During human childhood, brain development and body growth compete for limited metabolic resources, resulting in a trade-off where energy allocated to brain development can decrease as body growth accelerates. This preregistered study explores the potential links between language skills, used as a proxy for brain development, and body mass index at 3 distinct developmental stages, serving as proxies for body growth. Analyzing longitudinal data from 2002 children in the EDEN mother-child cohort using structural equation modeling, we identified a compelling pattern of associations: girls with a delayed adiposity rebound, signaling slower growth rate, demonstrated better language proficiency at ages 5-6. Importantly, this correlation appears specific to language skills and does not extend to non-verbal cognitive abilities. Exploratory analyses show that early environmental factors that contribute to enhanced cognitive development, such as higher parental socio-economic status and higher levels of cognitive stimulation, are positively linked to both language skills and the age at which adiposity rebound occurs in girls. Overall, our findings lend support to the existence of an energy allocation trade-off mechanism that appears to prioritize language function over body growth investment, in girls.