A high inclusion rate and low drop out rate was achieved. This study will be the first to determine whether school meals based on the NND improve children's diet, health, growth, cognitive performance, and early disease risk markers.

It is widely assumed that nutrition can improve school performance in children; however, evidence remains limited and inconclusive. In the present study, we investigated whether serving healthy school meals influenced concentration and school performance of 8-to 11-year-old Danish children. The OPUS (Optimal well-being, development and health for Danish children through a healthy New Nordic Diet) School Meal Study was a cluster-randomised, controlled, cross-over trial comparing a healthy school meal programme with the usual packed lunch from home (control) each for 3 months (NCT 01457794). The d2 test of attention, the Learning Rating Scale (LRS) and standard tests on reading and mathematics proficiency were administered at baseline and at the end of each study period. Intervention effects were evaluated using hierarchical mixed models. The school meal intervention did not influence concentration performance (CP; primary outcome, n 693) or processing speed; however, the decrease in error percentage was 0·18 points smaller (P,0·001) in the intervention period than in the control period (medians: baseline 2·03 %; intervention 1·46 %; control 1·37 %). In contrast, the intervention increased reading speed (0·7 sentence, P¼0·009) and the number of correct sentences (1·8 sentences, P,0·001), which corresponded to 11 and 25 %, respectively, of the effect of one school year. The percentage of correct sentences also improved (P, 0·001), indicating that the number correct improved relatively more than reading speed. There was no effect on overall math performance or outcomes from the LRS. In conclusion, school meals did not affect CP, but improved reading performance, which is a complex cognitive activity that involves inference, and increased errors related to impulsivity and inattention. These findings are worth examining in future trials.Key words: School meals: Nutrition: Cognition: School performance It is widely considered that a nutritionally balanced diet is beneficial for learning and school performance in children. The influence of diet is plausible, since a poor diet may result in deficiency of nutrients that could play a role in cognitive development (1) . In low-income countries where malnutrition is prevalent, school feeding programmes have consistently shown positive short-term effects on the achievement in academic tests (2) . The relatively high brain metabolism in children also means that children are more vulnerable to fasting compared with adults. In line with this, breakfast skipping has been shown to impair cognitive function and has also been associated with attention problems and lower school grades, especially in younger children (3) . Moreover, carbohydrate quality and glycaemic response of foods may influence shortterm cognitive performance (4) . The dietary composition has also been hypothesised to influence cognition and school performance through effects on neurological factors involved in learning and memory. Components of a 'Western diet', i.e. a diet high in salt, saturated fat and...

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Fe and n-3 long-chain PUFA (n-3 LCPUFA) have both been associated with cognition, but evidence remains inconclusive in well-nourished school-aged children. In the Optimal Well-Being, Development and Health for Danish Children through a Healthy New Nordic Diet (OPUS) School Meal Study, the 3-month intervention increased reading performance, inattention, impulsivity and dietary intake of fish and Fe. This study investigated whether the intervention influenced n-3 LCPUFA and Fe status and, if so, explored how these changes correlated with the changes in cognitive performance. The study was a cluster-randomised cross-over trial comparing school meals with packed lunch (control). At baseline and after each treatment, we measured serum ferritin, whole-blood n-3 LCPUFA and Hb, and performance in reading, mathematics and d2-test of attention. Data were analysed using mixed models (n 726) and principal component analysis of test performances (n 644), which showed two main patterns: 'school performance' and 'reading comprehension'. The latter indicated that children with good reading comprehension were also more inattentive and impulsive (i.e. higher d2-test error%). The intervention improved 'school performance' (P = 0·015), 'reading comprehension' (P = 0·043) and EPA + DHA status 0·21 (95 % CI 0·15, 0·27) w/w % (P < 0·001), but it did not affect serum ferritin or Hb. At baseline, having small Fe stores was associated with poorer 'school performance' in girls, but with better 'reading comprehension' in both boys and girls. Both baseline EPA + DHA status and the intervention-induced increase in EPA + DHA status was positively associated with 'school performance', suggesting that n-3 LCPUFA could potentially explain approximately 20 % of the intervention effect. These exploratory associations indicate that increased fish intake might explain some of the increase in reading performance and inattention in the study.Key words: School meals: Iron: n-3 PUFA: Cognitive performance: School performance Fe deficiency (ID) is one of the most prevalent nutrient deficiencies in the world, but it is mainly seen in high-risk groups -that is, infants, preschool-aged children and women (1) . Similarly, there is a potential risk of insufficient n-3 long-chain PUFA (n-3 LCPUFA) status in otherwise well-nourished children, as dietary intake is lower than recommended and endogenous synthesis is likely to be insufficient (2,3) . However, Fe and n-3LCPUFA are important nutrients for optimal brain development in childhood (4) . DHA (22 : 6n-3) is accumulated in the brain, and it has been shown to have an important role in neuronal growth, differentiation, myelination and monoamine neurotransmission (5,6) . Fish is the main source of n-3 LCPUFA, and maternal fish intake and blood DHA status during pregnancy has been associated with offspring neurological and cognitive development (7)(8)(9)(10) . that early n-3 LCPUFA intake may have beneficial effects on visual and cognitive development in early life (11) . In adolescents, fish consumptio...