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Undernutrition is a major health problem in the growing elderly population. It is estimated that one in ten Dutch community-dwelling older adults is suffering from undernutrition, and one in three Dutch older adults who receive home care. Undernutrition may lead to many negative consequences, ranging from fatigue and falls to impaired immune function and death. This makes undernutrition an obvious target for preventive measures. Undernutrition can be defined as “a state of nutrition in which a deficiency or excess (or imbalance) of energy, protein, and other nutrients causes measurable adverse effects on tissue/body form (body shape, size and composition) and function, and clinical outcome”. In addition, it is often described as protein energy malnutrition. Adequate protein intake may to some extent prevent and reverse this process. However, throughout ageing, it becomes increasingly difficult to reach adequate protein intake due to higher protein needs and lower protein intakes. Finding solutions to assist older adults in reaching their optimal protein intake is necessary. In our overall research project, we considered 1.2g protein per kg weight per day (g/kg/d) as adequate protein intake. In Dutch community-dwelling older adults, protein intake is around 1.0 g/kg/d, implying room for improvement. However, it is possible that many of these older adults deal with physiological changes, medical conditions, and physical and mental limitations that impair their appetite and food provision. For these older adults with higher protein needs, merely recommending that they eat more would not be realistic. It would be more realistic to explore strategies that increase protein intake without having to increase food intake. This calls for the exploration of instruments that match the needs and preferences of older adults: protein-enriched regular products. One particular group that can be identified as a target group for such products, are older adults who receive home care. Undernutrition prevalence is high in this group, which may be explained by their health problems that led to this dependence on home care. Likewise, many of these older adults also depend on meals-on-wheels. These meals-on-wheels recipients, regardless of whether they receive home care or not, often risk undernutrition too. In both these (overlapping) care-dependent groups, difficulties in adhering to energy and protein recommendations can be discerned. For this reason, enriching the readymade meals that these older adults receive may contribute to the prevention of protein undernutrition by increasing protein intake while keeping food intake the same. Here, protein enrichment instruments can be used to prevent undernutrition, but only when implemented in a timely manner. Adequate undernutrition management systems are therefore necessary to facilitate timely intervention, ensuring that the developed protein-enriched meals are actually offered and effective. For this reason, the overall aim of our research project was to gain insight into the current state of undernutrition management in community-dwelling older adults in the Netherlands and explore the role of protein-enriched regular products as a supportive instrument in protein undernutrition management. In Study 1 (chapter 2) we explored the experiences of 22 Dutch nutrition and care professionals and researchers with undernutrition awareness, monitoring, and treatment among community-dwelling older adults. This qualitative study among, for example, dietitians, general practitioners, nurse practitioners, and home care nurses provided insight into the current bottlenecks within the existing undernutrition management guidelines. In these telephone interviews, these experts also discussed the current dietary behaviour problems of older adults and their impact on undernutrition risk. The experts’ experiences implied that undernutrition awareness is limited, among both older adults and care professionals. In addition, the interviewees were unclear about which professionals are responsible for monitoring and which monitoring procedures are preferred. The dietitians feel that they become involved too late, leading to decreased treatment effectiveness. In general, the interviewees desired more collaboration and a coherent and feasible allocation of responsibilities regarding undernutrition monitoring and treatment. This implied that the available guidelines on undernutrition management require more attention and facilitation. In the following mixed-methods study (chapter 3), with interviews, we qualitatively explored the dietary behaviour and undernutrition risk of 12 Dutch elderly meals-on-wheels clients, one of the largest at-risk groups. We followed up on this information by quantifying the topics that emerged from the qualitative exploration of experienced bottlenecks in performing adequate dietary behaviour. For this, we used a survey among 333 meals-on-wheels clients. The interviews with elderly meals-on-wheels clients made clear that they have fixed and habitual eating patterns, while at the same time their appetite had decreased throughout the years. This was confirmed by the survey finding that regular portion size meals were perceived as too large by the oldest group aged over 75y. In addition, as the professionals suggested earlier, the interviewed elderly clients indeed showed limited awareness of undernutrition risk. Simultaneously, the survey showed that almost one in four elderly meals-on-wheels clients was undernourished. These findings led to the conclusion that staying close to the identified dietary habits may facilitate small yet effective modifications within these habits to prevent inadequate nutritional intake. Still, the limited awareness of undernutrition risk was expected to play a limiting role in whether clients believe they need dietary modifications. Consequently, informing them about this need could facilitate their motivation to implement modifications. After learning about the general dietary behaviour of these older adults, we used this information for Study 3 (chapter 4). We developed two kinds of protein-enriched readymade meals that are in line with the needs and preferences of older adults: one of regular size (450g) and one of reduced size (400g). We tested these meals in a lab setting in 120 community-dwelling older adults in a single-blind randomised crossover trial. One day a week at lunchtime, for four weeks, participants had to consume and evaluate a readymade meal. Overall, regardless of portion size, the protein-enriched meals led to higher protein intakes in vital older adults in a lab setting during lunch. In this crossover study, the participants liked the protein-enriched meals and the regular meals equally. However, we did not find the expected lower ratings of satiety after the reduced-size meals, while one reduced-size enriched meal and another regular-size enriched meal led to higher ratings of subsequent satiety. This higher satiety in the enriched meals could lead to compensational behaviour on the remainder of the day. After establishing that the protein-enriched meals were effective and acceptable in the lab setting, we moved to the homes of older adults to test the meals in a longer-term study in Study 4 (chapter 5). In this double-blind randomised controlled trial of two weeks, we also included protein-enriched bread to assess whether both this bread and the meals could increase daily protein intake to 1.2g/kg/d in 42 community-dwelling older adults to reach optimal protein intake. We found that the enriched products again led to higher protein intakes and a high liking. The mean protein intake per day was 14.6g higher in the intervention group, which amounted to a protein intake of 1.25g/kg/d, compared with 0.99g/kg/d in the control group. In addition, the meals scored 7.7 out of 10, while the bread scored 7.8 out of 10, which both were comparable with their regular counterparts. Lastly, we found no negative effect of compensational behaviour throughout the day. These promising findings indicated that we achieved a good match between older adults’ needs and preferences regarding protein intake. In the general discussion of this thesis (chapter 6), we combined our learnings from the four studies to reflect on protein undernutrition management in community-dwelling older adults and the possible role of protein-enriched regular products. We have discussed a conceptual framework consisting of three wheels of protein undernutrition management. In the first wheel regarding awareness, we proposed that limited awareness of adequate nutrition and body composition forms the largest bottleneck in undernutrition management. When this awareness is generated among both older adults and professionals, it will benefit the second wheel of monitoring. Here, we argued that a policy and the actual facilitation of that policy are required for this monitoring to succeed. When the monitoring is performed adequately, in the third wheel, the appropriate treatment can be carried out. We discussed that personalisation and evaluation of this treatment are important conditions. All in all, the public health implications that we have discussed on the basis of our findings can be summarised by the three key messages that could help us ace in adequate protein undernutrition management: address awareness in both older adults and professionals, facilitate continuous collaboration between professionals, and offer protein-enriched products expediently.