In this week’s health news, running will make you live longer. Maybe. Maybe not.
The National Post headline announced that “Running for five to 10 minutes a day may add three years to lifespans, study suggests“(7/29/2014). I shouldn’t blame the journalists for exaggerating when the journal article title oversells the study findings as well: Leisure-Time Running Reduces All-Cause and Cardiovascular Mortality Risk. (Lee D, Pate RR, Lavie CJ, Sui X, Church TS, Blair SN. J Am Coll Cardiol 2014; 64:472-81.)
The study basics:
- subjects = 55,137 people who visited a health clinic for a check-up between 1974 and 2002; mostly white, middle to upper class, college educated
- Data was gathered at the first visit and, where possible, at a second visit whenever that might have occurred.
- National Death Index used to identify death through 2003.
- Running was self-reported estimate of activity over previous six months including frequency, duration, distance and speed.
All of the graphs in the paper illustrate differences in hazard ratio (risk of death in one group compared to another group) rather than differences in actual mortality rates. The paper provides mortality rates adjusted for baseline age, sex, and examination year so I’ve graphed those rates for comparison.
We can make predictions based on this information. Each person-year is observing one person for one year. So 10,000 person-years could be following 10,000 people for one year, 5,000 people for two years, 1,000 people for ten years or some other combination. Comparing a group of 10,000 non-runners to a group of 10,000 runners, you would expect to observe 15 more deaths in the non-running group (45 vs. 30).
Another way to describe this is using Absolute Risk Reduction for all-cause mortality. The ARR = 0.146%. So, yes, runners did have a 30% lower risk of dying from any cause but the risk of dying was low anyway (0.45% in non-runners) so 30% isn’t really much of a difference.
The big problem with the conclusions stated by the researchers is that association does not equal causation. They found that runners had lower all-cause mortality. That does not prove that running reduces mortality. In fact, in another part of the study, non-runners at the first visit who became runners as of a later visit, did not have lower mortality. This study observed people who chose to run and chose how much they ran. Higher duration, frequency, and/or intensity of running did not result in lower mortality.
Which leads to the newspaper headline. Can running five minutes per day add three years to your life? Probably not. The study didn’t even attempt to answer that question. They estimate that the life expectancy of non-runners is three years shorter than runners.
This study is not proof that, if you start running, you will live longer.