Obesity is a worldwide epidemic with increasing prevalence, specially severe obesity (Body Mass Index (BMI) ≥ 35 kg/m2). It is a multifactorial disease that involves genetic and environmental factors that lead to increased mortality from cardiovascular disease, diabetes, cancer, among others and impairs life quality. Most research on severe obesity focuses on surgical alternatives and their results, thus this clinical trial aims to evaluate the effect of a non-pharmacological approach based on nutritional intervention and supplementation with a functional food, the olive oil. It will analyze the effectiveness of interventions on: weight loss, improvements on body composition and inflammatory profile (TNF-alfa, interleucins 1, 6 and 10, adiponectin), insulin resistance and serum lipids control, changing eating habits and physical activity practice, modification on bone mineral density and sarcopenia, and reduction of cardiovascular risk and other diseases. Also, it will be investigated the influence of polymorphisms (Pro12Ala of PPAR-γ gene, -174G>C of IL6 gene e Trp64Arg of ADRB3 gene) on nutritional intervention effectiveness with and without olive oil. This research looks for improving severely obese patient's care and contributing to effective results by reducing costs and risk treatment. The investigators believe that this informations will contribute significantly to the scientific field, expanding the knowledge about severe obesity.

After enrollment, patients undergo a diagnostic phase (baseline) with nutritional, anthropometric and body composition assessment (multifrequency bioelectrical impedance analysis and dual X-ray absorptiometry); investigation of clinical history; collecting blood samples; carotid artery intima-media thickness, heart rate variability and accelerometry assessments; and application of validated questionnaires to evaluate secondary outcomes. Patients will be randomized into three different treatment groups. Randomization will be stratified according to Body Mass Index (BMI) (BMI between 35 and 39,99 kg/m2 and BMI ≥ 40 kg/m2), allocation ratio 1:1:1 and parallel intervention. Then, trained nutritionists will deliver the intervention according to the patient group and they will be followed-up for 12 weeks, with visits each 4 weeks.

Interventions will be provided as follow:

Olive oil group: in order to evaluate the effect of only the extra virgin olive oil on primary and secondary outcomes, patients will be instructed to maintain habitual food consumption and consume 52 mL of extra virgin olive oil daily (4 sachets per day, 2 at lunch and 2 at dinner).

Nutritional intervention group: patients assigned to this group will receive nutritional intervention consisting of prescription of individualized food plan to reduce 5 to 10% of initial body weight, nutritional counseling for comorbidities and promoting healthy eating habits and physical activity. Resting Energy Expenditure (REE) will be calculated according to Horie-Waitzberg & Gonzalez equation developed for severe obese individual using current weight and lean body mass (Horie et al., 2011). Total energy expenditure (TEE) will be calculated multiplying REE by activity factor recommended by Institute of Medicine (Trumbo et al., 2005) and thermic effect of food (Hill, Wyat, Peters, 2012). Total energy value (TEV) of diet will be determined reducing 550 to 1100 kcal from TEE to reach 0.5 to 1.0 weekly weight reduction according to the patient's individual goal. Distribution of macronutrients will follow Dietary Reference Intake (DRI) recommendation: 45-65% of carbohydrates, 10-35% of proteins and 20-35% of lipids (Trumbo et al., 2002). Fat acids (FA) will be distributed as recommended by Sociedade Brasileira de Cardiologia (Sposito et al., 2007): ≤ 7% saturated FA, ≤ 20% monounsaturated FA and ≤ 7% polyunsaturated FA. Food plan prescription will distribute TEV in 4-6 meals. Motivation will be held to changing eating and lifestyle habits.

Nutritional intervention plus olive oil: this group will receive conventional intervention plus supplementation of 52 mL of extra virgin olive oil daily (4 sachets per day, 2 at lunch and 2 at dinner).

At the end of follow-up the same assessments performed at diagnostic phase will be held.

To detect a reduction of 10 kg, with standard deviation of 30, which is in agreement with the study of Rodrigues e Silveira (2011), with a two-sided 5% significance level and a power of 80%, a sample size of 50 patients per group will be necessary, given an anticipated dropout rate of 42,9%.

This research will be conducted at a capital of center Brazil, at Unidade de Pesquisa Clínica of Hospital das Clínica/UFG, Faculty of Medicine/UFG.

Inclusion Criteria:

• Body mass index ≥ 35 kg/m2
• Residence in Goiânia or metropolitan area
• Patients referred by Secretaria Municipal de Saúde to Severe Obese Nutrition Clinic

Exclusion Criteria:

• To have already performed bariatric surgery
• Pregnancy
• Reduction greater than 8% of body weight in the past 3 months
• To have been treated at Severe Obesity Nutrition Clinic or have received nutritional counseling in another place in the last 2 years
• Anti-obesity drugs use
• Regular utilization of anti-inflammatory drugs or corticosteroids
• HIV/AIDS, heart failure, kidney failure, hepatic insufficiency, chronic obstructive pulmonary disease and cancer