Effect of including a dietary supplement of raisins, a food rich in polyphenols, on cognitive function in healthy older adults; a study protocol for a randomized clinical trial oleh María J. Rodrigo-Gonzalo, José I. Recio-Rodríguez, Roberto Méndez-Sánchez, María C. Pablos-Hernández, Alfonso González-Ramírez, Jesús González-Sánchez, María J. Fermoso-Palmero, Ana S. Puente-González, María C. Sánchez-Sánchez, Fausto J. Barbero-Iglesias, María I. Rihuete-Galve, Susana González-Manzano