Strenuous Exerciser vs Total Couch Potato: Which Is Riskier?

Laird Harrison

Disclosures

September 09, 2015

In This Article

How Much Exercise Is Too Much?

James O'Keefe Jr, MD, loves exercise. He used to work out twice a day, compete in triathlons, and race from one state to another. But as he reached his mid-40s, Dr O'Keefe, an associate clinical professor of cardiology at the University of Missouri-Kansas City School of Medicine, had a change of heart. Literally.

"When I would push really hard, I would feel a vague discomfort in my chest," he says. "I could feel that maybe I was overdoing it. That's what made me stop and think about the whole issue of how much exercise is the right amount."

Today Dr O'Keefe, who is also director of preventive cardiology at the Mid America Heart Institute of St. Luke's Hospital in Kansas City, Missouri, has become a leading proponent of the notion that endurance athletes may be doing themselves more harm than good.

"If we had a drug that does everything exercise does—reduces heart attacks, strokes, cancer, dementia, and depression—it would be the most potent drug in the world," he says. "But as with any drug, you have to get the dose right. Too little a dose won't do anything, but too big a dose can be lethal."

That position has brought criticism from both runners and other experts researching the question of how much exercise is too much from a healthy perspective.

Says one researcher, Hannah Arem, PhD, MHS, a postdoctoral fellow in the Division of Cancer Epidemiology and Genetics at National Cancer Institute in Bethesda, Maryland, and lead author of one of the largest studies to look at this question, "Our study does not support previous findings that suggest that there's an overall danger with high levels of exercise."[1]

It's not just an academic point. In 2013, the number of marathoners reached a record of 541,000 in the United States[2]; in 2014, USA Triathlon had over 170,000 members.[3]

The Case Against Overexercising

The case for the danger to the health of these athletes rests on four types of evidence.

Case studies. The most famous example of the potential lethality of overexercise is the original marathon runner, Pheidippides. According to legend, he died in 490 BCE after running 25 miles to announce a victory in battle.[4] Dr O'Keefe also likes to cite the example of Micah True, a distance runner who died of an apparent cardiac dysrhythmia at age 58, three years after being featured in Christopher McDougall's best-selling book Born to Run: A Hidden Tribe, Superathletes, and the Greatest Race the World Has Never Seen.[5] But, of course, there are many counterexamples of endurance athletes living to a healthy old age.[6]

Animal studies. Rats forced to run with electric shocks have developed hypertrophy of the ventricles, diastolic dysfunction, dilation of the atria, increased collagen deposition, and fibrosis in both the atria and ventricles.[6] But the running didn't actually kill the rats. The fibrosis subsided when the rats were allowed to move at a gentler pace. And what happens to a rat doesn't necessarily happen to a human, particularly one who is running by choice.

The physiology of endurance athletes. Some findings in human endurance athletes resemble those in the rats.[6] Among them: enlarged ventricle volumes, increased left ventricle wall thickness and cardiac mass, increased left atrial size, increased coronary artery calcium, and increased risk for atrial fibrillation. Biomarkers suggesting harm in these athletes include cardiac troponin, creatinine kinase MB, and B-type natriuretic peptide. Some researchers have also found signs of renal dysfunction, such as elevations in serum urea nitrogen, serum creatinine, and cystatin C. But these may only be transient effects. Some may even be signs of healthy adaptation.

Observational studies of physical activity. Many researchers have surveyed groups of people about their physical activity and looked for correlations with morbidity and mortality. These studies have repeatedly found that more active people are healthier than less active people—but only up to a point. The debate centers on what happens after that point.

Dr O'Keefe was a coauthor in one of the most recent of the observational studies, a comparison of 1098 joggers and 3950 nonjoggers in the Copenhagen City Heart Study.[6] The researchers found that 31% of the nonjoggers who were totally sedentary—ie, couch potatoes—died over a period of 12 years. (Their average age was 61 years, so a high death rate is not altogether surprising.)

To compare the two groups, Dr O'Keefe and colleagues made statistical adjustments for age, smoking, and other factors. They found that people who jogged—but only once a week or less—were less likely to die. (Less than once a week could mean any amount of jogging less frequent than once every 7 days—for example, once every 8 days or once a month.) In fact, they had only 29% of the couch potatoes' risk for death (ie, a hazard ratio of 0.29).[6]

But more jogging didn't improve the odds of longevity of the joggers. People who jogged two or three times per week had 32% of the couch potatoes' risk for death.[6] People who jogged more than three times a week had more than double this risk—71% of the couch potatoes' risk.

The researchers found a similar curve when they looked at jogging pace. People who ran slowly cut their risk for death. Those who ran faster also cut their risk—but not as much. When they combined the two variables into one model, they calculated that those joggers who ran the fastest and most frequently were actually more likely to die than the couch potatoes. In fact, they had nearly double the risk.

Many other studies have generated similar curves, though not quite as startling as that in the Copenhagen City Heart Study, Dr O'Keefe says. Plotted on a graph, the curves take the shape of a U or backward J, with a steep drop in mortality followed by a gradual rise with increased amounts of exercise. "In virtually any well-done observational study, if you have enough people doing strenuous exercise, you will see this inverse J-shape or U-shape," Dr O'Keefe says. "That's probably not random."

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