Tuesday February 3 2015
Most of us could do with running more, not less
“Too much jogging 'as bad as no exercise at all',” BBC News reports. However, the results of the new Danish study on which this headline is based, are not as clear cut as the media has made out.
The study involved about 1,500 people in Denmark. It found that light to moderate jogging was associated with living longer compared with being sedentary, but strenuous jogging was not.
A major limitation to this study was that once the joggers were split into groups by duration, frequency, and pace, some individual groups – particularly the most active groups – were much smaller. These small numbers mean the analyses are less able to detect differences between these small groups and the sedentary group, even if they do exist.
Overall, the study does not impact the current physical activity recommendations for adults.
While it is important that people do not push themselves over their limit, in general, the more common problem is people not doing enough exercise to meet these recommendations.
Where did the story come from?
The study was carried out by researchers from the Frederiksberg Hospital in Denmark and other research centres in Denmark and the US. The study was funded by the Danish Heart Foundation.
The study was published in the peer-reviewed Journal of the American College of Cardiology.
The Daily Telegraph’s headline – “Fast running is as deadly as sitting on couch” – is too sensationalist given the limitations to the study, which are not mentioned.
While BBC News and the Daily Mail commit the journalistic sin of stating “too much of x is bad for you”; an entirely uninformative statement of the obvious. “Too much” of anything is bad for you. That is what “too much” means; a quantity that is so excessive it poses a threat to wellbeing.
A more useful statement would be explaining how much is too much, but unfortunately this study cannot conclusively provide this information.
What kind of research was this?
This was a prospective cohort study aiming to find out what the ideal “dose” of jogging would be for lengthening your life. The researchers report that although people who are physically active have longer lives, the ideal dose of exercise (in terms of intensity, duration and frequency) for achieving the greatest impact on lifespan is not known.
The researchers’ previous study on jogging suggested that jogging up to 2.5 hours in total a week, over up to three sessions, at a slow or average pace was associated with the lowest risk of death during follow-up. Jogging more or less than this was not associated with a reduced risk of death. The researchers investigated this further in the current study.
While people could be randomly assigned to different patterns of exercise, it is unlikely that they would continue to exercise as instructed over their entire lifetime. Therefore a cohort study is likely to be the most feasible way of comparing the effect of people’s normal physical activity patterns on a long-term outcome such as lifespan/risk of death. As with all studies of this type, the main limitation is that people who are physically active may also have other habits (such as healthy eating) that influence their likelihood of death. The researchers need to take these confounders into account in their analyses to try to isolate the effect of the physical activity pattern alone.
What did the research involve?
The researchers identified healthy joggers and healthy non-joggers taking part in the Copenhagen City Heart study. They followed these people up over two years to identify any people who died in that period. They then compared the risk of death in light, moderate and strenuous joggers with that of non-joggers.
The Copenhagen City Heart study took a random sample of nearly 20,000 white adults aged 20 to 93 living in Copenhagen in January 1976. Participants were sent a survey four times during follow-up. For the current study, researchers excluded people who had a history of heart disease, stroke or cancer.
The current study looked at data on physical activity collected from 2001 to 2003, the fourth time survey data had been collected from participants. The sample included original participants from 1976 and an additional sample of younger individuals. The recruitment of these additional people was reported in previous publications, and not in the current study.
The study assessed what type and how much physical activity people did in their leisure time. People were considered “sedentary” if they were almost entirely inactive in their leisure time e.g. reading, watching TV, or doing only very light activity such as gentle walking for less than two hours a week.
Those who jogged were asked about their pace, total time jogging per week, and frequency of jogging per week. This information was used to categorise them as:
- light joggers – less than two and a half hours a week at a slow or average pace (about five miles an hour) three times a week or less
- moderate joggers – between light and strenuous jogging in terms of pace, duration, and frequency – so for example, could be at a slow pace but more than three times a week, or up to a fast pace two and a half to four hours in total in up to three sessions a week
- strenuous joggers – for more than four hours a week at a fast pace (more than seven miles an hour) at any frequency, or two and a half to four hours a week at a fast pace more than three times a week
Participants were followed up to 2013, and researchers managed to follow up almost all participants. Anyone who died in this period was identified through a national death register.
