OLIVE OILS AND HEALTH 192 (e.g., changes in cognitive performance), and no preventive or therapeutic interventions are performed. It is in this last aspect where observational cohorts and clinical trials differ, given that in a nutritional intervention trial, researchers design the diet to be tested, and in one of the groups (intervention group) participants are educated to adhere to that particular diet during the intervention period. At the end of it, the frequency of events or changes of interest are compared between the intervention (active group) and control (receiving another type of diet) groups. The Three-City study, published by Berr et al. in 2009 is a French observational cohort from Bordeaux, Montpellier, and Dijon. Cognitive performance tests were performed in 6947 individuals aged more than 65 (60% from 65 to 74 years) at the beginning of the study and at 4-year follow-up. Results showed a 15% less cognitive decline in visual memory and verbal fluency tests (e.g., capacity for making word lists with an assigned letter in a short time interval) in individuals consuming olive oil for all purposes (raw and for cooking) as compared to non-consumers. Within the SUN (Seguimiento Universidad de Navarra; Navarra University Follow-up) cohort, in which 23000 Spanish university graduates are currently participating, cognitive performance was evaluated in a subsample of 806 participants older than 55 years (mean age 66 years). After a 6-year follow-up, a reduced risk of cognitive decline was observed in participants with high lignan intake. In these participants, lignans were phenolic compounds mainly ingested from olive oil and nuts. The results from the Three-City and SUN cohorts concur with other observational studies showing a preventive effect of the Mediterranean diet on cognitive decline and dementia. As mentioned before, randomised controlled intervention trials (field trials) are the epidemiological studies providing the highest level of scientific evidence and the most robust causal inferences on prevention issues. Findings from the PREDIMED (Prevention with Mediterranean diet; PREvención con DIeta MEDiterránea) randomised intervention trial are therefore extremely relevant. In this study, the effect of a Mediterranean diet supplemented with EVOO (first active group) or nuts (second active group) on cardiovascular events (e.g., myocardial infarction or stroke) prevention was compared with that of a low-fat diet (control group). This trial was performed in Spain between June 2005 and December 2010 with 11 Spanish research centres and 7447 high cardiovascular risk individuals. The participants, who were free from cardiovascular disease at commencement of the trial, generously accepted to take part in the study for around 5 years. The effects of the PREDIMED intervention on cognitive performance were evaluated in two centres (Figures 16.1 and 16.2).
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