21 May 2014

(See Larry in a Hangout we had last week with Jay Widmer MD, and Dave Albert MD about an mHealth study done by Mayo Clinic)

The issue of “too much exercise” is in the news again this week.  In last week’s edition of the medical journal, Heart, a group of German investigators headed by Dr. Ute Mons from the Division of Clinical Epidemiology and Aging Research at the German Cancer Research Center in Heidelberg reported on “A reverse J-shaped association of leisure time physical activity with prognosis in patients with stable coronary heart disease:  evidence from a large cohort with repeated measurements.”  This study extends our understanding of the importance of exercise in patients with known heart disease and the findings are very much worth noting.

In the popular press, this study received considerable attention in the past few days.  In Forbes, we had a short article by Larry Husten, entitled “Exercise:  Can There Be Too Much of a Good Thing?”  In the Wall Street Journal, we had a short article by Kevin Helliker, entitled “Too Much Exercise May be Harmful to Your Health.”  There were many other articles; these are representative.  The headlines were all similar.

There was also interest in social media and the blogosphere.  The research was shared along with admonitions about exercising too much, all in rather broad strokes.

Let’s take a look at the study….

The Study

Back in 1999-2000, just more than 1100 German individuals were enrolled in a longitudinal study.  These were individuals who were undergoing inpatient cardiac rehabilitation (a monitored exercise program) after some sort of aucte heart problem–acute coronary syndrome, heart attack or myocardial infarction (MI), or coronary revascularization (with a stent or bypass surgery).  As such, they were all known to have significant coronary artery disease (CAD).  At the time of enrollment, they ranged in age from 30 to 70 years.

Over the following 10 years, these individuals answered questionnaires about their health at 1, 3, 4.5, 6, 8, and 10 years.  At the time of the 1-year questionnaire, the median age of respondents was 61 years.  The typical participant was male, overweight, a current or former smoker, with a history of heart attack, and high blood pressure.  On each of the questionnaires, there was a question about physical activity:

“On average, how often have you engaged in physically strenuous and sweat-inducing activity in your leisure time in the past 12 months (ie, cycling, speedy hiking, gardening, sport)?”

And the possible answers included:

  • Daily
  • 5-6 times per week
  • 2-4 times per week
  • 1-4 times per month
  • Rarely or never.

The respondents were also asked to estimate the number of hours per week, on average, they spent doing strenuous physical activity.

In addition to physical activity, the investigators collected information about the important outcome measures:  major cardiovascular events (MI and stroke); cardiovascular mortality (death due to a heart-related cause); non-fatal cardiovascular events; and all-cause mortality.

The Results

At the time of the 1-year questionnaire, most individuals were physically active, with just 9.1% reporting “rarely or never” exercising.  Most (41.3%) were in the “2-4 times per week” category.  The “5-6 times per week” group accounted for 15.8% and the “daily” group accounted for 15.3%.  For context, the current American Heart Association (AHA) guidelines on physical activity (generally, and not specific to patients with known CAD) call for 3 to 5 days of exercise per week, depending upon the intensity of the exercise.  In terms of time, those guidelines suggest 150 minutes (2 1/2 hours) of moderate-intensity exercise OR 75 minutes of vigorous exercise per week.

Perhaps not surprisingly, as the individuals aged over the 10-year period of the study, their activity levels decreased.  Perhaps their interest in well-being or focus on their heart condition waned as well.  There was a relatively constant percentage of enrollees in the “2-4 times per week” category, but there was a decrease in those exercising more….and an increase in those exercising less.

Statistical analysis was used to try to isolate the influence of physical activity as a variable….and eliminate the influence of other confounding variables (employment status, smoking, obesity, self-reported “poor health,” history of MI, diabetes, high blood pressure, poor heart function, and number of coronary arteries that were diseased) as well as the influence of changes in the amount of exercise over time.  Many of these confounding variables would otherwise have influenced the outcome measures.

The primary finding was that individuals who exercised the least (either “rarely or never” or “1-4 times per month) were at greatest risk for all 4 of the outcome measures.  This isn’t surprising.  This study confirms the findings of many previous studies.

The investigators also found that there was a sweet spot in terms of the frequency of exercise, where there was the greatest benefit, and lowest risk for the outcome measures.  For all-cause mortality, cardiovascular mortality, and major cardiovascular events, the sweet spot was “2-4 times per week” of exercise.  Either more or less exercise was associated with greater risk.  For the outcome measure of non-fatal cardiovascular events, though, there was little association with the frequency of exercise.  These results are the ones that received attention in the press this week.

On the face of it, though, these findings about frequency of exercise might be deceiving.

The investigators also reported on the amount of exercise–the number of hours spent per week in physical activity.  Again, there appeared to be a sweet spot where the risk of the outcome measures was least:  10-11 hours per week, for all-cause mortality and cardiovascular mortality and ~9 hours per week for major cardiovascular events.  In each of these cases, a broad range in the amount of exercise, perhaps 5 to 16 hours per week, conveyed a benefit over no exercise at all.  And similar to their findings for the frequency of exercise, the investigators found very little relationship between the amount of exercise and the outcome measure of non-fatal cardiovascular events.  I don’t recall seeing these results reported in the media.

Take Home Messages

  • There’s always more to the story than the headlines suggest.
  • This study is a longitudinal, observational study.  It is not a prospective trial, or experiment.  Because of the study design, some will be critical about the results and conclusions.  But realize that, for a variety of reasons, there will never be a 10-year trial where one group is told to exercise and another group is told not to exercise.  As a result, the current study is the type of investigation that will continue to inform us about the issue of “too much exercise.”  We should pay attention to the results.
  • This study involves individuals with known CAD.  And recall that the typical enrollee was male, age 61, overweight, current/former smoker, with a history of MI and high blood pressure.  Remember to keep in mind, then, that the results and conclusions apply to THIS group of individuals.  This study does NOT speak to the issue of “too much exercise” for healthy individuals.
  • The greatest risk for the outcome measures was identified for those individuals who exercised the least.  This is the most important finding of the study.  This finding is consistent with my personal experience caring for such patients.  In this group of patients, like elsewhere in our society, we have a problem of too little exercise, not too much.  Don’t lose sight of this message.
  • CAD is common.  For the very large group of patients with CAD, there is a real and practical issue of how much exercise to recommend, or to “prescribe.”  We want patients to derive the most benefit possible.  And we want them to avoid unnecessary risk.  This study suggests that “2-4 times per week” is the sweet spot for frequency of exercise but that a rather generous ~9-10 hours per week is the sweet spot for the amount of exercise.  We should pay attention to these findings when we make recommendations to patients with CAD.
  • For patients with CAD who choose to exercise beyond these sweet spots, I’d advise caution.  Obviously, “2-4 times per week” and ~9-10 hours is quite a bit of exercise, and possibly far in excess of what is recommended by the AHA guidelines.  So there’s obviously room for considerable activity and sports participation up to these sweet spots.  For any level of planned exercise, individuals with known CAD ought to work with their doctor(s) to settle on what is safe and appropriate given their particular circumstances, realizing that the benefits and risks will not be the same for every individual.  Beyond these sweet spots, though, the motivation for additional exercise is probably something other than one’s health.  In that situation, judgements need to be made about the trade-off between additional exercise and additional risk.

Related Posts:

1. Don’t Stop Running Yet!

2. Interesting Research from the ACC Meeting 2014

3. Do Elite Athletes Live Longer?


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