Only four countries included in the first study of 18 high-income nations — Australia, Japan, Denmark and Norway — showed increasing rates for both men and women in 2015.
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The other 14, including Australia, Japan, Spain, Sweden, the United Kingdom and the United States all saw declines between 2014 and 2015.
According to the latest figures from the Atlanta-based Centers for Disease Control and Prevention, the life expectancy for men and women in the U.S. in 2016 was 76.1 and 81.1 years.
"This is the first time in recent decades that these many high income countries simultaneously experienced declines in life expectancy for both men and women, and the size of these declines were larger than in the past," Science Daily reported.
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Furthermore, according to the researchers, deaths in the U.S. were concentrated among younger adults in their 20s and 30s, whereas declines in overall life expectancy outside the U.S. focused on people 65 and older.
“The USA now has the lowest life expectancy levels among high income developed countries, and Americans fare poorly across a broad set of ages, health conditions, and causes of death compared with their counterparts in these countries,” according to the study.
This concentrated decline among a younger cohort is strongly related to the “ongoing, large scale drug overdose epidemic stemming from misuse of prescription opioids, heroin, and fentanyl,” researchers wrote, noting that compared to other countries, the U.S. actually performs relatively well at older ages.
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Since 2000, the opioid epidemic has claimed more than 300,000 lives and every day, 115 Americans die from prescription opioids and illicit opioids, according to the CDC.
The number of overdose deaths involving opioids in 2016 was five times higher than in 1999.
The second study featured in the journal, based on 1996-2016 death rate data for people aged 25-64 years, suggested “the opioid epidemic is the tip of an iceberg,” study author Steven Woolf said in a statement.
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“No single factor, such as opioids, explains this phenomenon” of dramatic declines in mortality rates across multiple racial and ethnic groups in the U.S. and U.K., researchers wrote.
Woolf and his colleagues acknowledged that drug overdoses, suicides and alcoholism are the leading cause of added deaths, but heart disease, influenza, lung disease, Alzheimer’s, mental disorders and “deaths of despair” signal “a systemic cause” of worsening health conditions in America.
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While their findings revealed overall death rates were higher among men, fatal drug overdoses and suicides were greater among women, adding to previous research about health care disadvantages among women.
And, according to social gerontology expert Holly Nelson-Becker, who was not involved in either study, countries which increasing environmental threats may be more susceptible to influenza and respiratory problems, she told CNN.
"The fact that modern healthcare systems in the most advanced high-income countries were unable to cope with this unexpected challenge, resulting in the first reductions in longevity for decades, is striking and might signal more profound problems," Domantas Jasilionis, researcher at the Max Planck Institute for Demographic Research in Germany, wrote in an accompanying editorial.
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Jasilionis believes the declines in the U.K. and U.S. are perpetuated by social inequality, poverty and declining health care quality.
“More effort must be made to convince international and national agencies to invest in robust register based systems that allow timely and accurate monitoring of changes in longevity,” he added.
Because both studies were observational in nature, the authors note that they could not establish cause and effect.
Read the full research article at bmj.com.