Have you ever noticed that it’s kind of hard to guess the age of a toddler until they open their mouths and speak? I’ve got a behemoth of a nephew who could have passed for four at age two based on size alone. But at the same time, he was a bit late to the language game, and could potentially have been mistaken for a younger age around his fourth birthday (at least, if he hadn’t been such a behemoth).
There’s a good reason for this, according to a new study from Northwestern University. A human child’s physical growth slows to a snail’s pace between the ages of two and five. That’s why it’s so hard to guess age based on size at that time. So where is all of that energy going to?
Straight to their noggins.
After analyzing a pool of existing PET and MRI brain scan data–which measure glucose uptake and brain volume, respectively–Christopher Kuzawa, professor of anthropology at Northwestern, found that a toddler’s brain sucks up a staggering 66 percent of the energy normally consumed by the entire body at rest. That’s more than 40 percent of a kid’s total energy expenditure during the day.
It’s at this age where a person’s brain soaks in the most information, busily pruning synapses and strengthening connections based on learning and experience. And with the brain soaking up that much energy, there isn’t really any left to fuel physical growth.
It was previously believed that the brain’s resource burden on the body was largest at birth, when the size of the brain relative to the body is greatest. The researchers found instead that the brain maxes out its glucose use at age five.
“At its peak in childhood, the brain burns through two-thirds of the calories the entire body uses at rest, much more than other primate species,” said William Leonard, professor and chair of Northwestern’s Department of Anthropology. “To compensate for these heavy energy demands of our big brains, children grow more slowly and are less physically active during this age range. Our findings strongly suggest that humans evolved to grow slowly during this time in order to free up fuel for our expensive, busy childhood brains.”
The paper, “Metabolic costs and evolutionary implications of human brain development,” was published in the Proceedings of the National Academy of Sciences by Kuzawa and Leonard as well as Harry T. Chugani, Lawrence I. Grossman, Leonard Lipovich, Otto Muzik, Patrick R. Hof, Derek E. Wildman, Chet C. Sherwood and Nicholas Lange.