Date Published: September 9, 2014
Source: Proceedings of the National Academy of Sciences of
the United States of America
Institutions Involved: Northwestern University, Children's Hospital of Michigan, Wayne University School of Medicine, Harvard University, George Washington University, Icahn School of Medicine, and University of Illinois
Summary: A study reveals that the rate of glucose uptake by
the brain peaks during childhood, not infancy as previously believed. This new
finding has implications regarding the evolution of a slowed period of growth.
The human brain is proportionally large compared to the
brains of other animals. Developing and maintaining a large brain is energy
intensive; previous research suggests that brain development in infancy accounts
for 87 percent of the baby’s resting metabolic rate. Consequently, scientists
previously believed that the rate of glucose uptake by the brain was greatest
at infancy.
A new study claims that the absolute and relative rates of
glucose uptake actually peak during childhood, the age when neural metabolic
processes are maximal. Researchers measured the rate of glucose uptake by using
Positron Emission Tomography (PET Scan). Researchers accounted for variables by
calculating the brain’s rate of glucose uptake as different percentages of
daily energy requirements and resting metabolic rate. These percentages show how
much of daily energy needs could be met by breaking down the glucose that the
brain takes up.
The results showed that at birth, the glucose uptake is 52.5
percent of resting metabolic rate in males and 59.8 of resting metabolic rate
in females. However, at around age four-and-a-half, the rate of glucose uptake
by the brain is 66.3 percent of resting metabolic rate in boys and 65.0 percent
of resting metabolic rate in females. After childhood, glucose uptake rates
begin to decrease. From this data, researchers suggest that peak uptake of
glucose happens in childhood. The measurements reveal that the peak rate of
glucose uptake by the brain happens specifically around year five of human
development.
The second idea in the study deals with human growth; in
this study, human growth is measured in terms of change in height and weight.
Humans grow quickly during infancy, slow down during childhood, speed up again
after they reach puberty, and slow down as they exit puberty. The primary
interest is childhood, and why it experiences a slowed physical growth rate.
The data collected in this study support the hypothesis that
slowed physical growth offsets the metabolic costs of increased brain development.
Researchers explain that slowing childhood growth and increased brain
metabolism probably coevolved. The study makes the point that the evolution of
a slow childhood growth rate probably required cultural influences such as
transitioning into a hunter-gatherer society and having social networks to care
for young children. Brain development is an energy intensive process that is
vulnerable to shortfall of resources. The transition into the hunter-gatherer
society provided the calorically rich diet and the social networks helped
buffer shortfall. Researchers state that the period of slowed physical growth
allows the brain to develop the mental processes that give humans their
cognitive ability.
Jean Piaget, a developmental psychologist explains that mental
development occurs in five stages. This research article maintains that peak glucose
uptake and maximal neural metabolic processes occur at age five. According to
Piaget, age five marks the midpoint of the preoperational stage. During the
preoperational phase, children learn how to think symbolically, and they develop
memory and imagination. These basic mental processes are the foundations for
the abstract thinking and rationality that makes humans unique. Researchers
conclude that slowed childhood growth evolved in order to allow the brain to
develop these cognitive fundamentals.
This research provides insight into the development of the
human brain and into the evolution of human life history. By learning more
about how human life history evolved, scientists can better understand the
evolution and speciation of other primates and mammals.
Citation:
Kuzawa, C. K., H. T. Chugani, L. I. Grossman, L. Lipovich,
O. Muzik, P. R. Hof, D. E. Wildman, C. C. Sherwood, W. R. Leonard, and N.
Lange. 2014. Metabolic Costs and Evolutionary Implications of Human Brain
Development. Proceedings of the National Academy of Sciences of the United
States of America 111:13010-13015
