Humans have managed to pull off an interesting trick: although we evolved a large and energetically costly brain, our body’s basal energy expenditure in adulthood is the same as predicted by metabolic scaling laws for a mammal of our size. During infancy and childhood, the brain is especially large relative to body size, implying that it requires an even larger portion of the body’s energy pool at these ages. I and my collaborators used PET and MRI data to show that the fraction of the body’s resting metabolic rate devoted to the brain is 50% at birth, drops to 35% by 6 months, then rises to a lifetime peak of 66% of RMR at 4 years of age, reflecting the high energetic costs of synaptogenesis. We further showed that brain energetics and body weight growth rate are tightly, linearly, inversely correlated, establishing that the high costs of brain development help explain our species’ unusually slow and protracted period of childhood growth. These findings also highlighted the likely importance of our uniquely high brain costs (which rely primarily upon glucose) to the developmental etiology of diabetes. Other collaborative work has shown that the human pattern of brain energy expenditure is an embellishment of a similar underlying pattern of costly, prolonged synaptogenesis that is also present in our closest living relative, the chimpanzee. We have also shown that peak brain energetics coincides with the timing of the adiposity rebound – the age of slowest growth and lowest body fat stores in childhood. Because the timing of the adiposity rebound is predictive of long-term body composition trajectories, this work points to an important role of the brain and brain energetics in lifelong body composition trajectories.
Figure: Concordance between peak brain energetics in childhood (total cerebral blood flow, red line) and the lifetime nadir in adiposity (the adiposity rebound). This work shows that the age of peak brain energy needs is also an age of lowest adiposity, and also reveals an inverse relationship between these traits across the full life cycle (Aronoff et al International Journal of Obesity, 2022).