A recent study reported that joggers who exercise strenuously have the same life expectancy as people who do
barely any exercise at all. But the author has now admitted he hasn't
actually proved this.
"Training very hard 'as bad as no exercise at all,'" reported the BBC. "Fast running is as deadly as sitting on
couch," agreed the Daily Telegraph and countless other newspapers around
the world, striking fear into the hearts of hardcore runners.
The headlines were based on the results of a large Danish study - more than
1,000 healthy joggers and almost 4,000 healthy non-joggers were
followed up over 12 years, as part of the Copenhagen City Heart Study.
The results were published in the prestigious Journal of the American College of Cardiology. The headline conclusion was that light and moderate joggers live longer
on average than people who have sedentary lifestyles and don't exercise
at all, whereas - and this was the surprise - strenuous joggers don't.
Strenuous joggers were defined as people who run at a fast pace - more than 7mph -
for more than four hours per week, or more than three times per week
for at least two-and-a-half hours in total.
But, although the overall number of people studied was large, the number of strenuous
joggers was not. Only 36 people fitted the strenuous jogging category -
two of them had died.
Critics say these numbers are too small to be statistically significant.
And the researchers don't know how the two strenuous joggers died - whether
they had succumbed to an illness such as cardiovascular disease or
whether they had been involved in some sort of accident. They could have
been knocked down by a bus.
The lead author of the study, clinical cardiologist Dr Peter Schnohr, now concedes that he didn't have
the evidence to say that strenuous jogging is bad for you.
"We should have said we suspect that it is so, but we can't say for sure. Everybody makes some mistakes in papers," he says.
But Schnohr thinks experienced readers of research papers would have realised this - he says it was obvious from the statistical analysis
that you couldn't have confidence in the claim that strenuous joggers
have the same average life expectancy as those with a sedentary
"It shouldn't have been misunderstood," he says, because if you go into the statistics the limits of the research are
clear. "If you normally read papers you could say 'Ah! This is not good
statistically - this is too thin.'"
But most journalists around the world weren't going to be delving into the statistical tables - they
take their lead from the researcher's conclusions, according to Alicia
White, who analysed the study for the UK's National Health Service website.
"In today's world, research findings can rapidly be disseminated across the
globe - particularly when they capture the public imagination due to a
shocking finding," she says.
"It's not realistic to assume that everyone who hears a headline will track down the research paper, read
and critically appraise it before they decide to act or share it with
their friends. That's why it is so important that research findings,
their implications and limitations are communicated in a clear way right
from the start."
924,741 runners have completed the London Marathon since it was set up 1981
The Journal of the American College of Cardiology, which published the research, stands by the paper.
"Some news articles appear to have misinterpreted and exaggerated" the story
leading to "misleading headlines," says editor-in-chief Valentin Fuster.
But the fact remains that the paper's headline conclusion included the statistically insignificant finding about strenuous jogging
- something Peter Schnohr admits shouldn't have been highlighted.
Even so, he doesn't regret the shock-horror headlines, and isn't worried that they might have put people off jogging.
"I don't think so… you always have deaths in marathons and so on," he says
- and suggests that with regular check-ups, cases of heart disease may
have been detected and some of these fatalities "could have been
Schnohr remains convinced that although he hasn't proved it this time, strenuous jogging might be bad for you.
"We're not saying that you should not do the marathon, but we're saying that
maybe every other year you should do an investigation -
echocardiography, and so on - to look at your heart."
He says that his findings that light jogging is associated with significantly lower
mortality rates is robust though, and that this important health message
was picked up by a number of media outlets.