Short fasts do not impair thinking ability in healthy adults, study says

Skipping breakfast or practicing intermittent fasting is unlikely to cloud most adults’ thinking in the short term, according to research published in the journal Psychological Bulletin.

Fasting, which can involve abstaining from food for several hours to days, has become one of the more popular contemporary eating regimens, often promoted for potential health benefits such as improved insulin sensitivity, cellular repair and weight management.

“While fasting has become trendy over the years, there is widespread concern, often reflected in common sayings like, “You’re not you when you’re hungry,” that going without food might acutely impair mental sharpness,” said David Moreau, Ph.D., associate professor of psychology at the University of Auckland and the study’s lead author.

“Given how important it is to sustain adequate levels of cognitive performance for professional and personal activities, the potential side effects warrant careful, systematic examination.”

Researchers conducted a meta-analysis of 71 studies that compared cognitive performance in healthy adults who were either fasting or had recently eaten.

The studies assessed abilities such as memory recall, decision-making, and response speed and accuracy. Altogether, the analysis included a total of 3,484 participants. Most fasting periods were short-term, with a median duration of 12 hours.

“Our main finding was that there is generally no consistent evidence that short-term fasting impaired mental performance,” Moreau said.

“Individuals who fasted performed remarkably similarly to those who had recently eaten, suggesting cognitive function remains stable in the absence of food intake.”

While the study found no meaningful overall difference, the researchers noted some nuances. Cognitive performance showed modest reductions for fasting intervals longer than 12 hours, and children, who made up a small portion of the dataset, exhibited greater performance deficits compared with adults.

“We were certainly surprised in one sense, as our results contradict the widespread assumption that fasting inherently compromises thinking ability,” Moreau said. “Across a broad range of diverse tasks, cognitive performance remained remarkably stable. Many people believe that missing a meal leads to immediate declines in mental acuity, but our synthesis of the evidence suggests otherwise.”

One of the most intriguing findings was that fasting’s effects depended on context.

“Performance deficits were often evident only in tasks involving food-related stimuli, such as looking at pictures of food or processing food-related words,” Moreau said.

“In contrast, performance on tasks using neutral content was largely unaffected. Hunger might selectively divert cognitive resources or cause distraction only in food-relevant contexts, but general cognitive functioning remains largely stable.”

The researchers also highlighted differences by age.

“Age was a powerful and salient moderator,” Moreau said. “Children showed noticeable declines in performance while fasting, echoing earlier studies that highlighted the steady cognitive advantages of eating breakfast in younger age groups.

“Our data support the idea that pediatric populations may require particular consideration when evaluating fasting interventions, suggesting that the developing brain has a heightened vulnerability to energy unavailability.”

Beyond the laboratory, the findings carry broad implications for public health and fasting practices, according to Moreau.

“The primary takeaway is a message of reassurance: Cognitive performance remains stable during short-term fasting, suggesting that most healthy adults need not worry about temporary fasting affecting their mental sharpness or ability to perform daily tasks,” he said.

“Physiologically, fasting triggers important metabolic shifts. When glycogen stores are depleted, the body uses ketone bodies produced from fat tissue as an alternative energy source. Emerging evidence suggests that relying on ketones may confer broad health benefits, modulate hormonal systems, and activate cellular repair processes linked to longevity.”

According to the researchers, these findings support the feasibility of intermittent fasting as a health intervention for adults while underscoring the importance of tailoring fasting practices to specific populations, such as children or individuals with medical conditions.