There is a much-quoted saying, attributed to the epidemiologist Geoffrey Rose: “A large
        number of people exposed to a small risk may generate many more cases than a small number
        exposed to a very high risk.” This is true for many individual risk factors such as salt
        intake (linked to high blood pressure and cardiovascular disease) and speeding on the
        highway (linked to injuries and accidents). Does it apply to many other global health
        risks? The study by Anthony Rodgers and colleagues suggests that it does.
        To develop effective health policies, one must understand the existing health risks and
        disease burdens. On a worldwide scale, this is a tough challenge. The Global Burden of
        Disease Database, maintained by the World Health Organization (WHO), collects data from
        countries around the world on risk factors such as tobacco, malnutrition, childhood abuse,
        unsafe sex, childbirth, and cholesterol levels, as well as on disease burdens, for example
        depression, blindness, and diarrhea. A large group of scientists from all over the world
        has developed a framework to analyze these data. To compare different risks or burdens,
        they calculate disability-adjusted life-years, or DALYs—the number of healthy life years
        lost because of a particular disease or risk factor.
        Rodgers and colleagues used data from the WHO database for 26 risk factors and from 14
        epidemiological subregions of the world to calculate the proportion of
        risk-factor-attributable disease burden in different population subgroups defined by age,
        sex, and exposure level. For being underweight in childhood, for example—the leading risk
        factor for global loss of healthy life—they found that only 35% of the disease burden
        occurred in severely underweight children, the rest occurred in those only moderately
        underweight. The relative risks for the moderately underweight are much lower, but the
        number of children in that category is so large that the total attributable burden amounted
        to almost two-thirds of the total global burden of disease for that risk factor.
        The analysis confirms—and extends to a global level—previous research showing that many
        major health risks are important across the range of exposure levels, not just among
        individuals exposed to high levels of risk. It also points to risk factors that are
        particularly prevalent among specific populations and age groups, and for which highly
        targeted interventions could be effective.
        Despite numerous caveats and limitations of studies like this one, such analyses are
        essential aids in guiding the distribution of limited funds to lower the burden of life
        years lost to premature death and disability.