Both in free-ranging and captive-housed animals, food availability and quality has been repeatedly identified as an important factor influencing animal’s survival chances, physical health and wellbeing. Yet even today, the importance of providing captive animals with adequate diets is often being overlooked or underestimated. In this study, we evaluated the impact of improving the diets of two chimpanzee groups (N=13) housed at the primate sanctuary Fundació MONA. Here, their diet has been corrected for nutritional imbalances and adapted to the chimpanzees' increasing age, i.e., reducing sugar, increasing fibre while also increasing the food volume. To evaluate the impact of the new diet, we analysed 2 years of behavioural data as well as data on body weight and body condition. Furthermore, before and after the diet change, faecal samples for microbiome analyses and opportunistic blood samples (N=4) for blood analysis were collected. As expected, our preliminary results already indicate a desired impact on the animal’s behaviour and physical health. Specifically, overweight chimpanzees lost weight, while lean animals maintained their body condition. Behaviours, such as aggression, vigilance, abnormal and self-directed behaviour decreased while alimentation and relaxed resting activities increased. The gut microbiome showed a significant change in beta- but not alphadiversity, indicating a shift in composition of the bacterial communities, rather than 91 overall diversity. Moreover, blood parameters related to commonly problematic health issues in great apes, such as cardiovascular diseases, diabetes and/or obesity improved. Specifically, we found blood triglycerides and the blood sugar level (HbA1C) to decrease, whereas the cholesterol ratio greatly improved. Finally, in initially overweight chimpanzees, leptin concentrations noticeably decreased. Our results highlight the importance of providing captive-housed chimpanzees with an adequate and well-balanced diet, as well as demonstrate the clear positive impact on the chimpanzee´s health, behaviour and general welfare.