Experimental, double-blind, randomized, prospective, placebo-controlled study. Patients aged 4-17 years old are selected from the endocrine outpatient clinic of the Hospital de Pediatria del CMN "Siglo XXI", and both patients and parents sign informed consent. They are scheduled for a fasting oral glucose tolerance test (OGTT) with 1.75 mg/kg of oral glucose. A blood sample is taken from antecubital vein at baseline and 120 minutes.

A high carbohydrate diet is indicated for 3 days prior to the test. All patients diagnosed with glucose intolerance are invited to participate. All followed an aerobic program of 30 to 45 minutes every day, an isocaloric diet from the moment of inclusion throughout the study period, in order to decrease the effect of nutrition modification in the metabolic and vascular parameters. A complete medical history is taken, recording family background of diabetes plus blood pressure, heart rate and Tanner stage13. Age, body weight, height, waist circumference are measured and recorded in percentile according to age and gender, and the body mass index is calculated (kg/m2), and recorded in percentile according to reference values of the Center for the Disease Control, Atlanta, GA14. Subjects are randomized for metformin or placebo. Both metformin and placebo are placed in capsules with gelatin to blind their aspect. Treatments are administered during 3 months. Children are evaluated monthly, measuring transaminases, glucose, creatinine, and uric acid. Measurements of C reactive protein, cytokines, adipokines, insulin concentrations, HbA1c levels and lipid profile are made at the beginning and end of the study. Insulin resistance was assayed by HOMA-IR index. Beta cell function is determined by using HOMA-β. These are complemented with an evaluation of diet, physical activity, and physical examination.

Treatment assignment: Two groups are formed, randomly assigned using a table of random numbers previously designed with the Epistat statistic package (Round Rock, Texas). When the screening finished, every child receives a consecutive number. This number corresponds to an envelope with a card identifying his/her medication flasks, containing 60 tablets of 850 mg or placebo. The content of the capsules is not identified, so both the physician and the patient are blinded. A blinded member of the study prepared the flasks and labeled them.

Participants take the capsules twice daily, at breakfast and dinner. They are instructed to take medication with meals in order to decrease the possibility of adverse effects (nausea, meteorism, diarrhea), which are checked during every visit. Hepatic function is monitored during the study.

FPG, total cholesterol, HDL-cholesterol, triglycerides, liver function tests and creatinine are measured using the Synchron CX analyzer (Beckman Systems, Fullerton CA), according to the standard protocols. The coefficients of variation for cholesterol and HDL-cholesterol are 3.3% and 2.5% respectively. Plasma leptin, adiponectin and insulin concentrations are measured in duplicate by radioinmmunoassay (Linco Research Inc, St Charles, MO). HbA1c is determined in whole blood using ion exchange high performance liquid chromatography (normal range 4-6). Plasma resistin is measured using Human Resistin Elisa Kit (PeproTech, Rock Hill, NJ, USA) read by Multiscan EX, Lab Systems, USA). Plasma IL-6, IL1-b, TNF-α concentrations are determined by ELISA using Quantike HS Human Immunoassay Kits (R\&D Systems, Minneapolis, MN, USA), plasma C-reactive protein (hs-CRP) is measured using a highly sensitive human CRP ELISA Kit (Alpha diagnostic international, San Antonio, TX, USA ) according to the manufacturer's instructions and are read by an ELISA reader (Sunrise, Tecan USA, Durham NC,USA).