Review: Your Hardcore Run May Be as Dangerous as Sitting Around Doing Nothing

Study Goals and Design:

This study uses data from the Copenhagen City Heart Study. The goal of the study was to look for

associations between exercise quantity/quality and risk of cardiovascular-related mortality. The design of the

Copenhagen City Heart Study was an observational, prospective cohort study that examined 20,000 men and

women. This research looked at 5,048 of the 20,000 people and further narrowed it down to 1,098 healthy joggers

and 413 healthy but sedentary non-joggers. The researchers excluded those with heart disease, stroke, and diabetes.

Study Quality – Strengths and Weaknesses:

One strength of a cohort study is that it is clear that the exposure (exercise) preceded the outcome (death).

The individuals in the study were also picked in such a way to reduce potential biases. For example, those already

diagnosed with heart disease, stroke, and diabetes were not picked in order to control for self-selection bias. People

with these diseases may choose to exercise more after their diagnosis, which may bias the sample because the

exercise group may have a greater proportion of people likely to die sooner, thus skewing the data.

Because the study is a single observational study, it is difficult to establish causality. The wording in the

paper hints at a causal relationship between strenuous jogging and mortality rates - “vigorous running … could be

almost as harmful as sitting around doing nothing.” The news article suggests that cardiovascular-related mortality

is the intended measurement, given that the article noted the “stress that intense exercise can put on the

cardiovascular system.” However, the results report all-cause mortality risk. Thus, the deaths reported may have

nothing to do with jogging status leading to cardiovascular problems (e.g., cancer). To show that strenuous jogging

leads to increased all-cause mortality, one must come up with a biologically plausible explanation for why

strenuous jogging impacts non-cardiovascular related death.

One of the biggest weaknesses of the study is the potential for confounding variables that occur with

observational studies and alternative explanations to the data. The paper does not mention alternative explanations.

Even though strenuous jogging may be associated with increased mortality rates, those strenuous joggers may also

have other lifestyle habits or genetic factors that may have caused the increased mortality rates. For example, the

group of strenuous joggers may have a higher proportion of “type A” personalities, who tend to have higher blood

pressure and a higher risk for cardiovascular disease. So although the study tried to control for potential

confounders like diabetes, stroke, and heart disease, there are other potential confounders that may affect the

outcome. Since the researchers excluded those with certain diseases, it also limits the generalizability of the

findings – those with the excluded diseases may in fact benefit from strenuous running.

Another potential problem is the study may have been a “fishing” expedition. The original study looked at

“thousands of people that have been the basis for many reports on cardiovascular health … sifted through 12 years’

worth of data.” This suggests that the original study looked at many measures that might relate to cardiovascular

health. The researchers may have sifted through many variables, each with a 5% probability of showing up as

significant by chance alone, given an alpha value of 0.05. Perhaps the relationship between jogging and mortality

was just due to chance.

According to the news article, we only know that 128 out of the 413 healthy (.3) sedentary non-joggers

died, and 28 out of the 1098 (0.02) healthy joggers died. The article does not mention absolute risk values of

mortality for the further subdivisions within the joggers (light, moderate, and strenuous joggers) and only displays

relative numbers (via hazard ratio) in a figure in the beginning of the paper. The paper does not explain what the

hazard ratio is. The news article should have reported absolute risk values for mortality, which is more informative

because they show the total number of people in each subcategory.

It is crucial to report absolute risk values. For example, the strenuous joggers group may have contained

very few people compared to the non-jogger group. Because we are measuring all-cause mortality, a few number of

deaths in the strenuous jogger group could be due to non-cardiovascular related incidents. For example, the deaths

could have been due to car accidents entirely unrelated to either cardiovascular problems or jogging. Because there

is a small sample size, those few deaths could artificially inflate the risk of mortality for strenuous joggers. If this is

the case, the title of the news article, “Your Hardcore Run May Be as Dangerous as Sitting Around Doing

Nothing” – is misleading. Furthermore, a small sample size inherently contains a large variability in the data. This

is reflected in a large confidence interval.1

1 In fact, the confidence interval for strenuous joggers reported in the actual study was 1.97 [95% CI 0.48 to 8.14], which includes values (< 1) that indicate a protective effect!

Additionally, the news article does not state any details about the method of gathering details about

jogging status. If there is a large time gap between each questionnaire, the participants may simply forget how

often they jog, or there may be a social desirability bias to report more exercise. Over such a long period of time

for the study, it is likely that the jogging status fluctuated. If so, it is unclear from the news report what jogging

status the individual is ultimately placed into. This may distort the number of mortalities observed in each category

of jogging status.

Quality of the Source and Reporting:

The paper, for which the news article derives, is sourced from the Journal of the American College of

Cardiology, which is a peer-reviewed journal. This establishes some credibility on the results of the study. The

news article does not report the source of funding for the research study. The main problem is that the title of the

news report and the claims toward the beginning of the article imply a causality that is not justified by the results

and observational study design. The news article does not provide any information about how the participants were

selected – were they randomly selected to minimize bias?

The news report does a good job providing context for the study by presenting different viewpoints and

criticisms. Throughout the news report, the editors point to other research that supports either side of the argument.

For example, they point out that other research has shown that “taxing workouts may put excessive strain on the

heart,” but also note that “plenty of studies have also underscored the many health benefits of exercise.” They also

bring in third party experts, such as David Bach, a cardiologist, and Duck-chul Lee, who wrote an accompanying

editorial. These comments moderate the overall stance of the news article, but are strategically placed toward the

end, after the extreme introduction, “vigorous running …. could be almost as harmful as sitting around doing

nothing.” Since most news-browsers focus on the headlines and the first page or two of the article, the presentation

of the information is likely biased to favor the more newsworthy stance of “too much exercise is bad,” rather than

the conventional “exercise is good” stance.