<qa>  What is the main aim of the study?Positive effects of n-3 long chain polyunsaturated fatty acids PUFAs, as provided by fatty fish intake, on cognitive functions are widely accepted, but so far there is no clear evidence from randomized intervention studies in children and young adults, although fish intake has been positively associated with cognitive functions in observational studies. The main aim of this study is to test the effect of diet on improvement in cognitive performance in kindergarden children aged 4-6 years. In the intervention study, children will be randomly allocated into two groups, one consuming prepared meals with herring or mackerel for four months, and one group consuming meals containing meat as a control group. The children n200 will consume the prepared meals three times a week. Cognitive performance of the children will be tested at study start and end using the the Wechsler Preschool and Primary Scale of Intelligence WPPSI_III and nine hole peg test. Children will be screened for mental health problems in the Strengths and Difficulties Questionnaire SDQ, which is a global measure of childrens mental health. Compliance will be monitored by documentation of study fish intake and measuring fatty acid status. For a more detailed investigation of effects of increased herring and mackerel intake, before and after intervention blood and urine will be collected for lipidomic analysis metabolites, vitamin D and iodine determination and genotyping fatty acid metabolism and inflammation. 