You can spend a fortune on omega-3 supplements, eat a green smoothie with imported seeds for breakfast, and still fail the exam if your daily diet is a parade of shiny packaging. The brain nutrition industry moves billions, but the solid evidence points to things that are simpler and less photogenic. Let’s separate what works from what just sounds good.
First, two concepts. Sustained attention is the ability to keep your focus on a task for minutes without your brain wandering off. Working memory is the mental space where you manipulate information for a few seconds, like holding a number in your head while you dial the phone. Both are executive functions that depend on glucose, oxygen and a network of neurotransmitters that your diet modulates, but not as directly as the ads promise.
And a third, because it will come up a lot: an ultra-processed food, according to the NOVA classification, is an industrial formulation made mostly from substances extracted from foods (protein isolates, refined oils, modified starches) plus cosmetic additives: colorings, flavorings, emulsifiers. Packaged sliced bread is ultra-processed; bread from a bakery is not. The criterion is the process, not the nutrition label.
The importance of breakfast
Studies don’t say that any breakfast improves school performance. They say that habitually skipping it is associated with worse attention and memory, especially in food-insecure populations. The systematic review by Hoyland, Dye and Lawton gathered 45 studies published between 1950 and 2008 and found very small but consistent positive effects on memory and attention tasks, with greater benefit in undernourished or low-socioeconomic-status children [1].
In Jamaica, Simeon and Grantham-McGregor took the design to the extreme. They admitted the children twice to a metabolic ward and, after the overnight fast, gave them breakfast on one visit and a cup of tea on the other, reversing the order for half of the group. They divided them into three categories: stunted, previously severely malnourished, and well-nourished controls. The malnourished groups did worse on verbal fluency and on the coding test when they missed breakfast. In the children with wasting, working memory also dropped, measured with backward digit span. And the well-nourished group? It scored better on arithmetic without breakfast [2].
That’s not a typo. The effect of breakfast depends critically on what state you arrive in. If a child is well fed, skipping a single meal doesn’t wreck anything.
Now, the type of breakfast. The idea that a low-glycemic-index breakfast, with protein and fiber, sustains attention better is reasonable and is everywhere. The meta-analysis by Álvarez-Bueno and colleagues compared low- versus high-glycemic-index breakfasts in children and adolescents: 0.13 for immediate memory, 0.07 for delayed memory and −0.01 for attention, all with confidence intervals that cross zero [3]. It barely makes a difference.
An Australian crossover trial shows it even better. Thirty-nine children aged ten to twelve, three isocaloric breakfasts with high, medium and low glycemic load, and a continuous glucose monitor attached to the body for three hours. The glucose curve did exactly what the theory predicts: with the high-load breakfast, levels fell below the starting point at 83 minutes. The famous crash. And cognition? No differences. And satiety? No differences. And what they ate at the lunch buffet three hours later? No differences [4].
That’s the point: the sugar crash happens, it can be measured, and the child’s brain doesn’t even notice. On the morning of the exam, the advice is not “choose slow carbohydrates.” It’s “eat something you already know sits well with you.”
The “magic” omega-3
The brain contains a lot of docosahexaenoic acid, DHA. It’s tempting to conclude that taking more DHA improves cognition. The evidence doesn’t support this for the general population, and the best example is the story of DOLAB.
In 2012, an Oxford team led by Alex Richardson published the DOLAB study: 362 British children aged 7 to 9 with reading below the 20th percentile, randomized to 600 mg daily of algal DHA or placebo for 16 weeks. The poorest readers improved significantly [5]. The study made the press around half the world. It was funded by DSM Nutritional Products, which also supplied the capsules.
And then the same team did something uncommon: it tried to replicate it. DOLAB II recruited 376 children in 84 schools across five counties, with the same criteria, the same dose and the same duration. It found nothing. Not in reading, not in working memory, not in behavior [6].
The effect size on reading was 0.05 standard deviations. The authors calculated that, to detect something like that with decent statistical power, more than 11,500 children would be needed. In other words: even if the effect exists, it is so small that no school in the world would notice the difference. That is the story marketing doesn’t tell, and also the reason we only trust studies that have been replicated and funded by independent sources.
