A journal club member (Dr RB) saw a middle aged woman ask her husband to walk up a hill to their car and drive down to pick her up. She probably saved herself one minute of walking.
We know that exercise prolongs life. The question therefore is:
How much would that one minute’s walk have added to that woman’s life?
Or, more generally:
What advice could we give to sedentary adult patients about the life-prolonging effect of taking up exercise?
Population: Adults who are mainly sedentary.
Intervention: Increase in exercise (e.g., walking)
Comparator: Remaining sedentary.
Outcome: Life expectancy.
Exercise, dose-response, life expectancy, mortality, sedentary
Cochrane library – no suitable reviews found
PubMed database – hundreds/thousands of articles for each combination of search terms. Because we are a time-limited journal club, when the first 50 hits are all irrelevant, we chew another sandwich and move on to another search mechanism! Which in this case was…
Google scholar – Using search terms ‘exercise dose response mortality‘ the first hit was highly relevant. Just goes to show that sometimes formal PICOs are beaten by a quick-and-dirty Google!
Lee, I and Skerrett, P. Physical activity and all-cause mortality: what is the dose-response relation? Medicine and science in sports and exercise, 2001
Wen, CP et al. Minimum amount of physical activity for reduced mortality and extended life expectancy: a prospective cohort study. Lancet 378:9798;1244-53 Oct 2011
The Lee paper is a lit review of 44 studies. “There is clear evidence of an inverse linear dose-response relation between volume of physical activity and all-cause mortality rates in men and women, and in younger and older persons.”
So it is reasonable for us to divide the dose into one-minute intervals and expect that each minute will prolong life.
The Wen paper was a prospective cohort study (an apt study for this question, as an RCT would be unethical and impractical) of 416,000 Taiwanese people. Participants who exercised moderately for just 15 mins per day had a three-year increase in life expectancy compared to those who didn’t. Further longevity gains continued with more minutes of exercise, although with reducing efficiency.
This three-year life gain (or loss, from the perspective of a sedentary person) is good enough for our purposes. We don’t require a precise figure – we are looking for a ‘ballpark’ figure to tell patients.
My own subsequent reading also found Leisure time physical activity of moderate to vigorous intensity and mortality: a large pooled cohort analysis by Moore, S. et al [PLoS Med. 2012 November; 9(11): e1001335] which pooled 650,000 participants in six US cohort studies. For adults over 40 years of age, 11 minutes exercise per day gained 1.8 years of life.
Let’s do the maths – or skip straight to the conclusion if you prefer.
Assume we are talking about a 30-year-old exercising for 15 minutes per day for the next 40 years. Total exercise minutes = 15 x 40 x 365 = 219,000 minutes
The Wen study tells us they can expect to live an extra three years = 3 x 365 x 24 x 60 = about 1.58 million minutes of life gained.
The ratio is 1576800 / 219000 = 7.2
To check this, a similar result comes from the US study by Moore. If we assume that their 40-year-olds did 11 minutes exercise per day for 35 years, the minutes of exercise is 11 x 35 x 365 and the minutes of life gained is 1.8 x 365 x 24 x 60.
The ratio is 946080/140525 = 6.7
Therefore one minute of exercise equals about seven minutes of life gained.
Health professionals, patients and the health-reading public love catchy, easy messages. This conclusion is so beautiful and simple (and admittedly simplistic!) it is worth repeating.
Our message for the woman whose husband walked a minute up the hill for her is that she has just missed out on the chance to live seven minutes longer. And to think her husband thought he was doing HER a favour!
Every minute of exercise you do as an adult gains you seven minutes of life.
Although we made a few assumptions, I still think that this one line is the most useful take-home message we have yet published from the IPCJC journal club.
Feel free spread the word via your favourite medium, social or otherwise.
By Dr Justin Coleman. Read my blog for further comments on this subject.