BACKGROUND: There are growing concern globally regarding fast food consumption and its related cardiometabolic outcomes. In this study we investigated whether fast food consumption could affect the occurrence of metabolic syndrome (MetS) after 3-years of follow-up in adults. METHODS: This longitudinal study was conducted in the framework of Tehran Lipid and Glucose Study on 1476 adults, aged 19-70 y. The usual intakes of participants were measured using a validated semi-quantitative food frequency questionnaire at baseline. Biochemical and anthropometric measurements were assessed at baseline (2006)(2007)(2008) and 3 years later (2009)(2010)(2011). Multiple logistic regression models were used to estimate the occurrence of the MetS in each quartile of fast food consumption. RESULTS: The mean age of participants was 37.8 ± 12.3 y, and mean BMI was 26.0 ± 4.5 kg/m 2 at baseline. Participants in the highest quartile of fast food consumption were younger (33.7 vs 43.4 years, Po0.01). Higher consumption of fast food was accompanied with more increase in serum triglyceride levels and triglyceride to HDL-C ratio after the 3-year follow-up. After adjustment for all potential confounding variables, the risk of metabolic syndrome, in the highest quartile of fast foods compared with the lowest, was 1.85 (95% CI ¼ 1.17-2.95). The effects of fast food consumption on the occurrence of MetS were more pronounced in younger adults (o30 years), and participants who had greater wait to hip ratio, consumed less phytochemical-rich foods or had low-fiber diet (Po0.05). CONCLUSION: We demonstrated that higher consumption of fast foods had undesirable effects on metabolic syndrome after 3-years of follow-up in Iranian adults. Keywords: metabolic syndrome; fast foods; abdominal obesity; insulin resistance; dyslipidemia; hypertension INTRODUCTION Metabolic syndrome (MetS) is a common multi-factorial disorder including insulin resistance, abdominal obesity, dyslipidemia and hypertension, all of which directly promote the development of diabetes and cardiovascular disease. 1,2 The prevalence of obesity, metabolic syndrome and related cardiometabolic risk factors have increased over the past two decades in developing countries. 3 Major factors contributing to this alarming rising trend are sedentary lifestyles and the on-going nutrition transition with progressive shift to a westernized diet, in particular higher consumption of processed foods, sweetened beverages and fast foods. 4 Epidemiological and clinical studies reported that frequent consumption of fast foods is related to weight gain, insulin resistance and other metabolic abnormalities. [5][6][7] Reports indicated that mortality and hospital admission for acute coronary heart disease in regions with providing greater numbers of fast food services were significantly higher. 8 Out-of-home meals and fast foods have large portion sizes and contain highly processed meat and refined carbohydrate as well as sodium; fast foods are also energy dense, and rich sources of total fat, saturated...