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. conditions: Nutrition Poor conditions: Overweight and Obesity conditions: Mobile Phone Use conditions: Pain, Chronic studyType: INTERVENTIONAL phases: NA allocation: NA interventionModel: SINGLE_GROUP interventionModelDescription: Repeated measures, single-subject withdrawal design. All subjects who enroll in the study will receive the intervention and there is no randomization. primaryPurpose: TREATMENT masking: NONE count: 20 type: ESTIMATED name: Nutrition care name: Dialogue and education (part of Interdisciplinary Pain Rehabilitation Program) name: activity training (part of Interdisciplinary Pain Rehabilitation Program) name: meetings (part of Interdisciplinary Pain Rehabilitation Program) name: Cognitive behavioral therapy (part of Interdisciplinary Pain Rehabilitation Program) name: Relaxation techniques (part of Interdisciplinary Pain Rehabilitation Program) name: Physical exercise (part of Interdisciplinary Pain Rehabilitation Program) measure: Pain intensity (numeric rating scale) measure: Pain interference (one aspect in Multidimensional Pain inventory) measure: Dietary habits (a lifestyle questionnaire) measure: Perceived Usability (System Usability Scale) measure: Perceived benefit (numeric rating scale) measure: Adherences/ compliances of nutritional care (numeric rating scale) measure: Difficulty sleeping (Insomnia Severity Index) measure: Emotional distress (The Hospital Anxiety and Depression Scale, HAD) measure: Impact of chronic pain (Multidimensional Pain inventory) measure: Health- related quality of life 1 (EQ-5D) measure: Health- related quality of life 2 (RAND-36) measure: Pain coping (Chronic Pain Acceptance Questionnaire, CPAQ) measure: Fear of movement (Tampa scale for Kinesiophobia) measure: Physical activity and sedentary behavior (self-estimated time, questionnaire) measure: Body fatness (physiological parameter) measure: Body weight (physiological parameter) measure: Biomarkers explored from blood samples (physiological parameters such as proteins, lipids, metabolites and micro-RNA) sex: ALL minimumAge: 18 Years stdAges: ADULT stdAges: OLDER_ADULT hasResults: False