The present study is a consensus document of two Spanish scientific associations, FESNAD (Spanish Federation of Societies of Nutrition, Food and Dietetics Associations) and SEEDO (Spanish Association for the Study of Obesity), about the role of the diet in the prevention and treatment of overweight and obesity in adults. In this part of the document the methodology and the classification of levels of evidence are described. In order to reach the strongest evidence possible, a systematic review of 543 medical studies dealing with these issues published in the last 15 years (from January 1st 1996 to January 31st 2011) has been conducted. In the executive summary, along with the obtained evidences, a set of degree-classified recommendations are established. These recommendations could constitute a useful tool to design food guides addressed to the nutritional counseling for obesity and overweight treatment.