To save a starving child, aid workers have long used one obvious treatment: food. But a new study suggests feeding their gut bacteria may be as important—or even more important—than feeding their stomachs. In a head-to-head comparison against a leading treatment for malnutrition, a new supplement designed to promote helpful gut bacteria led to signs of improved growth and more weight gain, despite having 20% fewer calories. The study also highlights how important gut bacteria—the so-called microbiome—can be to human health.