Fasting and Intelligence: Does Fasting Make You Smarter or Slower?
Fasting and intelligence are barely linked. Across 3,484 healthy adults, short fasts shifted cognitive test performance by only about 0.3 IQ points.
Dr. Russell T. WarneChief Scientist
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Fasting has no meaningful effect on intelligence. A 2025 meta-analysis of 3,484 healthy adults found that people who had fasted (for a median of 12 hours) performed almost exactly as well on cognitive tasks as people who had eaten, a difference of g = 0.02, or about 0.3 IQ points.
The picture changes a little at the edges. Longer fasts and younger participants showed modest drops, children who skip breakfast tend to do slightly worse on the same morning's tasks, and a month of daytime fasting during Ramadan has lowered exam grades in one well-designed study. This page covers what the best studies show, how the brain fuels itself when you stop eating, why intermittent fasting is marketed as a brain booster, and who should talk to a clinician first.
Does fasting affect intelligence?
The strongest evidence comes from Christoph Bamberg and David Moreau, who pooled experimental studies comparing fasted and fed healthy adults on standardized psychological tasks. Across 222 effect sizes, the overall difference was g = 0.02, with a 95% credible interval from -0.05 to 0.10. Here "g" is Hedges' g, a standardized difference between two groups (not to be confused with the general intelligence factor), and multiplying by 15 converts it to IQ points. They saw modest reductions for longer fasts and for younger participants, but no reliable differences between domains such as attention and inhibitory control.
Earlier reviews were less settled. Erik Benau and colleagues reviewed 10 fasting studies in 2014 and called the results equivocal: several found no differences, while others found specific deficits in psychomotor speed, executive function and mental rotation. Their 2021 update of 21 newer studies found that fasting was more often associated with deficits than benefits and noted that samples, fasting methods and tasks varied too much to draw firm conclusions. The 2025 meta-analysis, with far more data, suggests those deficits average out to almost nothing for fasts of around half a day.
The best single trial of intermittent fasting tells the same story. Bamberg, Julia Reichenberger and Jens Blechert randomly assigned 122 adults to 10 days of 16-hour, breakfast-skipping fasting or a waitlist. The fasting group did not perform worse on repeated cognitive tests or report worse mood. They did feel less concentrated before noon while fasting than after eating in the afternoon, so how sharp you feel when hungry is not a reliable guide to how well you actually perform.
None of this touches intelligence in the trait sense. An IQ score reflects reasoning ability that is stable over years; a fast changes your state for a few hours. Many of these tasks are also shorter and simpler than a full IQ test, so these results are good but indirect evidence about IQ.
Glucose, ketones and the hungry brain
The worry that fasting starves the brain has a real starting point. In humans the brain is about 2% of body weight but uses about 20% of the body's glucose-derived energy, as Philipp Mergenthaler and colleagues summarize. It cannot store much fuel, so it depends on a steady supply from the blood.
The body is built to protect that supply. Between meals the liver releases glucose, and during longer fasts the brain switches partly to "ketone bodies," fuels made from fat. In a classic 1967 study, Owen and colleagues sampled blood vessels serving the brain in three patients with obesity during 5 to 6 weeks of starvation and found that ketones had replaced glucose as the brain's main fuel. A skipped lunch is nowhere near that extreme.
A related popular idea was that self-control runs on blood sugar, so a sugary drink restores willpower. When Miguel Vadillo, Natalie Gold and Magda Osman examined the glucose studies with a technique called p-curve analysis, they found the evidence weak and possibly inflated by publication bias, and advised against drawing any conclusions about glucose and self-control.
Intermittent fasting and calorie restriction over months
Claims that fasting makes you smarter usually come from animal research and wellness marketing. In a 2021 review, Jip Gudden, Alejandro Arias Vasquez and Mirjam Bloemendaal described promising results in animal models but found "no clear evidence of a positive short-term effect" of intermittent fasting on cognition in healthy people. Clinical benefits in conditions such as epilepsy are a separate question from raising intelligence in healthy adults.
