And I would walk 500 more

From Stuff

Ten thousand is often touted as the golden number of steps required in a day to live a healthy life.

It’s a goal many Kiwis don’t reach, with one survey from the Ministry of Transport highlighting 80 per cent of respondents don’t walk more than 100 metres a day.

But new research has found the sweet spot to be significantly lower, especially for older women wanting to live longer.

The study – published in JAMA Internal Medicine by researchers from Harvard Medical School – found just 7500 steps a day, for women aged over 70, could cut their risk of death.

This is a good description of the study — and it even links to the research.  There are two aspects that are not really clear in the story, but they’re not really clear in the text of the research paper either.

The researchers asked 18000 women over 60 to wear accelerometers (like Fitbits, but less cool) for a week, to measure how much they walked, and then waited for a bit more than four years to see who died.  Women who walked more were less likely to die during that period. Here’s the graph comparing rates of death by amount of walking.  The blue curve is the study estimate of the reduction in rate of death, but the shaded area is an uncertainty interval — the data are consistent with the curve being basically anywhere in the shaded region

As you can see, the estimated curve flattens out at about 7500 steps per day.  If we just looked at the blue line we’d say any women who walks less than 7500 steps per day could cut her risk of death by walking even a bit more, but that 7500 is the maximum that’s worthwhile. Looking at the shaded area shows that we really don’t have much idea about what happened to women who walked more than 7500 steps. Their risk could have been quite a bit lower, or about the same, or higher.  The problem is that only 49 of the women who walked more than 7500 steps per day ended up dying during the study; that’s great for them, but it’s a real limitation for the statistical analysis.

Also, you might worry (you should worry) which way cause and effect goes.  The headline would sound less interesting if it said women within four years of dying walked less than women with a longer life expectancy, but the correlation is the same either way. I’d guess it’s mixture of both: walking does make you healthier, but being healthier also makes you walk more.

PS: I should note that the Stuff journalist, Brittney Deguara, also got the story on WHO classification of ‘burnout’ correct (it’s not being listed as a medical condition), vs the somewhat misleading story in the Herald that begins “Burnout” is officially a disease, according to the World Health Organisation.