A new meta-analysis in Psychological Bulletin pooled 63 studies involving more than 3,400 participants to examine how short-term intermittent fasting (fasts under 24 hours) affects thinking and mental performance. Typical fasting windows in the analyzed studies centered around a median of about 12 hours (most commonly between 8 and 24 hours). Overall, these brief fasts neither consistently improved nor reliably worsened cognitive performance for most healthy adults.
Main findings
– Most adults: Short fasts (median ≈12 hours) produced no large changes in cognitive test performance. Effects were generally small and inconsistent across studies.
– Children and adolescents: Several studies found declines in cognitive function when young people fasted, supporting the importance of breakfast for school-aged children and teens.
– Time of day: Adults sometimes performed worse on tests given later in the day, suggesting fasting can amplify normal circadian dips in alertness.
– Moderators: Outcomes varied with fasting duration, testing context (time of day and task difficulty), and individual characteristics such as age and body composition.
Expert perspective
The paper’s senior author, David Moreau, PhD (University of Auckland), interprets the results as evidence that humans can usually preserve cognitive function during short fasts because of metabolic adaptations. Mir Ali, MD, said the findings may reassure people worried about concentrating while fasting. Debra Safer, MD (Stanford), emphasized that the review addressed short-term fasting only and that more data are needed on longer or repeated fasts, people who are underweight or overweight, and those with medical or psychiatric conditions, including eating disorders.
Why cognition is often preserved
Glycogen (stored glucose) declines during a fast, but the body shifts to alternative fuels—most notably ketone bodies derived from fat. This metabolic flexibility, an evolutionary adaptation to periods of scarce food, helps maintain brain energy and likely explains why short fasts typically do not reduce cognitive performance in adults.
Implications and limitations
– For most healthy adults, brief fasts (commonly around 12 hours) are unlikely to impair mental sharpness.
– Context matters: testing near the end of a fast, later-in-day assessments, younger age, and certain individual factors can increase the chance of measurable decline.
– The review does not answer questions about long or repeated fasting regimens, effects on people with medical conditions or disordered eating, or long-term cognitive outcomes. More research is required in these groups.
What is intermittent fasting?
Intermittent fasting describes patterns that alternate eating and fasting windows and focuses on when to eat rather than what to eat. Common approaches include:
– 16/8: 16-hour fast with an 8-hour eating window (often skipping breakfast or dinner).
– 5:2: Very low calorie intake (~500–600 kcal) on two nonconsecutive days each week.
– Eat Stop Eat: A 24-hour fast once or twice weekly.
Potential benefits reported in short-term studies
Short-term fasting has been associated with weight loss, improved insulin sensitivity, reduced markers of inflammation, rises in human growth hormone, and cellular repair processes such as autophagy. Improved insulin sensitivity during fasting may help prevent or manage type 2 diabetes for some people, though individual responses vary.
Safety, side effects, and practical tips
– Common short-term effects: hunger, and sometimes temporary changes in mood, energy, or concentration—especially in younger people or with longer fasts.
– Who should avoid or be cautious: pregnant or nursing people, anyone with a history of eating disorders, and individuals with certain medical conditions unless supervised by a clinician.
– Fluids: Water, coffee, and tea are typically allowed during fasting windows.
– How to start safely: Gradually extend fasting duration (for example, begin with 6–8 hours, move to 8–10, then 12–14 hours). Focus on nutrient-dense meals, adequate protein and vegetables at the last meal before the fast, moderate carbohydrates to help limit hunger, regular exercise, and good sleep.
Bottom line
Short-term intermittent fasts up to about 24 hours generally do not impair cognitive function in most healthy adults, though children and adolescents appear more vulnerable and cognitive testing late in a fast can reveal performance dips. The findings do not settle questions about longer or repeated fasting, effects in people with health issues, or long-term outcomes—areas that need further study.
