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 and have a Body Mass Index (BMI) ≥ 25 kg/m2 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.