Does intermittent fasting affect mental performance? A global review weighs the evidence

The health benefits of intermittent fasting are proven – but are you a little less sharp mentally when you’re deprived of food?

A review of 71 studies from around the world offers reassurance for adults, finding no meaningful difference in mental performance when going without food for between about eight and 24 hours.

“People often worry that if they fast they won’t be able to concentrate at work or study effectively,” says Dr David Moreau, a neuroscientist at the University of Auckland who was the senior author of the paper in Psychological Bulletin.

“Our results show that, for most adults, short-term fasting is unlikely to have a major impact on mental sharpness.”

However, children and adolescents tended to perform more poorly, reinforcing the importance of a proper breakfast before school.

There were also some nuances.

Fasting people tended to perform worse in tests later in the day, suggesting the lack of food may amplify natural dips in circadian rhythm.  

Tasks involving food-related cues, such as judging portion sizes or responding to food images, seemed to impede performance, perhaps because hungry individuals get distracted.   

“It seems the brain is quite resilient in the face of temporary food shortages,” says Moreau, who works in the University’s School of Psychology, leading the Brain Dynamics Lab.

“Humans evolved with periods of food scarcity, so it makes sense our cognitive systems can function well without constant refuelling,” he says. “That said, children and teenagers appear more vulnerable, which fits with what we know about their high energy demands.”

In the studies, the median fasting duration was 12 hours and only a few extended beyond 24 hours.

Lead author Dr Christoph Bamberg was a PhD student at the University of Auckland and is now at Lodron University Salzburg.

“These results don’t mean fasting is risk-free for everyone,” says Moreau. “People with health conditions, children, and those fasting for extended periods may still experience negative effects. Our review highlights the need for careful consideration in these groups.”

The study was supported by the Marsden Fund and an Early Career Research Excellence Award from the University.