The rest of the literature points the same way. The meta-analysis by Jiao and colleagues, which covered infants through the elderly, concluded that omega-3 supplementation improves cognitive development in infants but not cognitive performance in healthy children, adolescents or adults [7]. The Oxford-Durham study, well designed, showed improvements in reading and behavior in children with developmental coordination disorder, but it involved 117 children from a very specific clinical population, and they received omega-3 together with omega-6, so the effect can’t even be attributed to DHA alone [8].
The only health claim the European Union authorizes for DHA sums it all up: “DHA contributes to the maintenance of normal brain function,” at 250 mg per day [9]. In other words, it doesn’t claim it improves anything.
Iron and vitamins
Around 25% of school-age children worldwide have anemia, and iron deficiency accounts for roughly half of the cases. The meta-analysis by Low and colleagues gathered 32 randomized trials with 7,089 children, almost all in low- and middle-income countries: daily iron supplementation improved cognitive performance, including IQ, in those who were anemic [10]. In those who weren’t, the benefit fades.
An Ethiopian trial published in 2024 confirms it the uncomfortable way. Five hundred and four schoolchildren in southern Ethiopia, a 2×2 factorial design, intermittent iron and high-dose vitamin A versus placebo. Iron supplementation improved nothing, and it had a negative effect on a visual search task: −0.17 standard deviations, a small effect but one that doesn’t include zero. The detail that explains it is in the baseline: only 3.9% of those children had iron-deficiency anemia [11]. In a population without a deficit, giving iron was not neutral.
The same trial found that high-dose vitamin A did improve working memory, measured with digit span, by 0.30 standard deviations [11]. It’s a striking finding, but you should know that the benefit was concentrated in specific subgroups (girls, children with helminth infections, food-secure households), and subgroups are slippery terrain until someone replicates the result.
Ultra-processed foods
Ultra-processed foods are not just “junk food”: they are cookies, breakfast cereals, salty snacks, soft drinks, frozen pizzas, bottled sauces. Kevin Hall’s trial admitted 20 adults to an NIH metabolic unit for four weeks. Two weeks on an ultra-processed diet, two on an unprocessed diet, in random order, matched for calories offered, sugar, fat, fiber, sodium and macronutrients. Participants ate whatever they wanted. On the ultra-processed diet they consumed 508 kcal more per day and gained 0.9 kg; on the other they lost 0.9 kg [12]. Same content on paper, different behavior. They don’t fool your willpower: they fool your satiety.
At the epidemiological level, a meta-analysis of 43 observational studies found consistent associations between higher consumption of ultra-processed foods and higher risk of depression, cardiovascular disease and mortality [13]. In the French NutriNet-Santé cohort, with more than 26,000 adults and repeated dietary records, an increase in the proportion of ultra-processed foods was associated with a higher risk of developing depressive symptoms during follow-up [14]. These are observational data, with possible confounders, but the convergence between cohorts from different countries is remarkable.
And attention in class? There is less here than the press suggests, and it’s worth saying so. The EHDLA project assessed 788 Spanish adolescents aged 12 to 17 and found a dose-response association between ultra-processed food consumption and worse grades [15]. It is cross-sectional and uses self-reported diet: it describes an association, not a direction.
Where nutrition unmistakably moves the needle is where food is lacking. India has the largest school meal program in the world. A 2001 Supreme Court directive forced the states to implement it and each did so at its own pace, which left a natural experiment. Chakraborty and Jayaraman analyzed about 1.23 million children: those who had been receiving the meal for three or four years scored 18% higher in reading and 9% higher in mathematics than those who had received it for less than one year [17]. And in Kenya, twelve rural schools with 554 children were randomized to receive a local stew with meat, with milk, with oil, or nothing for five school terms. The meat group improved the most in school exam scores [18]. Real food, in children with documented deficiencies of iron, zinc and B12, changing measurable academic results.
