Too much jogging “as bad as no exercise at all”
“Fast running is as bad as sitting on a couch”
These are just two of the headlines being splashed around the papers this morning, claiming that too much running is bad for us.
As a runner and doctor, of course this grabbed my attention.
I certainly wasn’t willing to accept that running is “bad” for me until I’d seen the evidence for myself, and of course, shared it with you.
So, what are they talking about?
They’re referring to a paper recently published in the Journal of the American College of Cardiology, called the Copenhagen City Heart Study “Dose of Jogging and Long-term Mortality.”
The aim of the study was to investigate whether jogging had an affect on death rates, and in particular, whether the pace, quantity and frequency of jogging affected how likely you were to die in comparison with a sedentary person.
Since 2001 they’ve been following 1098 healthy joggers and 3950 healthy non-joggers to look for a relationship between jogging and death rates.
What did they find?
They found that people who jogged at a slow pace, 2-3 times a week for a total of 1 – 2.5 hours had the lowest death rates.
So far, so good. The study confirms what we already know; exercise is good for you.
They conclude that “strenuous joggers have a mortality rate not statistically different from that of the sedentary group,” which basically means that you’re just as likely to die if you jog strenuously than if you do no exercise at all.
I’m not sure about you, but I find this quite hard to believe.
Let’s talk statistics
When a study makes huge claims like this, we need to look more closely and work out how they reached that conclusion.
Only when we’ve done that can we decide whether the claims are valid, and if we want to believe them.
So, let’s let’s look more closely at this study:
Number of runners:
They study followed 1098 over a 12 year period, and categorised them into light, moderate, and strenuous joggers. You’d be right to expect that there should be an equal number of runners in each category to give a fair result.
There were 576 light joggers, 262 moderate joggers, and 40 strenuous joggers.
Yes, that’s right, just 40 strenuous joggers out of 1098.
Number of deaths:
There were 7 deaths in the light joggers, 8 in the moderate joggers, and 2 in the strenuous joggers.
So quite small numbers all around, especially when you think of all the reasons you could die, apart from running of course.
So, what do they do with these numbers?
- Hazard Ratio
This is a term used in statistics to describe the chance of something happening in one group compared to another. So in this study , they’re looking at the chance of dying in the sedentary group compared to the jogging groups.
Each jogging group is given a number (hazard ratio), to compare the chance of death in their group compared to the sedentary group.
If the hazard ratio is 1, there’s no difference, so you’re not more likely to die.
If the hazard ratio is less than one, you’re less likely to die.
If the hazard ratio is more than one, you’re more likely to die.
- Confidence Intervals
Ideally, a study like this would follow every strenuous jogger in the world to assess the risk between jogging and death.
Obviously, this is never going to happen, so they have to make do with the numbers they have to reach a conclusion.
Statisticians use something called a confidence interval to work out a range between which the real value is likely to be, and say we can be 95% sure that the real value is within that range.
So in this case, the confidence interval is saying that if we followed every strenuous jogger in the world, we could be 95% sure that the likelihood of dying as a result would be within that range.
As you can imagine, the more people you have in the study, the more accurate you can be with the confidence intervals.
It’s not surprising then that the confidence interval for the large group of light joggers was quite small (0.11-0.80), whereas the confidence interval for the small group of strenuous joggers was big (0.4-3.67).
Essentially, it’s hard to estimate how likely something is to happen when you haven’t got many numbers to work with. The larger the confidence interval, the less reliable.
What does this all mean?
The confidence interval for the group of light joggers is very small, i.e., very reliable. The hazard ratio (the chance of dying compared to the sedentary group) is less than one, so shows you are less likely to die.
Because the confidence interval is small and the entire range (0.11-0.80) is below one, we can be 95% sure that light jogging really does reduce your risk of dying.
On the other hand, the confidence interval for strenuous jogging group is huge, going all the way up to 3.67, with a hazard ratio that’s about the same as the sedentary group.
This is why they claim that you’re just as likely to die if you jog strenuously compared to if you’re sedentary.
So, they’ve come to this conclusion using the data of just 40 runners and two deaths.
If the group of strenuous runners had been bigger, they could have been much more accurate when calculating the confidence interval and hazard ratio, and therefore, provided a more realistic figure.
If the group sizes had been the same it may well have showed that strenuous jogging was even more beneficial than light jogging, but we’ll never know, as we don’t have the data.
This study clearly shows that regular exercise is good for you and can reduce your risk of death by 30% compared to people who lead a sedentary lifestyle.
However, given the data provided, I believe this study can not state that strenuous jogging has no effect on your long-term survival in comparison to being sedentary, or that there is a significant difference in death rates between light, moderate and strenuous joggers.
What do you think about this study?
Has this study changed the way you think about jogging and exercise?
Are you going to change your exercise habits as a results?
I think it’s great this study shows that keeping active is good for you, but until they come up with some stronger evidence, I’m going to continue with my “strenuous jogging.”