Chronic undernutrition kills over 3 million children under 5 every year and causes permanent harm, like growth failure, in many others. At least 1/3 of newborns are not receiving optimal feeding with colostrum, the first milk produced after birth. Colostrum is very rich in bioactive compounds that may be critical in shaping the microbiota and promoting growth. Researchers tested this hypothesis using a colostrum deprivation model where newborn mice were nursed by mothers producing mature milk instead of colostrum. This ‘age-inappropriate feeding’ led to growth failure and changes in the gut microbiota. The growth failure after colostrum deprivation was due to growth hormone resistance, despite absence of the two main known causes of this resistance, undernutrition and infection. And the benefits of colostrum persisted in experiments with germ-free mice. So, while colostrum did influence gut microbiota development, those changes were not related to colostrum’s growth support. Research in human cohorts is needed to confirm these findings, but they suggest that consuming colostrum right after birth is critical for healthy growth and that nutrition interventions need to be developmentally appropriate. Research building on these findings could lead to new, better ways to protect vulnerable newborns from the harms of undernutrition.