Hydration: cheap, sensible and less miraculous than they tell you
The most cited study is by Edmonds and Burford: 58 British children, half of whom received 250 ml of water between two tests. Those who drank improved on two visual attention tasks. On story memory and visuomotor tracking, nothing [19]. Parallel-group design, small sample, no measurement of baseline hydration status.
A randomized trial in five rural schools in Zambia, none with a water point within 500 meters, gave refillable bottles to half of 292 pupils. The intervention worked spectacularly on physiology: afternoon dehydration went from 67% in the control group to 10% in the intervention group. Across the six cognitive tests, the authors found no clear effect, only a suggestive relationship in visual attention [20].
Drinking water is free, has no side effects, and dehydration in classrooms is real. As a performance lever, don’t expect much.
Practical guide
For breakfast, combine a protein source (egg, plain yogurt, cheese, legumes) with a carbohydrate you like and water. Avoid sugary cereals, industrial juices and pastries, but not because of their glycemic index, since you’ve seen there is no measurable difference there: avoid them because they displace more nutrient-dense foods and are less filling. If you’re not hungry when you wake up, don’t force it. A glass of water and a piece of fruit or a handful of nuts mid-morning is better than a huge breakfast at seven with nausea. And on exam day, don’t try anything new: any change in your digestive routine adds noise and no signal.
With supplements, the rule is simple. Don’t buy omega-3 unless a blood test or a doctor indicates a deficiency, or you follow a strict vegan diet with no marine sources. Oily fish twice a week covers most people’s DHA and EPA needs. Walnuts, ground flax and chia provide the precursor, but conversion to DHA is low [23], so don’t take them as complete substitutes. And if you are chronically tired, pale, or short of breath climbing stairs, ask for a blood test with ferritin before buying anything. Correcting a diagnosed iron-deficiency anemia is one of the very few nutritional interventions with a demonstrated cognitive effect in trials. Taking iron without a deficit not only doesn’t help: it can make you feel worse.
With ultra-processed foods, try to replace them so that they stop being the foundation of your diet. Swap the breakfast cereal for oats, the soft drink for water, the cookie for a handful of almonds. These are boring changes. They are the ones with backing.
One last fact
Everyone repeats that the adult brain is 2% of body weight and consumes 20% of energy at rest. That’s true. What almost nobody knows is what happens in childhood: brain expenditure peaks around age four or five, when it consumes 66% of resting metabolism, or 43% of total daily energy [25]. Two to three times more than in an adult.
That explains why childhood malnutrition hits so hard, and why school meals in India and Kenya move grades while DHA in well-fed British children would need eleven thousand participants to be detected. Food matters enormously when it is lacking and fairly little when it is plentiful. Marketing works exactly the opposite way: it sells where it is least needed.
REFERENCES
[1] Hoyland A, Dye L, Lawton CL. A systematic review of the effect of breakfast on the cognitive performance of children and adolescents. Nutrition Research Reviews. 2009;22(2):220-243. DOI: 10.1017/S0954422409990175. Reliable. First systematic review on the question: 45 studies described in 41 articles, published between 1950 and 2008. Includes acute experimental studies, school feeding programs and habitual intake studies, with separate analysis of well-nourished children and those at nutritional risk. High heterogeneity between studies.
[2] Simeon DT, Grantham-McGregor S. Effects of missing breakfast on the cognitive functions of school children of differing nutritional status. American Journal of Clinical Nutrition. 1989;49(4):646-653. DOI: 10.1093/ajcn/49.4.646. Reliable. Crossover design with counterbalanced order in a metabolic ward; breakfast versus a cup of tea after an overnight fast, in three nutritional groups. Notable feature: by comparing different nutritional states, it detects that skipping breakfast harms the malnourished but not the control group, which even scored better on arithmetic.
