Extended childhood: Brain volume

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  • Published: Sep 1, 2014
  • Author: David Bradley
  • Channels: MRI Spectroscopy
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Brain versus body

Metabolic costs and evolutionary implications of human brain development

Pooled data from positron emission tomography (PET) and magnetic resonance imaging (MRI) scans of children's brains suggests that the brain increases in overall volume rapidly and using a large proportion of ingested nutrients during the period of lowest body growth in children at age about 4-5 years. The finding hints at a possible explanation as to why humans grow so slowly and have such a long childhood compared with most other mammals.

Anthropologists at Northwestern University in Evanston, Illinois, USA, reckon a five-year old child's brain is "an energy monster". It uses twice as much glucose as that of a fully grown adult, the monstrous finding could help explain in part why human children seem to take so long to grow up when compared with our closest ape relatives. The work detailed in the last issue in August of the journal Proceedings of the National Academy of Sciences suggests that energy is funnelled to the brain early in life so that brain development predominates the human body's metabolic profile rather than nutrition and energy being expended primarily in the rapid race to reach reproductive adulthood as is seen in other mammals.

Precocious peaking

The results emerge from pooled data from existing PET and MRI brain scans, which measure glucose uptake and brain volume, respectively. The developing brain gobbles up glucose fast and furious during the toddler years and brain activity is at a peak from about 4 to 5 years when body growth is at its slowest. Indeed, the brain uses the equivalent of 66 percent of what the entire body uses at rest at this age.

"Our findings suggest that our bodies can't afford to grow faster during the toddler and childhood years because a huge quantity of resources is required to fuel the developing human brain," explains study leader Christopher Kuzawa. "As humans we have so much to learn, and that learning requires a complex and energy-hungry brain." This finding lends credence to a long-standing hypothesis in anthropology that children grow so slowly, and are dependent on their parents or others for so long, because the human body needs to shunt a huge fraction of its resources to the brain during childhood, leaving little to be devoted to body growth. It also helps explain some common observations that many parents may have.

Toddler estimates

For instance, it is difficult to estimate the age of a toddler after a certain age based on their size because they grow so slowly for many months at the ages when the brain is developing rapidly. However, if you listen to their speech, watch their behaviour, the pictures they draw, the games they play you will see significant differences as the months roll by. "Our study suggests that this is no accident," says Kuzawa. "Body growth grinds nearly to a halt at the ages when brain development is happening at a lightning pace, because the brain is sapping up the available resources."

Earlier studies using different methods posited that the brain's appetite for glucose was greatest at birth at the point at which the organ's size relative to the body as a whole is at its greatest. But, the brain seems to achieve maximum glucose dependence at age 5 years, during the prior year it will account for more than 40 percent of the body's total energy expenditure.

"The mid-childhood peak in brain costs has to do with the fact that synapses, connections in the brain, max out at this age, when we learn so many of the things we need to know to be successful humans," Kuzawa explains. During this time children are their least physically active too having gone through the tearaway twos and threes.

"The study gives us insights into what we believe are some of the forces that have shaped important characteristics of childhood in all humans," Kuzawa told SpectroscopyNOW. "As a next step, we hope to explore newer imaging methods that could allow brain metabolism to be measured repeatedly in the same child as they age, which would allow us to track changes in brain metabolism in individual children and give us insights into how variable these developmental processes are within populations."

Related Links

Proc Natl Acad Sci (USA), 2014, online: "Metabolic costs and evolutionary implications of human brain development"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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