Abstract Background Metabolic syndrome (MetS) has become a growing risk factor of some non-communicable diseases. Increase of greenhouse gas emissions affects the planet. Aims To assess the association between MetS severity and amount of carbon dioxide (CO₂) emitted in an adult population. Design Cross-sectional study (n = 6646; 55-76-year-old-men; 60-75-year-old-women with MetS). Methods Dietary habits were assessed using a pre-validated semi quantitative 143-item food frequency questionnaire. The amount of CO₂ emitted due to the production of food consumed by person and day was calculated using a European database, and the severity of the MetS was calculated with the MetS Severity Score. Results Higher glycaemia levels were found in people with higher CO₂ emissions. The risk of having high severe MetS was related to high CO₂ emissions. Conclusions Low CO₂ emissions diet would help to reduce MetS severity. Advantages for both health and the environment were found following a more sustainable diet. Trial registration ISRCTN, ISRCTN89898870. Registered 05 September 2013.