Cancer has become the leading cause of death in rich nations, overtaking heart disease, according to the results of two landmark, decade-long global surveys of health trends released Tuesday.
Heart disease remains the leading cause of mortality among middle-aged adults globally, accounting for more than 40 percent of deaths, the data showed.
It was thought to have been responsible for around 17.7 million deaths in 2017.
But in richer countries, cancer now kills more people than heart disease, according to the twin studies published in The Lancet medical journal.
"The world is witnessing a new epidemiologic transition among the different categories of non-communicable diseases, with cardiovascular disease no longer the leading cause of death in high-income countries," said Gilles Deganais, emeritus professor at Laval University, in Quebec.
He said his team's study showed that cancer was the second most common cause of death globally in 2017, accounting for just over a quarter (26 percent) of all deaths.
Deganais said that as heart disease rates fell globally, cancer could become the leading cause of death worldwide "within just a few decades".
The study followed more than 160,000 adults, in high-, middle-, and low-income countries over the course of decade. It determined that people in poorer nations were on average 2.5 times more likely to die from heart disease than those in richer ones.
It conversely found that non-infectious diseases such as cancer and pneumonia were less common in low-income states than in richer ones.
A second study, also by researchers in Canada, and looking at data from patients in the same 21 countries, found that so-called "modifiable risk factors" accounted for 70 percent of heart disease cases globally.
These included diet, behavioural and socioeconomic factors, they said.
Metabolic risk factors -- high cholesterol, obesity or diabetes -- caused more than 40 percent of all heart disease, and were by far the biggest determinant of disease in richer nations.
But there was also a strong link between heart disease in developing countries and household air pollution, poor diet and low education levels.
"A change in tack is required to alleviate the disproportionately high impact of cardiovascular disease in low- and middle-income countries," said Salim Yusuf, professor of medicine at McMaster University.
"Governments in these countries need to start investing a greater portion of their gross domestic product in preventing and managing non-communicable diseases including cardiovascular disease, rather than focusing largely on infectious diseases."