A hungry brain slurps up a kid’s energy

Ask a parent of adult children and they’ll say that their kids grew up too fast, in a blink-and-you’ll-miss-it kind of way. But ask an anthropologist, and you’ll get the opposite answer. Biologically speaking, human children actually take their sweet time growing up. Compared with other animals, even other primates, the long expanse of human childhood stands out as particularly sluggish.

In a new study, researchers offer an explanation for our extended childhoods: A child’s body remains small for so long because the brain hogs all the energy. A 5-year-old’s brain actually burns more glucose than an adult’s, anthropologist Christopher Kuzawa of Northwestern University in Evanston, Ill. and colleagues found.

For years, scientists have kicked around the idea that humans’ “expensive brains” delay body growth. But hard data has been hard to come by. Sorting out exactly how much energy a child’s brain consumes required some creativity. “It took us years to figure out how to do this,” Kuzawa says. PET imaging, a method that estimates the brain’s energy consumption, involves getting a radiolabeled tracer into the brain, a technique that carries risks that make it unacceptable to use on children for research alone. The researchers instead used an older dataset of PET scans on 29 children suspected to have a brain disorder and who later turned out to be healthy.

The researchers combined the energy consumption measurements in those children with brain sizes and body weights of other children. That combination allowed the scientists to calculate how much energy each gram of brain tissue burned, and more importantly, how those numbers relate to body weight.  

When Kuzawa finally sat down with all the data and plotted the trends, two gorgeous, opposing curves materialized: The brain was burning the most energy exactly when the body’s growth slowed, he found. “I was blown away when I plotted that,” he says. “I couldn’t believe how clean it was. We hypothesized that it would be there, but I didn’t think the evidence would be that clean.” 

Around age 4, the brain accounts for a whopping 43 percent of the body’s total energy expenditure, the team estimates, a massive allotment that might divert resources from a growing body. Interestingly, this age might also be around the time children physically slow down a little bit. Activity levels of preschoolers seem to be a bit lower than those of older children, earlier studies have suggested. The body might be taking a rest and instead sending its resources to the energy-hogging brain.

This brain-body tradeoff emerges around 6 months as the brain starts slurping up more glucose and the body’s weight gain starts to slow, the team reports August 25 in the Proceedings of the National Academy of Sciences. The trend starts to reverse near the beginning of puberty, in boys around ages 12 or 13 and in girls around ages 9 or 10.

By pinpointing when the brain is demanding the most resources, the new study offers more than interesting playground chatter for parents. The results could have serious policy implications, Kuzawa says. Knowing exactly when the brain needs the most energy could help inform programs aimed at fighting childhood hunger. “If the brain requires so much energy at that age, what are the implications of malnutrition?” Kuzawa says. Hunger is never good, but it may be especially damaging in the preschool years when the brain is hungriest.

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