This study is a consensus document of two Spanish scientific associations, FESNAD (Spanish Federation of Nutrition, Food and Dietetetic Associations) and SEEDO (Spanish Association for the Study of Obesity), about the role of the diet in the prevention and of overweight and obesity in adults. It is the result of a careful and systematic review of the data published in the medical literature from January 1st 1996 to January 31st 2011 concerning the role of the diet on obesity prevention. The conclusions obtained have been classified according several evidence levels. Subsequently, in agreement with these evidence levels, different degree recommendations are established. These recommendations could be potentially useful to design food guides as part of strategies to prevent overweight and obesity.