While they may not have found the fountain of youth – at least, that we know of – a handful of countries are expected to outlive the rest of us.

Using data compiled from the Global Burden of Disease study analyzing 250 causes of death, researchers forecasted the average life expectancy in 295 nations in the year 2040 under a number of alternative scenario predictions. Best case, all countries will probably experience a slight increase in lifespan over the next two decades. Worse case? Nearly half of nations represented could face lower life expectancies.

The researchers suggest there will be an increase in death from injuries, such as car accidents, and non-communicable diseases, like chronic obstructive pulmonary disease (COPD), chronic kidney disease, diabetes, lung cancer, and other health issues linked to obesity. The top driving factors contributing to these deaths include high blood pressure, body mass index, blood sugar, tobacco and alcohol use, and air pollution. But these gains and falls will be felt disproportionately across the world.

"The future of the world's health is not pre-ordained, and there is a wide range of plausible trajectories," said lead author Dr Kyle Foreman in a statement. "But whether we see significant progress or stagnation depends on how well or poorly health systems address key health drivers."

In 2016, China’s average life expectancy was 76.3 years, putting the nation at 68th in the world. In 2040, life expectancy could jump to 81.9 years for a rank of 39th. Syria is expected to rise the most, from 137th to 80th, probably because of the author’s conservative model for resolving conflict. Other countries with significant gains include Nigeria (157 to 123) and Indonesia (117 to 100).  

On the other hand, the US could see the highest dip of all the high-income countries with an increase of just 1.1 years (79.8), dropping it 21 spots to 64th, followed by Canada, Norway, Belgium, and the Netherlands.

Spain takes the cake as the healthiest country in the world with an average lifespan of 85.8, just barely beating Japan (85.7 years). Next up are Singapore (85.4), Switzerland (85.2), Portugal (84.5), Italy (84.5), Israel (84.4), France (84.3), followed by Luxembourg and Australia (84.1).

The sum of their findings suggests that life expectancy could decrease in almost half of the countries, with just 57 seeing an increase of one year or more. A “better” scenario sees 158 countries with a jump in life expectancy of at least five years and 46 nations with as many as 10.

Here’s where the disparity kicks in. Even in those top 10 countries, their worst-case scenario still puts the average lifespan above 80 years. Other bottom-ranked countries – such as Lesotho, Swaziland, Central Africa Republic, and South Africa – could see a drop in as many as 30 years from best to worse case. This is largely due to socioeconomic factors like fertility, per capita income, access to clean water, and education rates – but the authors say it’s not too late to change these outcomes.

"The range of 'better' and 'worse' scenarios enables stakeholders to examine potential changes to improve health systems – locally, nationally, and globally," Murray said. "These scenarios offer new insights and help to frame health planning, especially regarding long lag periods between initial investments and their impacts, such as in the research and development of drugs."