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. 

"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 one quarter 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 a 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.