Energy use by the brain is quite severe, considering how little of our body is made up by grey matter. According to a report published in 2012, the brain weights only 2 per cent of the total body, yet requires 20 per cent of our daily energy needs. And even older research supports the theory that thinking harder burns more glucose.
A notable 2001 study out of Northumbria University had half of their test subjects complete complex verbal and maths problems, while the other half mindlessly pressed a key on repeat. Those who were completing the verbal tasks showed a significant drop in blood glucose levels, as a suggested result of greater energy expenditure.
"A period of intense cognitive processing leads to a measurable decrease in levels of peripherally measured blood glucose, which may be linked to increased neural energy expenditure," says the research.
“If we were to put you in a scanner and we looked at what’s going on [in your brain] while in front of the TV or doing a crossword, your brain’s activity would change if we gave you a demanding task, and it would use more energy,” said Dr. Marcus Raichle from Washington University School of Medicine in St. Louis in Time's investigation into the matter.
The reemerging research is a another revelation for those looking to cheat the system and burn calories without exercise, following on from another study this week, which suggests that regular saunas mimics the effects of cardio training sessions.