxiety
  • Decreased risk of developing type 2 diabetes, helps to control type 2 diabetes
  • Decreased risk of developing hypertension, helps to control hypertension
  • Decreased risk of developing some cancers (e.g., colon, breast)
  • Decreased risk of heart attack and stroke
  • Decreased risk of several types of mortality (death)
Register for our Personal Training Education to learn more about fitness guidelines and exercise programs. Doesn’t matter if you don’t plan on training anyone but yourself - all individuals are welcome to attend.



1O’Keefe J.H., Patil H.R., Lavie C.J., Magalski A., Vogel R.A., McCullough P.A. (2012). Potential Adverse Cardiovascular Effects from Excessive Endurance Exercise. Mayo Clinic Proceedings, 87(6), 587-595.