Manipulating what kinds of bacteria live in the gut might lead to a new way to treat millions of children suffering chronic malnutrition, researchers say.
They say the right microbes in the gut can help get the most out of a poor diet.
Researchers from Washington University in St Louis culled intestinal bacteria from babies and toddlers in Malawi, where malnutrition is a serious problem, and transferred them into mice for study.
Tweaking those gut microbes improved growth - even though the animals didn't eat more, or more nutritiously.
We share our bodies with trillions of bacteria, a customised set called a microbiome that starts building at birth, and Thursday's work is the latest to illustrate how crucial it is to develop a healthy one.
Among the findings: Certain nutrients in breast milk may help that happen.
"If we could hammer home a key point, microbiota count," said Dr Jeffrey Gordon of Washington University, who led the series of experiments published in the journals Science and Cell.
"Building healthy gut microbiota we think is important for health in the course of one's life."
Gut bacteria do more than simply break down food for digestion. They synthesise particular vitamins and micronutrients, and influence immune responses, for example.
"A healthy microbiome will allow us to access calories we might not have been able to use before," explained Dr Ilseung Cho, a gastroenterologist and gut bacteria specialist at New York University School of Medicine, who wasn't involved in the new work.
More research is needed before testing the approach in children, but Cho said the findings suggest there may be "very precise bacteria or very precise nutrient interventions that can unlock the microbiome and help it combat malnutrition".
While providing special "therapeutic foods" and vitamin supplements helps reduce deaths from malnutrition, Gordon said children still experience stunted growth and neurodevelopmental problems.
His team turned to Malawi, where according to UNICEF almost half of children under 5 have growth stunted by malnutrition.
The researchers already suspected gut bacteria played a role, based on previous research with pairs of Malawian twins, only some of whom were affected.
This time, working with more than 250 healthy or undernourished children, Gordon's team defined how a healthy gut microbiome normally develops - and found that the chronically malnourished tots harboured an immature one, too young for their age.
