expert opinion

Communicating the effects of lifetime habits on health

March 15, 2013


David Spiegelhalter, professor of biostatistics, University of Cambridge, UK

“We are bombarded by advice about the benefit and harms of our behaviors, but how do we decide what is important? I suggest a simple way of communicating the impact of a lifestyle or environmental risk factor, based on the associated daily pro rata effect on expected length of life. A daily loss or gain of 30 minutes can be termed a ‘microlife’ (micro = a millionth), because 1,000,000 half hours (57 years) roughly corresponds to a lifetime of adult exposure. From recent epidemiological studies of long term habits the loss of a microlife can be associated, for example, with smoking two cigarettes, taking two extra alcoholic drinks, eating a portion of red meat, being
5 kg overweight, or watching two hours of television a day. Gains are associated with taking a statin daily (1 microlife), taking just one alcoholic drink a day (1 microlife), 20 minutes of moderate exercise daily (2 microlives), and a daily intake of at least
5 servings of fresh fruit and vegetables respectively blood vitamin C concentrations above 50 nmol/L (4 mi-crolives).

Demographic associations can also be expressed in these units – for example, being female rather than male (4 microlives a day), being Swedish rather than Russian (21 a day for men) and living in 2010 rather than 1910 (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.

Quantities such as hazard ratios, standardized mortality ratios, and population attributable fractions arise naturally from standard epidemiological study designs. For example, a recent study reported that consump-tion of an extra portion of red meat (85 g) a day was associated with a hazard ratio for all-cause mortality of 1.13. This was greeted in the popular media with exaggerated headlines and little comprehension – for ex-ample, ‘if people cut down the amount of red meat they eat … to less than half a serving a day, 10% of all deaths could be avoided’ (Daily Express 4 March 2012). Such relative risk terminology is known to commu-nicate a greater size of risk than measures of absolute risk. Current guidelines from the Association of the British Pharmaceutical Industry state that relative risks should not be used without absolute risks when re-porting the results of clinical trials. Absolute risks are sometimes provided in terms of the numbers of early deaths delayed: for example, the recent estimate that a 40% reduction in alcohol consumption to a median of 5 g/day would delay 4,500 deaths a year in England.

An alternative absolute measure is change in life expectancy, for example, an estimated average two year extension by reducing excessive sitting in the US population to less than three hours a day. Hazard ratios can be converted to changes in life expectancy if a lifelong effect from a specified age is assumed, so if the above hazard ratio for eating red meat is applied to UK life tables (showing the probability of death) from, say, age 35, a lifetime habit of an extra portion of red meat per day is associated with a reduction in male life expectancy of around one year, from age 80 to 79. This does not look very impressive, as people tend to dismiss effects that are perceived to lie in the distant future. But the loss of one year over 45 years is 1/45th, which pro rata is roughly one week a year or half an hour per day. So an alternative, possibly more engaging, narrative is that a lifelong habit of eating burgers for lunch is, when averaged over the lifetimes of many people, associated with a loss of half an hour a day in life expectancy. Which is, unless you are a quite a slow eater, longer than it takes to eat the burger.

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. An average 35-year-old man and woman in England has a life expectancy of 45 and 48 years respectively (394,000 and 420,000 hours) assuming current mortality rates. Since life expectancy has been increasing by three months a year for decades, a current 35-year-old might realistically be expected to live another 55 years, which is 481,000 hours – or nearly a million half hours.

Specific hazard ratios can be converted to life expectancies by linking estimated effects of long term lifestyle and demographic risk factors to change in life expectancy for men and women aged 35 years, and to corres-ponding change in microlives (30 minutes of life expectancy) per day of exposure. These assessments are very approximate and based on numerous assumptions. The hazard ratios are primarily derived from recent meta-analyses, but inevitably they rely on published results that may be contentious, particularly in dietary studies. Averaged over a lifetime habit, a microlife can be ‘lost’, for example, from smoking two cigarettes, being 5 kg overweight, having the second and third alcoholic drink of the day, watching two hours of tele-vision, or eating a burger. On the other hand microlives can be “gained” by drinking 2–3 cups of coffee a day, eating fruit and vegetables regularly, and 20 minutes of moderate exercising daily. Also demographic factors have effects: the survival penalty of being male is roughly equivalent to smoking eight cigarettes a day, living in Russia rather than Sweden is equivalent to smoking 40 cigarettes a day for a man and 20 a day for a woman. The idea of microlives encourages a metaphor of ‘accelerated ageing’ due to harmful exposu-res: for example, smoking 20 cigarettes a day (10 microlives) is as if you are rushing towards your death at 29 hours a day instead of 24.

However, there are limitations of the concept: microlives are averaged over populations and lifetimes; they ignore variability between people in their response to harmful or beneficial exposures and do not apply either to specific ages or a single exposure (who knows what damage a few cigarettes do?). In addition, they are based on the epidemiological difference between groups of people and not necessarily the causal effect of the behavior. Neither do they necessarily express the effect of change of behavior in individuals. Further-more, the estimates are rough, both due to sampling variability and, more importantly, the inevitable limitations of epidemiological studies. Moreover, there is no consideration of quality of life, so behavior is evaluated only in terms of adding years to life, rather than life to years. However, a ‘healthy microlife’ could be adopted were data available.

The metaphor of speed of ageing and use of the term microlife are intended for popular rather than scientific consumption, but they could also be useful for health professionals. They could perhaps best be communica-ted with phrases such as ‘When averaged over a lifetime habit of many people, it is as if each burger were taking 30 minutes off their life.’ These quantities bring long term effects into the present and help counter temporal discounting, in which future events are considered of diminishing importance. Of course, evaluation studies would be needed to quantify any effect on behavior, but one does not need a study to conclude that people do not generally like the idea of getting older faster.”

Based on: Spiegelhalter D. Using speed of ageing and “microlives” to communicate the effects of lifetime habits and environment. BMJ. Published online December 2012.