In recent years, emerging approaches to chatbot-guided food coaching and dietary management, while innovative and promising in nature, have often lacked long-term studies. Therefore, with this work, we pursued a participatory approach within a design case study to the co-design and development of a nutrition chatbot for elderly people. Overall, 15 participants were directly involved in the study, of which 12 participated in the initial co-design phase, seven in the first real-world evaluation study over four weeks, and three in the second evaluation study over seven weeks. We contribute to the fields of Human-Computer Interaction by showing how the long-term use of such a chatbot in the area of nutrition looks like, which design implications arise for the development of nutrition chatbots, and how a participatory design approach can be realized to design, evaluate and develop nutrition chatbots.