Lower-income countries are increasingly facing a double blow from the impact of both undernutrition and obesity, which often coexist within the same community - and are even found within the same families or individuals.
That's the warning from a report published in The Lancet, which found that 14 countries with among the world's lowest incomes had newly developed this "double burden" of malnutrition by the 2010s compared with the 1990s, even though the prevalence fell in higher-income developing countries.
More than a third of low and middle-income countries are estimated to experience these overlapping effects, with around 2.3 billion people overweight globally, and more than 150 million children having stunted growth.
The global health community has been "slow to acknowledge" this dichotomy, with the two types of malnutrition often considered as separate issues rather than interconnected, according to the report.
However, a lack of nutritious food is at the root of both problems, with poor families increasingly exposed to low-quality, ultra-processed foods and drinks amid rapid changes in food systems. That also means people can be exposed to both forms of malnutrition at different points in their lifetimes, or a child can be both stunted and overweight, the report says.
Lead author Francesco Branca, director of the department of nutrition for health and development at the World Health Organization (WHO), which commissioned the report, said: "We don't have undernutrition and overweight in separate places, generated by different causes in different populations. They're actually coexisting in the same countries and communities, and often in the same individuals."
As the lines have become more blurred, it is no longer clear-cut that the key dietary issue will be undernourishment in low-income countries and obesity in high-income ones, he added.
The report found Indonesia to be the largest country with a severe double burden, but the trend was also prevalent in other countries in Sub-Saharan Africa, southern and eastern Asia, and the Pacific. However, improvements were observed in Latin America and the Caribbean, as well as the Middle East and North Africa.
The two types of malnutrition were seen within the same family in more than a quarter of households in some countries, including Azerbaijan, Guatemala, Egypt, and Comoros.
Branca hopes the report will have a significant impact in raising awareness of this double burden, pointing out that while the link has been recognised before, it has not always been acted on. That means countries may have a programme for undernutrition that doesn't take into account obesity, potentially exacerbating the problem.
"Without understanding the intimate interaction between these different conditions, you end up developing separate policies, which is not cost-effective," says Branca. "Malnutrition affects not only health, but human capital and the potential for development."
The report calls for a fresh approach to the issue, requiring "societal shifts that can be scaled up and sustained over decades". It recommends actions ranging from improved antenatal care and breastfeeding practices to new agricultural policies geared towards healthy diets, and modelling how school breakfast programmes can both health and the economy.
Whitney Schott, a demographer at the University of Pennsylvania in Philadelphia, welcomed the report, saying it highlighted that the world's population was "at greater risk of not only stunting and communicable diseases, but simultaneously at risk of non-communicable disease such as heart disease". She also stressed the importance of taking into account differences between the sexes, which may require gender-specific interventions.
Studies by her team have found a trend towards overweight in more well-off households in countries such as Ethiopia, India, Peru and Vietnam, Schott said.
"We can be relatively confident that the trajectories in less developed economies will closely follow those of more developed economies, unless interventions are successful to stop the spread of overindulgence in consumption of energy-dense fatty, salty and sugary foods with poor nutritional content," she warned.