New York, Aug 26 (IANS): Know why your five-year-old kid demand chocolate or ice cream or any other sugary food all the time? To feed his/her “energy monster” brain.

According to a fascinating study, our bodies cannot afford to grow faster during the toddler and childhood years because a huge quantity of resources is required to fuel the developing human brain.

It shows that energy funneled to the brain dominates the human body's metabolism early in life and is likely the reason why humans grow at a pace more typical of a reptile than a mammal during childhood.

“As humans, we have so much to learn and that learning requires a complex and energy-hungry brain,” said Christopher Kuzawa, a professor of anthropology at Northwestern University's Weinberg College of Arts and Sciences.

A five-year old's brain is an energy monster. It uses twice as much glucose (the energy that fuels the brain) as that of a full-grown adult.

The researchers found that the brain maxes out its glucose use at age five.

At age four, the brain consumes glucose at a rate comparable to 66 percent of the body's resting metabolic rate (or 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 explained.

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.

“To compensate for these heavy energy demands of our big brains, children grow more slowly and are less physically active during this age range,” added co-author William Leonard from Northwestern University.

The findings suggest that humans evolved to grow slowly during this time in order to free up fuel for our expensive, busy childhood brains.

The study appeared in the journal Proceedings of the National Academy of Sciences.