Over the last century, there has been a rapid decrease in the incidence of many infectious diseases. Over the same period, the prevalence of several noninfectious diseases, e.g. obesity, type-2 diabetes, and coronary heart disease, has increased dramatically to the point that we are facing an epidemic of noncommunicable diseases. These diseases are clearly observed more frequently in ‘rich’ countries: they are favored by the availability of abundant foods, motorized transportation and other riches and can be denominated ‘diseases of affluence’ . Prominent among these diseases of affluence is a constellation of pathological findings, including obesity, type-2 diabetes mellitus, dyslipidemia, high blood pressure and other related metabolic or vascular abnormalities, which has been called successively the syndrome X, the insulin resistance syndrome or, more recently, the metabolic syndrome . Although the pathogenesis of this syndrome remains debated, it is recognized that insulin resistance is one of its central components and might indeed be instrumental in the development of several of the associated metabolic alterations . Genetic factors are generally thought to be involved in the pathogenesis of obesity, type-2 diabetes, and dyslipidemia. Genetic factors may directly intervene in the pathogenesis of insulin resistance. The recent epidemiological development of these disorders, which showed a several-fold increase in their rates of prevalence over only a few decades, i.e. during which it is unlikely that major changes in the genetic background occurred, clearly indicates that other factors play a prominent role. It is likely that these factors are to be searched for in the important environmental changes which have occurred in the recent history of the world. Allison SP, Go VLW (eds): Metabolic Issues of Clinical Nutrition. Nestlé Nutrition Workshop Series Clinical & Performance Program, vol 9, pp 19–35, Nestec Ltd., Vevey/S. Karger AG, Basel, © 2004.