Exclusion criteria

The exclusion criteria were (1) documents that studied pharmacological and surgical treatments with no nutritional approach for CNCP, (2) acute pain, and (3) as this systematic review is focused only on nutritional interventions, pain derived from surgical interventions or oncological processes was also excluded.

The search and screening of documents were carried out by two researchers independently and the discrepancies regarding the selected documents were resolved by consensus of the researchers. Registration was made in the International Prospective Register of Systematic Reviews (PROSPERO) with the code CRD42021226431.

A data extraction table was created for the documents included in the review (Table 1), with the following items: first author, year, type of pain, objectives, method, sample, duration, measuring instruments, intervention design, and results.

Main characteristics of the studies included in this systematic review.

Antiox, antioxidants; DAS28, Disease Activity Score 28; DS, dietary survey; FCFQ, food consumption frequency questionnaire; FIQ, Fibromyalgia Impact Questionnaire; HAQ, Health Assessment Questionnaire; ICOAP, Intermittent and Constant Osteoarthritis Pain; KOOS, Knee Injury and Osteoarthritis Outcome Score; OA, osteoarthritis; OS, observational study; PROMIS-57, Patient-Reported Outcomes Measurement Information System-57; RA, rheumatoid osteoarthritis; RCT, randomized clinical trial; SF-36, short form health survey-36; SF-8, Short Form Health Survey-8; VAS, visual analog scale; vit, vitamin; WOMAC, Western Ontario and McMaster Universities Osteoarthritis Index.