[3] Álvarez-Bueno C, Martínez-Vizcaíno V, Jiménez López E, Visier-Alfonso ME, Redondo-Tébar A, Cavero-Redondo I. Comparative effect of low-glycemic index versus high-glycemic index breakfasts on cognitive function: a systematic review and meta-analysis. Nutrients. 2019;11(8):1706. DOI: 10.3390/nu11081706. Reliable. Random-effects meta-analysis on immediate memory, delayed memory and attention in children and adolescents. No evidence of publication bias on Egger’s test for any of the three domains.
[4] Brindal E, Baird D, Danthiir V, Wilson C, Bowen J, Slater A, Noakes M. Ingesting breakfast meals of different glycaemic load does not alter cognition and satiety in children. European Journal of Clinical Nutrition. 2012;66(10):1166-1171. DOI: 10.1038/ejcn.2012.99. Reliable. Randomized crossover trial, 39 children aged 10-12, three isocaloric 1.3 MJ breakfasts with glycemic loads of 33, 24 and 18, with a continuous glucose monitor and a computerized cognitive battery every hour for three hours, plus an ad libitum buffet at the end. Notable feature: confirms that the glucose curve behaves as the theory predicts (drop below baseline at 83 minutes with high load) and that even so nothing changes in cognition or satiety. Partially funded by the Dairy Health and Nutrition Consortium, whose commercial interest ran counter to the result obtained.
[5] Richardson AJ, Burton JR, Sewell RP, Spreckelsen TF, Montgomery P. Docosahexaenoic acid for reading, cognition and behavior in children aged 7-9 years: a randomized, controlled trial (the DOLAB study). PLoS ONE. 2012;7(9):e43909. DOI: 10.1371/journal.pone.0043909. Problematic. Randomized, double-blind, placebo-controlled trial; 362 Oxfordshire children below the 20th percentile in reading, 600 mg/day of algal DHA for 16 weeks. Funded by DSM Nutritional Products. See note 1.
[6] Montgomery P, Spreckelsen TF, Burton A, Burton JR, Richardson AJ. Docosahexaenoic acid for reading, working memory and behavior in UK children aged 7-9: a randomized controlled trial for replication (the DOLAB II study). PLoS ONE. 2018;13(2):e0192909. DOI: 10.1371/journal.pone.0192909. Reliable. Preregistered direct replication; 376 children in 84 schools across five counties, same dose and duration. Protocol and anonymized data published on the Open Science Framework. Also funded by DSM, with a contract that excluded the funder’s involvement in design and analysis.
[7] Jiao J, Li Q, Chu J, Zeng W, Yang M, Zhu S. Effect of n-3 PUFA supplementation on cognitive function throughout the life span from infancy to old age: a systematic review and meta-analysis of randomized controlled trials. American Journal of Clinical Nutrition. 2014;100(6):1422-1436. Reliable. Meta-analysis restricted to randomized trials with at least three months of treatment. Conclusion: improvement in cognitive development in infants, no improvement in cognitive performance in the other healthy groups.
[8] Richardson AJ, Montgomery P. The Oxford-Durham study: a randomized, controlled trial of dietary supplementation with fatty acids in children with developmental coordination disorder. Pediatrics. 2005;115(5):1360-1366. With reservations. Randomized double-blind placebo-controlled trial in 117 children with developmental coordination disorder, over three months. The reservation is not about design quality: the sample is clinical and very specific, and the intervention combined omega-3 with omega-6, which prevents attributing the effect to DHA separately.
[9] EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific opinion on the substantiation of a health claim related to DHA and maintenance of normal brain function. EFSA Journal. 2011;9(4):2078. DOI: 10.2903/j.efsa.2011.2078. The authorized claim appears in Regulation (EU) No 432/2012. Regulatory framework. Assessment of a dossier by a scientific panel and subsequent regulatory decision; not a primary study.
[10] Low M, Farrell A, Biggs BA, Pasricha SR. Effects of daily iron supplementation in primary-school-aged children: systematic review and meta-analysis of randomized controlled trials. CMAJ. 2013;185(17):E791-E802. DOI: 10.1503/cmaj.130628. Reliable. Meta-analysis of 32 randomized and quasi-randomized trials with 7,089 children, mostly from low- and middle-income countries, with separate analysis of hematological, cognitive and adverse effects. The cognitive benefit was concentrated in anemic children.
