Design: Randomized, double-blind, placebo-controlled study with 4 weeks of treatment with either fiber supplements or placebo.

Primary outcome: variation in SCFA between baseline and week 4. Secondary outcomes : variations in gut microbiota, disease activity (DAS28, RAID), immune cells (regulatory T and B cells, Th17) and heart-rate variability.

Population: 29 patients with rheumatoid arthritis treated with only csDMARDs and a moderate disease activity will be recruited in each arm and 29 controls with mechanic disease (osteoarthritis, tendinitis).

Collection: Blood, feces, disease activity and heart rate variability will be collected at baseline and week 4.

General

Inclusion criteria:

• Aged 18 to 85
• Be affiliated to or beneficiary of a French social security scheme
• o No desire for pregnancy for the duration of the study (Effective contraception for women of reproductive age, for the duration of the study (surgical sterilization, hormonal contraceptives, barrier method, intrauterine device))

Exclusion criteria:

• Lack of written informed consent after a period of reflection
• Patient involved in other research or research for which the exclusion period has not ended
• Pregnant or nursing woman
• Type I or II diabetes
• Patient receiving more than 10 mg/d corticosteroids at time of inclusion
• Patient who received corticosteroid infusions in the month prior to randomization
• Unusual consumption (>50% higher than usual consumption) of dietary fibre in the last 72 hours (assessed by food questionnaire)

Patient with Rheumatoid polyarthritis

Inclusion criteria:

• Rheumatoid arthritis meeting ACR/EULAR 2010 criteria
• With moderate clinical activity: DAS28-CRP 3.2 and 5.1
• Patient receiving conventional background treatment at stable doses and should not be modified during 4-week follow-up

Exclusion criteria:

• Patient treated by targeted RA treatment in the year prior to inclusion
• Patient who received antibiotic therapy within 3 months prior to randomization

Control subjects

Inclusion criteria:

• Patient consulting in the rheumatology department for a mechanical pathology: osteoarthritis of the limbs or spine, osteoporosis, tendinopathy, fibromyalgia, algodystrophy or capsulitis.
• Matched to a gender and age PR topic 5 years

Exclusion criteria:

· Patient with autoimmune disease, infection or progressive cancer