Objective: We here studied how energy is allocated between brain and body both during the ontogenetic development from a child to an adult and during weight loss. Methods: We investigated 180 normal weight female and male children and adolescents (aged 6.1-19.9 years) as well as 35 overweight adolescents undergoing weight reduction intervention. 52 normal weight and 42 obese adult women were used for comparison. We assessed brain mass by magnetic-resonance-imaging and body metabolism by indirect calorimetry. To study how energy is allocated between brain and body, we measured plasma insulin, since insulin fulfils the functions of a glucose allocating hormone, i.e., peripheral glucose uptake depends on insulin, central uptake does not. We used reference data obtained in the field of comparative biology. In a brain-body-plot, we calculated the distance between each subject and a reference mammal of comparable size and named the distance "encephalic measure." With higher encephalic measures, more energy is allocated to the brain. Results: We found that ontogenetic development from a child to an adult was indicated by decreasing encephalic measures in females (r=-0.729, P<0.001) and increasing plasma insulin concentrations (F=6.6, P=0.002 in females and F=8.6, P<0.001 in males). Weight loss of about 5 kg in females and about 9 kg in males resulted in reduced insulin concentrations and increased encephalic measures. Conclusion: Our results indicate that the share of energy allocated to the brain increased with weight loss, but decreased during the ontogenetic development from childhood to adolescence. These developmental changes in brain-to-body energy allocation appear to be driven by increasing plasma insulin concentrations. Am. J. Hum. Biol. 25:725-732, 2013.