LONDON, Feb 29 (Reuters) - More than a billion people globally are now considered obese, a condition linked to an increased risk of numerous serious health problems, according to updated estimates from the World Health Organization and an international group of researchers.

Obesity is so prevalent it has become more common than being underweight in most nations, including many low and-middle income countries that have previously struggled with undernourishment.

“A staggering number of people are living with obesity,” said Majid Ezzati, senior author of the paper published in The Lancet on Thursday and a professor at Imperial College London.

The findings, considered among the most authoritative of independent estimates, are based on data from more than 220 million people in more than 190 countries.

While obesity rates are plateauing in many wealthier countries, they are rising rapidly elsewhere, Ezzati added. And while being underweight is becoming less common globally, in many countries it remains a significant issue, leaving increasing numbers of countries facing what is known as the “double burden” of malnutrition.

“In the past, we have been thinking of obesity as a problem of the rich. Obesity is a problem of the world,” said Francesco Branca, head of nutrition at the WHO, in a press conference.

Obesity rates for adults more than doubled between 1990 and 2022, and more than quadrupled among children and adolescents aged 5-19, the paper said.

Over the same period, the proportion of girls, boys and adults considered underweight fell by a fifth, a third and half, respectively, the analysis found.

Ezzati called the rise in obesity rates among children “very concerning”, mirroring a trajectory seen with adults since even before 1990. At the same time, he said, hundreds of millions still do not have enough to eat.

Being severely underweight can be very detrimental to childrens' development and, at its most extreme, the condition can cause people to starve to death. Obese people are also at risk of premature death and disability given the link to the early onset of diabetes, heart and kidney disease, and a slew of other serious health conditions.

The rise in the double-burden has been greatest in some low- and middle-income countries, the paper said, including parts of the Caribbean and the Middle East.

In these countries, obesity rates are now higher than in many high-income countries, particularly in Europe. In some European countries like Spain, there are indications obesity rates could be starting to decline or at least stagnate, Ezzati added.

The updates are the first from the team since 2017, and were compiled by more than 1,500 scientists in the Non-Communicable Disease Risk Factor Collaboration. At that point, around 774 million people above the age of 5 were estimated to be living with obesity, a similar proportion - around 1 in 8 people - as the new figures.

WHO Director-General Dr. Tedros Adhanom Ghebreyesus said implementing measures such as taxes on high sugar products and promoting healthy school meals were needed to help tackle obesity rates.

“Importantly, it requires the co-operation of the private sector, which must be accountable for the health impacts of their products,” he added.

Branca and Ezzati said potent new obesity drugs like Novo Nordisk's Wegovy and Eli Lilly's Mounjaro and Zepbound were tools that may help, but their cost and low availability risked further increasing inequality.

There were some limitations to the study, including a lack of data from after the COVID-19 pandemic, and use of body mass index (BMI) to determine obesity, described as an “imperfect” measure by the researchers. (Reporting by Jennifer Rigby Editing by Bill Berkrot)