The investigators' purpose is to develop and test the feasibility, usability, and satisfaction of the Diet4painrelief app as a platform for implementing nutrition care in a specialist pain rehabilitation clinic. The Diet4painrelief will consist of two components: a) a screening tool for nutrition status b) an individually tailored behavior change program aiming to improve the dietary habits and behaviors of patients living with chronic pain. The study is planned to include 20 patients with complex chronic pain and non-optimal BMI (underweight or overweight/obesity) to examine the feasibility and outcomes of evidence-based Interdisciplinary Pain Rehabilitation Program (IPRP) integrated with nutrition care. The Diet4painrelief includes a screening tool to assess basic nutritional status as well as their intake of key unhealthy and healthy foods and drinks (through three 24hour-dietary recordings or food diary). Thereafter, the patients receive a personalized behavior change program for dietary optimization. The investigators will design and adapt 6 modules in the digital platform (Diet4painrelief app) based on the International Association for the Study of Pain (IASP) recommendations (6 aspects about 'nutrition and pain') and Sweden's food culture. Clinical outcomes using patient-reported data on socio-demographics, pain aspects, psychometric data, physical disability, and quality of life will be measured at three occasions: first appointment (Pre-IPRP), immediately after completing the rehabilitation program (Post-IPRP), and at a 12-month follow-up (FU-IPRP). Blood samples will be taken to identify biomarkers at the same occasions that provide objective information on metabolic and nutritional abnormalities and further to evaluate the effect of the dietary intervention on changing of pain rehabilitation outcomes.