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.

Part I: Included 20 patients with complex chronic pain 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.

Part II: Included 192 patients with complex chronic pain. Half of the participants receive IPRP and nutrition care (described above) and the other half receive standard IPRP.

Potential participants are patients referred to a specialized pain and rehabilitation clinic due to complex chronic non-malignant pain conditions. A bio-psycho-social assessment revealed that these patients presented psychological symptoms and other comorbidities, reported large difficulties with their pain, and their condition severely affected their working life and participation in social activities. They often did not respond to routine pharmacological/ physiotherapeutic treatments delivered in a mono-disciplinary fashion. The potential participants received written information about the Swedish Quality Registry for Pain Rehabilitation (SQRP) (www.ucr.uu.se/nrs/) research and gave their written consent. The SQRP is mainly based on patient reported outcome measures (PROM) about socio-demographics, pain aspects, body weight and height, psychometric data, physical disability, and quality of life. Patients will complete the questionnaires of SQRP on three occasions: (1) before assessment on the first visit to the clinic (baseline or Pre-IPRP); (2) immediately after IPRPs (Post-IPRP); and (3) at the 12-month follow-up after IPRP discharge (FU-IPRP). Information on body height and weight were self-reported or measured and registered during the clinical assessment. All patients who respond to SQRP will be asked to participate in the study and may submit blood samples (see below).

After completing the informed consent to participate, the participants will fill out a food diary. For those who fail to complete food diaries, a dietician will book an individual meeting to go through three 24hour-dietary recordings. They will be introduced to get access to Diet4painrelief app by one research assistant. One dietician will make individualized plans of nutrition care in the app based on the information from the screening (through three 24hour-dietary recordings or food diary). The patient will receive dietary recommendations based on each patient's need for nutrition. The investigators will design and adapt 6 modules in the digital platform (Diet4painrelief app) based on the IASP recommendations (6 aspects about 'nutrition and pain') and Sweden's food culture. Overall, the app consists of the several features: weekly, push notifications, self-monitoring and feedback features of pain, diet, and recipe feature. The users will receive a text message prompting a brief weekly screening of the nutrition aspects, followed by feedback on individual screening results in comparison with recommendations they initially have received from the dietician. The feedback will be delivered on a graduated colored scale addressing diet aspects (agreement with recommendations). All users will then access a personal interactive dashboard with pictures representing nutritional behavior.

Through the Diet4painrelief platform, the dietician will follow up the progress of each participant in different timepoints during pain rehabilitation process: 4 weeks before standard pain rehabilitation program, 1-2 meetings integrated in rehabilitation process, and 4 weeks after the rehabilitation (via chat function). Participants' perceptions of using eHealth (digital healthcare applications) will be evaluated through individual interviews with the valid questionnaires (personal feelings, utility, and technical issues).

Blood samples will be collected in P100 tubes for all participants at Pre-IPRP, Post-IPRP and FU-IPRP. The sample will be centrifuged at 2500 g for 20 min at room temperature within 2-4 hours. Plasma will be extracted by carefully removing the upper part of the supernatant in fractions to a 10 mL tube, and after mixing gently will be aliquoted into 200 µl in 0.6 mL eppendorf tubes and stored at -86°C. The cell fraction will be removed to a new tube and stored at -86°C. The samples (marked with a code number) will be sent to the clinic´s research laboratory for centrifugation and storing in -86°C. The omic analysis aims to identify biomarkers that provide objective information on metabolic and nutritional abnormalities that can cause or worsen pain, further to objectively evaluate the effect of the dietary intervention on changing of pain rehabilitation outcomes.

Inclusion Criteria:

• Self reported Body Mass Index (BMI)>=25;
• Written consent to participate and attend to the pain rehabilitation program due to disabling chronic pain (on sick leave or experiencing major interference in daily life due to chronic pain)
• Agreement not to participate in other parallel treatments for weight reduction
• Not currently using medications of weight reduction
• Available to get access to internet via own mobile/cellphone.

Exclusion Criteria:

• Health conditions such as pregnancy, active metabolic disease, rheumatoid arthritis, and serious psychiatric disease (investigator's judgement, eg, psychosis or suicidal ideation);
• Coagulation disorders with predisposition to bleeding, medication with anticoagulants (low- dose aspirin is permitted);
• hypersensitivity to anaesthetic
• Difficulties in understanding the Swedish language.

Although we received full ethical authorization for this study, we do not have authorization to make the raw data available. Even though data are anonymized, these registry data are still considered sensitive information by the Swedish authorities and restrictions apply to making the data publicly available. The data are available upon request, and data requests should be sent to the Swedish Ethics Committee (registrator@etikprovning.se).