[11] Gutema BT, Levecke B, Sorrie MB, Megersa ND, Zewdie TH, Yesera GE, De Henauw S, Abubakar A, Abbeddou S. Effectiveness of intermittent iron and high-dose vitamin A supplementation on cognitive development of school children in southern Ethiopia: a randomized placebo-controlled trial. American Journal of Clinical Nutrition. 2024;119(2):470-484. DOI: 10.1016/j.ajcnut.2023.11.005. Registry: NCT04137354. Reliable. 2×2 factorial design with 504 children with a mean age of 9.6 years, assessed with Raven’s progressive matrices, digit span, Tower of London and visual search. Notable feature: documents the baseline prevalence of deficiency (only 3.9% with iron-deficiency anemia), which is exactly what allows the result to be interpreted. The vitamin A subgroup effects are exploratory.
[12] Hall KD, Ayuketah A, Brychta R, et al. Ultra-processed diets cause excess calorie intake and weight gain: an inpatient randomized controlled trial of ad libitum food intake. Cell Metabolism. 2019;30(1):67-77.e3. DOI: 10.1016/j.cmet.2019.05.008. Reliable. Randomized crossover trial with 20 adults admitted to the NIH Clinical Center for four weeks, with diets matched for calories presented, energy density, macronutrients, sugar, sodium and fiber. Participants lived in the unit the whole time, which eliminates the problem of self-reported diet. Small sample.
[13] Lane MM, Davis JA, Beattie S, et al. Ultraprocessed food and chronic noncommunicable diseases: a systematic review and meta-analysis of 43 observational studies. Obesity Reviews. 2021;22(3):e13146. DOI: 10.1111/obr.13146. Reliable. Meta-analysis of observational studies, mostly prospective cohorts, with adjustment for confounders. Does not prove causality; the weight comes from the coherence between independent cohorts from different countries.
[14] Adjibade M, Julia C, Allès B, et al. Prospective association between ultra-processed food consumption and incident depressive symptoms in the French NutriNet-Santé cohort. BMC Medicine. 2019;17(1):78. DOI: 10.1186/s12916-019-1312-y. Reliable. Prospective cohort with more than 26,000 adults and a mean of five years of follow-up. Consumption was measured with repeated 24-hour dietary records, not a single food frequency questionnaire, which reduces measurement error.
[15] López-Gil JF, Cisneros-Vásquez E, Olivares-Arancibia J, Yañéz-Sepúlveda R, Gutiérrez-Espinoza H. Investigating the relationship between ultra-processed food consumption and academic performance in the adolescent population: the EHDLA study. Nutrients. 2025;17(3):524. DOI: 10.3390/nu17030524. With reservations. Cross-sectional secondary analysis of 788 Spanish adolescents aged 12 to 17, with diet from a self-administered questionnaire and official grades provided by the schools. Cross-sectional design: describes an association, not a causal direction.
[16] dos Santos JVL, de Melo ISV, Costa CACB, de Almeida LC, Silva DR, Ferro DC, Paula DTC, Macena ML, Bueno NB. Association between ultra-processed food consumption and cognitive performance among adolescent students from underdeveloped cities in Brazil: a cross-sectional study. International Journal of Public Health. 2024;69:1607658. DOI: 10.3389/ijph.2024.1607658. With reservations. Cross-sectional, 116 adolescents aged 15 to 18 from three public secondary schools in the interior of Alagoas, with three 24-hour recalls and a non-verbal general intelligence test. Observed difference of 26.5% versus 22.5% of energy from ultra-processed foods, P = 0.17. Small sample and a different outcome from [15].
[17] Chakraborty T, Jayaraman R. School feeding and learning achievement: evidence from India’s midday meal program. Journal of Development Economics. 2019;139:249-265. DOI: 10.1016/j.jdeveco.2018.11.011. Reliable. Natural experiment exploiting the staggered state-by-state rollout following the 2001 Indian Supreme Court directive, with about 1.23 million children from the ASER survey. The authors warn of a possible upward bias in the estimate.
