Obesity is a metabolic chronic disease of high complexity and multifactorial origin. Diet and life style are the main causes of the disease but the effect of food in the organism is not easy to measure. The use of mature erythrocyte membranes as representative site for all other body tissues, is an established protocol for membrane-based molecular diagnostics (1−3) . The aim of this study was to compare the fatty acid (FA) composition of erythrocyte membranes from obese children compared with normal weight controls together with the evaluation of the nutritional and behavioral profile.A prospective observational study was carried out on a cohort of 161 children (104 normal weight and 57 obese), from 6 to 16 years old, recruited at AZTI and Biocruces Bizkaia Health Research Institute. FA composition of mature erythrocyte membrane phospholipids was determined by gas chromatography-mass spectrometry. Nutritional data and life style were also obtained through validated questionnaires.Obese children showed a mid-quality diet (7.8 points, measured with KIDMED index) characterized by a lower intake of fish, vegetables and nuts and higher intake of sugars and processed meat compared with optimal quality diet observed in normal weight children. Fatty acid profile of obese children showed an inflammatory profile, evidenced by the excess of omega 6 FA (mainly arachidonic acid), as well as a prevalence of saturated and trans FA (Fig. 1).