Expert calculates effect of daily habits on life expectancy
Using speed of ageing and "microlives" to communicate the effects of lifetime habits and environment
It may be the season to eat, drink and be merry, but each day of over-indulging can take several hours off your life, according to a Christmas article published on bmj.com today.
Activities like smoking, having a couple of drinks, eating red meat and watching television anytime of year can each knock at least 30 minutes off your life expectancy for every day you indulge. In contrast, each day of sticking to just one alcoholic drink, eating plenty of fruit and vegetables, and exercising can be expected to add up to two hours to your life.
Professor David Spiegelhalter, a statistician at the University of Cambridge, wanted to find a simple way of communicating the impact of our behaviours on expected length of life.
He suggests using the concept of ageing faster or slower, by expressing the daily effect of lifestyle habits as "microlives" (half hours of life expectancy). A half hour of adult life expectancy can be termed a microlife as it is loosely equivalent to one millionth of life after age 35, he explains.
Using data from population studies he calculates that, averaged over a lifetime habit, a microlife can be "lost" from smoking two cigarettes, being 5 kg overweight, having a second or third alcoholic drink of the day, watching two hours of television, or eating a burger.
On the other hand, microlives can be "gained" by sticking to just one alcoholic drink a day, eating fresh fruit and vegetables, exercising, and taking statins.
Demographic factors can also be expressed in microlives. For example, being female rather than male (a gain of 4 microlives a day), being Swedish rather than Russian (a gain of 21 a day for men), and living in 2010 rather than 1910 (a gain of 15 a day).
This form of communication allows a general, non-academic audience to make rough, but fair comparisons between the sizes of chronic risks, and is based on a metaphor of "speed of ageing," which has been effective in encouraging cessation of smoking, says Professor Spiegelhalter. "So each day of smoking 20 cigarettes (10 microlives) is as if you are rushing towards your death at 29 hours rather than 24."
He points to several limitations and stresses that these assessments are very approximate and based on numerous assumptions. However, he says they "bring long term effects into the present and help counter temporal discounting, in which future events are considered of diminishing importance."
In spite of the limitations, he concludes that "a reasonable idea of the comparative absolute risks associated with chronic exposures can be vividly communicated in terms of the speed at which one is living one's life."
He adds: "Of course, evaluation studies would be needed to quantify any effect on behaviour, but one does not need a study to conclude that people do not generally like the idea of getting older faster."