The longest human trial of eating less is CALERIE, which randomly assigned 220 non-obese adults to two years of 25% calorie restriction or their usual diet. Emilie Leclerc and colleagues analyzed a spatial working memory task from that trial and found a significantly greater improvement in the restriction group, which they described as a slightly positive effect. That is one task, in one trial, with improvement linked mostly to lower protein intake, which is a long way from evidence that fasting raises IQ.
Skipping breakfast and children's test scores
Children are where hunger shows up most clearly. Katie Adolphus and colleagues at the University of Leeds reviewed 45 intervention studies in 2016. Eating breakfast, compared with fasting, had a short-term (same morning) positive effect on tasks requiring attention, executive function and memory, with clearer effects in undernourished children. The evidence on breakfast composition and on long-term school breakfast programs was too thin and inconsistent for firm conclusions.
A school-based trial by the same group randomly assigned 234 children aged 11 to 13 to breakfast cereal with milk or no breakfast. The breakfast group did better on a memory task, reaction time and sustained attention later that morning, and children with weaker baseline performance gained most. The trial was funded partly by Kellogg's and the 2016 review by a Kellogg's grant, which is worth knowing, though the trial's result fits the wider literature.
Studies of habitual breakfast skipping and school grades are harder to interpret. Adolphus's 2013 review found that eating breakfast more often was consistently associated with better academic performance, but that these associations could partly be explained by socioeconomic status. Families where children skip breakfast differ in many ways that also affect schooling.
Ramadan fasting and exam results
Ramadan offers a natural experiment, because observant Muslims avoid food and drink from sunrise to sunset for 29 or 30 days. Hessel Oosterbeek and Bas van der Klaauw studied an introductory economics course at VU University Amsterdam, where the number of Ramadan weeks falling during the course varied from zero to four across years. Each additional Ramadan week lowered Muslim students' final grades by almost a tenth of a standard deviation, about 1.5 points on an IQ-style scale per week.
Two cautions apply. The researchers did not observe who actually fasted, so this is the effect of Ramadan falling during the course, which also brings changes in sleep and meal timing. Our page on how sleep affects IQ and cognitive performance shows why disrupted sleep alone can matter. A drop in course grades over several weeks of studying is also a different thing from a change in intelligence.
Who should not fast without medical advice
Some people should get medical advice before any fast. People with diabetes, especially those who use insulin or other glucose-lowering medicines, people who are pregnant or breastfeeding, children and teenagers, and anyone with a history of an eating disorder should talk to their doctor or diabetes care team before fasting, including for religious fasts. Low blood sugar in people with diabetes can impair thinking in the moment; our page on IQ and type 1 diabetes covers what is known about longer-term effects.
Frequently asked questions
Does fasting make you smarter?
No. A 2025 meta-analysis of 3,484 healthy adults found essentially no difference between fasted and fed performance, and reviews of intermittent fasting find no clear short-term cognitive benefit in healthy people.
Does fasting lower your IQ?
Not in any lasting way. Short fasts change task performance by about 0.3 IQ points on average, though longer fasts and younger people show modest dips.
Should I eat before an IQ test?
Eating normally is sensible, mainly so hunger does not distract you. Our guide on how to prepare for an IQ test covers sleep and other practical steps.
Does skipping breakfast hurt children's school performance?
Breakfast gives children a small same-morning boost in attention and memory, especially in undernourished children. Long-term effects on grades are less certain because breakfast habits track family income.
Does the brain need sugar to think?
The brain runs mainly on glucose, but the body keeps blood glucose steady between meals, and during long fasts the brain shifts partly to ketones. The idea that a sugary drink restores willpower has not held up.
The takeaway
Fasting and intelligence are essentially unrelated. In healthy adults, typical fasts leave cognitive performance almost unchanged; children do a little better on a morning's tasks after breakfast, and a month of daytime fasting during term can cost students some grades, likely through sleep and fatigue as well as hunger. Nothing in the research suggests fasting raises IQ. If you want an accurate measure of your reasoning ability, eat as you normally would and, once you are well rested, take the RIOT IQ test.