[18] Hulett JL, Weiss RE, Bwibo NO, Galal OM, Drorbaugh N, Neumann CG. Animal source foods have a positive impact on the primary school test scores of Kenyan schoolchildren in a cluster-randomised, controlled feeding intervention trial. British Journal of Nutrition. 2014;111(5):875-886. DOI: 10.1017/S0007114513003310. Reliable. Cluster-randomized trial; 12 rural schools in Embu, 554 children, four isocaloric groups over five school terms between 1999 and 2001. Notable feature: the baseline documented widespread deficiencies of iron, zinc, vitamin A and B12, which makes the study a test of correcting deficiencies, not of supplementation on an adequate diet. The outcome was real school exam grades.
[19] Edmonds CJ, Burford D. Should children drink more water? The effects of drinking water on cognition in children. Appetite. 2009;52(3):776-779. DOI: 10.1016/j.appet.2009.02.010. With reservations. Acute parallel-group trial (not crossover): 58 children aged 7 to 9, 250 ml of water between two tests. Small sample, no measurement of baseline hydration status, and an effect on only two of the four tasks assessed.
[20] Trinies V, Chard AN, Mateo T, Freeman MC. Effects of water provision and hydration on cognitive function among primary-school pupils in Zambia: a randomized trial. PLoS ONE. 2016;11(3):e0150071. DOI: 10.1371/journal.pone.0150071. Registry: NCT01924546. Reliable. Randomized trial in five rural schools without a water point within 500 meters, 292 pupils, with hydration measured by urine specific gravity using a portable refractometer and six cognitive tests. Notable feature: it is the first trial of water provision and cognition in a low-income country, and the authors openly report the null result and its power limitations.
[21] Chassaing B, Van de Wiele T, De Bodt J, Marzorati M, Gewirtz AT. Dietary emulsifiers impact the mouse gut microbiota promoting colitis and metabolic syndrome. Nature. 2015;519(7541):92-96. DOI: 10.1038/nature14232. Animal model. Mouse study with emulsifier doses higher than usual human consumption. Establishes a plausible mechanism; it is not direct evidence in people.
[22] Chassaing B, Compher C, Bonhomme B, et al. Randomized controlled-feeding study of dietary emulsifier carboxymethylcellulose reveals detrimental impacts on the gut microbiota and metabolome. Gastroenterology. 2022;162(3):743-756. DOI: 10.1053/j.gastro.2021.11.006. With reservations. Randomized controlled-feeding trial with only sixteen participants. Pilot study: needs replication with more power before drawing clinical conclusions.
[23] Burdge GC, Calder PC. Conversion of alpha-linolenic acid to longer-chain polyunsaturated fatty acids in human adults. Reproduction Nutrition Development. 2005;45(5):581-597. DOI: 10.1051/rnd:2005047. Reliable. Review of human stable isotope tracer studies. Conversion of ALA to DHA is consistently low, with notable differences by sex.
[24] Schaeffer L, Gohlke H, Müller M, et al. Common genetic variants of the FADS1 FADS2 gene cluster and their reconstructed haplotypes are associated with the fatty acid composition in phospholipids. Human Molecular Genetics. 2006;15(11):1745-1756. DOI: 10.1093/hmg/ddl117. Reliable. Genetic association study in the general population. It is not a clinical trial: it supports the plausibility of a differential response to supplementation, but does not demonstrate it.
[25] Kuzawa CW, Chugani HT, Grossman LI, et al. Metabolic costs and evolutionary implications of human brain development. PNAS. 2014;111(36):13010-13015. DOI: 10.1073/pnas.1323099111. Reliable. Combines prior PET and MRI data to calculate brain glucose use from birth to adulthood. The peak falls between 4.2 and 4.4 years: 66.3% of resting metabolic expenditure and 43.3% of daily energy requirement. These are two different denominators, not a range.