The prevalence of obesity is increasing rapidly in all age groups in most EU-countries and is one of the fastest growing epidemics, now affecting 10-40% of the adult population. Obesity increases the risk of serious co-morbidities such as type 2 diabetes, cardiovascular disease, certain cancers and reduced life expectancy, and these complications may account for 5-10% of all health costs in EU countries. The risk of diabetes is particularly increased by obesity, and 80-95% of the increase in diabetes can be attributed to obesity and overweight with abdominal fat distribution. There is robust evidence from cross-sectional and longitudinal studies to support that an energy-dense, high fat diet and physical inactivity are independent risk factors for weight gain and obesity. Furthermore, interaction between dietary fat and physical fitness determine fat balance, so that the obesity promoting effect of a high fat diet is enhanced in susceptible subjects, particularly in sedentary individuals with a genetic predisposition to obesity. Ad libitum consumption of diets low in fat and high in protein and complex carbohydrates, with a low glycaemic index, contributes to the prevention of weight gain in normal weight subjects. It also causes a spontaneous weight loss of 3-4 kg in overweight subjects, and has beneficial effects on risk factors for diabetes and CVD. To prevent obesity and diabetes there are grounds for recommending the combination of increasing daily physical activity level to a PAL-value of at least 1.8 and reducing dietary fat content to 20-25 energy-% in sedentary subjects, and to 25-35% in more physically active individuals.