The global rate of obesity has quadrupled in children and doubled in adults since 1990, according to a new analysis published in The Lancet, a medical journal, March 1, 2024.

About one billion people in the world — that's 1 in 8 of the global population — are obese: They have a body mass index (BMI) over 30.

The World Health Organization describes the BMI as "a simple index of weight-for-height that is commonly used to classify overweight and obesity in adults."

It is calculated by dividing a person's weight in kilograms by the square of their height in meters (kg/m2).

Francesco Branca, Director of the WHO's Department of Nutrition and Food Safety, said the organization had previously estimated that the global rate of obesity would hit a billion people in 2030. But that marker was hit eight years early — in 2022.

Speaking at a press conference about the new Lancet study, its co-author and professor of public health at Imperial College London, Majid Ezzati, said they were "taken aback" by how fast obesity rates had developed.

But this increase in obesity is not happening where you might expect — in rich countries.

The new data shows that while obesity rates are generally starting to plateau in many rich countries, they are growing rapidly among both adults and children in low-to-middle income countries, such as Egypt, Iraq, Libya, South Africa and Chile, with Syria, Turkey and Mexico not far behind.

"None of the traditional industrialized world or wealthy nations, except for the United States, appear in the top group [of the most obese nations]. It's almost exclusively covered by low-to-middle-income countries," said Ezzati.

The new data shows some progress in reducing the number of people who are starving across the globe.

Over the past 30 years, the global proportion of underweight adults has halved. It fell by a fifth in girls and a third in boys under the age of 18 years.

However, the study shows that in some countries the situation has not improved.

The proportion of underweight adults in countries like Ethiopia and Uganda, for example, has barely changed.

Other countries, such as India, Bangladesh and Pakistan, have seen the proportion of underweight adults decline sharply.

But Pakistan appears to have traded one form of malnourishment for another.

While the proportion of underweight adults has dropped from 27% to 7% since 1990, the proportion of obese adults has risen from 3% to 24% in the same period. That's a higher rate of obesity than most countries in the European Union.

Some sub-Saharan African countries have seen the same sort of swap, particularly among women. Where the number of people underweight has gone down, the number of those with obesity has gone up.

Branca said the increase in obesity has been so sharp in low-to-middle income countries compared to wealthier countries for a few reasons: There's been a transformation in food production, then there's "the biology of the double burden", and finally a lack of public health policies.

We explain the impact of each of these below:

First, the past 30 years have been marked by rapid industrialization in countries like Egypt and Mexico. They have transformed their food systems, especially in urban areas, Branca said.

"The sales of processed food or drinks or the number of supermarkets and outlets — that's where the largest increase is," said Branca. "[It's a] very rapid transformation of that food system and not towards the better."

Second, the biology of the double burden, said Branca, refers to people who had a low birth weight or did not get enough food to eat as children. They are often more prone to becoming overweight or obese as adults. This could potentially help explain the changes seen in sub-Saharan Africa.

And third, there's the lack of government policy and, as a result, a lack action by health departments to improve people's access to healthy food choices.

Unlike wealthier countries, said Branca, many low-to-middle income countries have very few or zero policies in place to curb the "enormous" pressure created by marketing that pushes foods that are high in fat, sugar and salt.

"The important story here is that in the past we [thought] of obesity as a problem of the rich," said Branca — but obesity is a problem for the whole world.