The trial will study the acute metabolic effects of three different types of diet: low-fat diet, low-carbohydrate diet and Mediterranean diet. About 20 patients with diabetes mellitus type 2 will be recruited and each of them will be given each type of diet on three different days. Menus with the same energy content but with different content of fat, carbohydrate and protein are designed. Breakfast and lunch are served for the patients each day of testing. Laboratory samplings are performed six times during the day. The order of the dietary intervention is randomized for each patient.