People are nowadays well aware that adopting healthy lifestyles, i.e., a combination of correct diet and adequate physical activity, may significantly contribute to the prevention of chronic diseases. We present the use of Semantic Web technologies to build a system for supporting and motivating people in following healthy lifestyles. Semantic technologies are used for modeling all relevant information, and for fostering reasoning activities by combining real-time user-generated data and domain expert knowledge. The proposed solution is validated in a realistic scenario and lessons learned from this experience are reported.