A third of the poorest countries in the world are dealing with high levels of obesity as well as under-nourishment, which leaves people too thin, according to a report in The Lancet.
It says the problem is caused by global access to ultra-processed foods, and people exercising less.
The authors are calling for changes to the "modern food system" which they believe to be driving it.
Countries in sub-Saharan Africa and Asia are most affected.
The report estimates that nearly 2.3 billion children and adults on the planet are overweight, and more than 150 million children have stunted growth.
And many low and middle-income countries are facing these two issues at once - known as the 'double burden of malnutrition'.
This means that 20% of people are overweight, 30% of children under four are not growing properly, and 20% of women are classified as thin.
Communities and families can be affected by both forms of malnutrition, as well as individual people at different points in their lives, the report says.
According to the report, 45 out of 123 countries were affected by the burden in the 1990s, and 48 out of 126 countries in the 2010s.
By the 2010s, 14 countries with some of the lowest incomes in the world had developed this 'double problem' since the 1990s.
Failing food systems
The report authors say action should be taken by governments, the United Nations and academics to address the problem, and it points the finger at changing diets.
The way people eat, drink and move is changing. Increasing numbers of supermarkets, easy availability of less nutritious food, as well as a decrease in physical activity, are leading to more people becoming overweight.
And these changes are affecting low and middle-income countries, as well as high-income ones.
Although stunted growth of children in many countries is becoming less frequent, eating ultra-processed foods early in life is linked to poor growth.
"We are facing a new nutrition reality," says lead author Dr Francesco Branca, director of the department of nutrition for health and development at the World Health Organization.
"We can no longer characterise countries as low-income and undernourished, or high-income and only concerned with obesity.
"All forms of malnutrition have a common denominator - food systems that fail to provide all people with healthy, safe, affordable, and sustainable diets."
Dr Branca said changing this needed changes in food systems - from production and processing, through trade and distribution, pricing, marketing, and labelling, to consumption and waste.
"All relevant policies and investments must be radically re-examined," he said.
What is a high-quality diet?
According to the report, it contains:
- lots of fruits and vegetables, wholegrains, fibre, nuts, and seeds
- modest amounts of animal source foods
- minimal amounts of processed meats
- minimal amounts of food and beverages high in energy and added sugar, saturated fat, trans fat and salt
High-quality diets reduce the risk of malnutrition by encouraging healthy growth, development, and the body's protection against diseases throughout life.