Giant Human Brain Sucks Energy, Stunts Growth for Years

2570892-sbigbrainHave 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.

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About bigkingken

A science writer dedicated to proving that the Big Ten - or the Committee on Institutional Cooperation, if you will - is more than athletics.
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One Response to Giant Human Brain Sucks Energy, Stunts Growth for Years

  1. John Skoyles says:

    I had a response to this work published in PNAS. Essentially they get the explanation wrong: the link is due to a homeostatic conflict between brain and brawn not energy saving.

    Skeletal muscle-induced hypoglycemia risk, not life history energy trade-off, links high child brain glucose use to slow body growth
    http://www.pnas.org/content/111/46/E4909.extract?sid=b4b9fd80-795a-43b2-949c-a3f33060fa64
    further comments
    http://goo.gl/0qU6VB

    The issue is one of the most important in science because Neanderthals grew faster than modern people. Thus this research lets us to put the finger on extended high energy neuromaturation as the critical innovation behind our evolution. Neanderthal and our brains might be the same size but our slower growth as children identifies our brains as undergoing longer energy expensive neuromaturation and so undergoing longer and deeper network refinement –something other research identifies with greater cognitive capacity. There is no bigger story in science.

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