Short-term intermittent fasting — periods without food lasting less than 24 hours — does not appear to impair cognitive performance in most adults, according to a new meta-analysis published in Psychological Bulletin. Researchers pooled 63 studies with more than 3,400 participants, finding that typical fasting windows (most commonly around 12 hours, and generally between 8 and 24 hours) neither consistently enhance nor reliably impair mental sharpness for adults.
Key findings
– For most adults, short fasts (median ~12 hours) had no major effect on cognitive performance.
– Children and adolescents showed some declines in cognitive function while fasting, underscoring the importance of breakfast for school-aged youth.
– Adults sometimes performed worse on cognitive tests later in the day, suggesting fasting may amplify normal circadian dips in alertness.
– Three factors influenced outcomes: fasting duration, the testing context (time of day and task demands), and individual characteristics such as age and body composition.
Expert perspectives
David Moreau, PhD (University of Auckland), senior author of the paper, said the results suggest humans can maintain cognitive function during short fasting periods, likely because of metabolic adaptations. Mir Ali, MD, noted the findings may reassure people concerned about concentration while fasting. Debra Safer, MD (Stanford), cautioned the review focused on short-term fasting and that more research is needed on longer or repeated fasting, people outside a healthy weight range, and those with medical or psychiatric conditions (including eating disorders).
Metabolic explanation
The researchers explain that while glycogen (stored glucose) falls during fasting, the body shifts toward alternative fuels such as ketone bodies derived from fat. This metabolic flexibility is an evolutionary adaptation that helps sustain brain function during periods of limited food. That flexibility likely explains why short fasting typically does not reduce adult cognitive performance.
Implications and caveats
– Most adults need not worry that short fasting will harm mental sharpness, but context matters: testing near the end of a fast and younger age increase vulnerability.
– The review does not assess long-term or repeated fasting regimens, effects in people with health conditions, or risks for those with disordered eating.
– Children and teenagers appear more sensitive to fasting-related cognitive declines.
What is intermittent fasting?
Intermittent fasting is an eating pattern alternating eating windows and fasting windows, focusing on when to eat rather than what to eat. Common approaches:
– 16/8: 8-hour eating window, 16-hour fast (often skipping breakfast or dinner).
– 5:2: Very low calorie intake (about 500–600 kcal) on two nonconsecutive days per week.
– Eat Stop Eat: 24-hour fast once or twice weekly.
Potential benefits
Short-term fasting has been associated with weight loss, improved insulin sensitivity, reduced inflammation, increased 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.
Safety, side effects, and practical tips
– Common side effects include hunger and potential short-term changes in mood, energy, or concentration, especially in younger people or when fasting is prolonged.
– Intermittent fasting may be inappropriate for pregnant or nursing people, those with a history of eating disorders, or individuals with certain medical conditions unless supervised by a clinician.
– Fluids such as water, coffee, and tea are typically allowed during fasting windows.
– To adopt fasting safely, clinicians recommend starting gradually (e.g., a 6–8 hour fast, progressing to 8–10, then 12–14 hours). Emphasize nutrient-dense meals, adequate protein and vegetables at the final meal, moderate carbohydrate intake to reduce hunger, regular exercise, and sufficient sleep.
Bottom line
For most healthy adults, short-term intermittent fasting up to 24 hours is unlikely to impair cognitive function, though younger people and late-in-fast testing times may show reduced performance. More research is needed on longer fasting durations, repeated/practice fasting, and effects in diverse clinical and demographic populations.
