Background: During the 1990s, the prevalence of the metabolic syndrome in the Netherlands ranged from 3% in women of 20-39 yrs to at least 33% in men 55 yrs and older and it is expected to increasing. Prevention is therefore warranted. In this respect the amount and type of fat in the diet deserves attention. Recently, an intervention study reported that a diet high in mono-unsaturated fatty acids (MUFA) such as from olive oil, increased insulin sensitivity in healthy subjects. However, additional beneficial effects can be expected from the Mediterranean diet as a whole. Hypothesis: Replacing saturated fatty acids (SFA) by mono-unsaturated fatty acids (MUFA) will improve hyperinsulinemia and dyslipidemia, and a typical Mediterranean diet will even have more pronounced effects. Study objectives: To investigate the impact of the Mediterranean diet, and especially the intake of MUFA, on markers of the metabolic syndrome in high-risk subjects. Methods: The controlled dietary intervention will include 60 subjects aged 40-65 years with moderate abdominal obesity. After a run-in diet for 2 weeks they will be assigned randomly to receive one of the three diets for a period of 8 weeks. Measurements of serum insulin concentration and other parameters will be carried out at weeks 2 and 10. Expected results: Our study will provide information on the role of MUFA and the expected beneficial impact of other factors of the Mediterranean type of diet on the metabolic syndrome.