References
1. Bamberg, C., & Moreau, D. (2025). Acute effects of fasting on cognitive performance: A systematic review and meta-analysis. Psychological Bulletin, 151(9), 1147-1169. [doi.org/10.1037/bul0000492](. doi.org
2. Benau, E. M., Orloff, N. C., Janke, E. A., Serpell, L., & Timko, C. A. (2014). A systematic review of the effects of experimental fasting on cognition. Appetite, 77, 52-61. [doi.org/10.1016/j.appet.2014.02.014](. doi.org
3. Benau, E. M., Makara, A., Orloff, N. C., Benner, E., Serpell, L., & Timko, C. A. (2021). How does fasting affect cognition? An updated systematic review (2013-2020). Current Nutrition Reports, 10(4), 376-390. [doi.org/10.1007/s13668-021-00370-4](. doi.org
4. Bamberg, C., Reichenberger, J., & Blechert, J. (2026). Stable cognitive performance while adapting to intermittent fasting: A randomised controlled trial. Journal of Health Psychology, 31(4), 1362-1377. [doi.org/10.1177/13591053251351204](. doi.org
5. Mergenthaler, P., Lindauer, U., Dienel, G. A., & Meisel, A. (2013). Sugar for the brain: The role of glucose in physiological and pathological brain function. Trends in Neurosciences, 36(10), 587-597. [doi.org/10.1016/j.tins.2013.07.001](. doi.org
6. Owen, O. E., Morgan, A. P., Kemp, H. G., Sullivan, J. M., Herrera, M. G., & Cahill, G. F., Jr. (1967). Brain metabolism during fasting. Journal of Clinical Investigation, 46(10), 1589-1595. [doi.org/10.1172/JCI105650](. doi.org
7. Vadillo, M. A., Gold, N., & Osman, M. (2016). The bitter truth about sugar and willpower: The limited evidential value of the glucose model of ego depletion. Psychological Science, 27(9), 1207-1214. [doi.org/10.1177/0956797616654911](. doi.org
8. Gudden, J., Arias Vasquez, A., & Bloemendaal, M. (2021). The effects of intermittent fasting on brain and cognitive function. Nutrients, 13(9), 3166. [doi.org/10.3390/nu13093166](. doi.org
9. Leclerc, E., Trevizol, A. P., Grigolon, R. B., Subramaniapillai, M., McIntyre, R. S., Brietzke, E., & Mansur, R. B. (2020). The effect of caloric restriction on working memory in healthy non-obese adults. CNS Spectrums, 25(1), 2-8. [doi.org/10.1017/S1092852918001566](. doi.org
10. Adolphus, K., Lawton, C. L., Champ, C. L., & Dye, L. (2016). The effects of breakfast and breakfast composition on cognition in children and adolescents: A systematic review. Advances in Nutrition, 7(3), 590S-612S. [doi.org/10.3945/an.115.010256](. doi.org
11. Adolphus, K., Hoyland, A., Walton, J., Quadt, F., Lawton, C. L., & Dye, L. (2021). Ready-to-eat cereal and milk for breakfast compared with no breakfast has a positive acute effect on cognitive function and subjective state in 11-13-year-olds: A school-based, randomised, controlled, parallel groups trial. European Journal of Nutrition, 60(6), 3325-3342. [doi.org/10.1007/s00394-021-02506-2](. doi.org
12. Adolphus, K., Lawton, C. L., & Dye, L. (2013). The effects of breakfast on behavior and academic performance in children and adolescents. Frontiers in Human Neuroscience, 7, 425. [doi.org/10.3389/fnhum.2013.00425](. doi.org
13. Oosterbeek, H., & van der Klaauw, B. (2013). Ramadan, fasting and educational outcomes. Economics of Education Review, 34, 219-226. [doi.org/10.1016/j.econedurev.2012.12.005](. doi.org
Hero image: Empty table setting plate fork knife, by Stock Catalog, CC BY 2.0. Via Wikimedia Commons.
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