The use of knowledge resources in the digital health domain is a trending activity significantly grown in the last decade. In this paper, we present HeLiS: an ontology aiming to provide in tandem a representation of both the food and physical activity domains and the definition of concepts enabling the monitoring of users' actions and of their unhealthy behaviors. We describe the construction process, the plan for its maintenance, and how this ontology has been used into a real-world system with a focus on "Key to Health": a project for promoting healthy lifestyles on workplaces. In turn, this project is part of the "Trentino Salute 4.0" framework: a long-term initiative promoted by the regional government that is responsible for the sustainability of the presented ontology and of the services connected with it.