This study is a consensus document of two Spanish scientific associations, FESNAD (Spanish Federation of Nutrition, Food and Dietetic Associations) and SEEDO (Spanish Association for the Study of Obesity), about the role of the diet in the treatment 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 treatment. The achieved conclusions have been classified into various evidence levels. Subsequently in agreement with these evidence levels, different degree recommendations are established being potentially useful to design food guides as part of strategies addressed to the treatment overweight and obesity.