Analyses compared 1,098 joggers versus 413 sedentary non-joggers. The researchers analysed the data in a way that took into account differences in age between joggers and non-joggers. Analyses were also adjusted for characteristics that the participants reported in the surveys:
- alcohol intake
What were the basic results?
Joggers tended to be younger (average age around 40 compared to 61 in non-joggers), have lower blood pressure and body mass index (BMI), and be less likely to smoke or have diabetes. The joggers ranged in age from 20 to 86 years, and the non-joggers from 21 to 92 years.
During follow-up there were 28 deaths among the 1,098 joggers (2.6%) and 128 deaths among sedentary non-joggers (31%).
Analyses by quantity of jogging found that those jogging between one and 2.4 hours a week were less likely to die during follow-up than sedentary non-joggers. Those jogging for more time each week did not differ from sedentary non-joggers in risk of death.
Analyses by frequency of jogging found that those jogging up to three times a week were less likely to die during follow-up than sedentary non-joggers. Those jogging more frequently did not differ from sedentary non-joggers in risk of death.
Analyses by pace of jogging found that those jogging at an average pace were less likely to die during follow-up than sedentary non-joggers. Those jogging at a slow or fast pace did not differ from sedentary non-joggers in risk of death.
Overall jogging “dose”
When combining all of these factors together, the researchers found that after adjusting for confounders, only light jogging was associated with significantly lower risk of death than sedentary non-joggers. Moderate joggers did have a slightly lower risk of death, but this difference was not large enough to rule out with a high level or certainty the possibility of the difference happening just by chance (it was not statistically significant).
How did the researchers interpret the results?
The researchers conclude that their findings show that light and moderate joggers have a lower risk of death than sedentary non-joggers during follow-up. However, strenuous joggers did not differ in their risk of death during follow-up to those who are sedentary. They note that more research is needed before this finding could be incorporated into physical activity recommendations for the general public.
This study has suggested that light to moderate jogging could be associated with living longer compared with being sedentary, but strenuous jogging might not be.
Due to the fact that this data was collected prospectively, there are considerable limitations. The main limitation is that although the total number of joggers was quite high (around 1,000), once these joggers were split up by duration, frequency and pace of jogging, some of the individual groups were much smaller. This was particularly the case in the most active jogging categories (those who jogged more often, for longer, and at a higher pace). This reduces the ability of the analysis to detect differences between these smaller groups and the sedentary group.
For example, there were only 36 people who were classed as “strenuous” joggers, and only two of these people died. These small numbers mean that we cannot say with certainty that there is definitely no difference between people in the most active jogging categories and the people who are sedentary.
The authors also note that even slow jogging would count as vigorous exercise, and strenuous jogging would be considered heavy vigorous exercise. This is important to bear in mind when considering the current UK physical activity recommendations for adults to be active daily and either:
- 150 minutes of moderate activity per week in bouts of 10 minutes or more
- 75 minutes of vigorous activity spread across the week
In addition, while the authors took various factors that could affect their results into account, such as age, these adjustments may not have removed their effect completely. They acknowledge that their study cannot determine whether the jogging patterns themselves directly caused the differences in risk of death seen. Jogging was also only assessed once in the study, and activity patterns may have changed over time. Also, death was the only outcome assessed, so we don’t know what the associations with other outcomes such as general fitness and quality of life were.
Overall, the study does not contradict the current physical activity recommendations and the issue of people not doing enough exercise is more likely to be a concern than people over-exercising.
Too many people in the UK are failing to meet the recommended levels of physical activity. This is reflected in the latest obesity statistics.
Still, it’s never too late to start – read more advice about how you can gradually increase your activity and fitness levels.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.