Inflammatory rheumatic diseases (IRD), such as rheumatoid arthritis, are characterized by adverse changes in body composition. Lean mass and bone mineral density are usually reduced while adiposity (total fat mass, visceral adiposity…) is increased in comparison with healthy controls. Many factors may influence the body composition of those patients such as aging, Disease Modifying Anti-Rheumatic Drugs (DMARDs), nutrition and physical activity.

However, data on body composition and adverse changes under DMARDs in patients with rheumatoid arthritis (RA) are actually scarce. This is the case with tofacitinib (targeted synthetic DMARD or tsDMARD) while preliminary data let us think that this treatment may influence body composition and bone mineral density.

This study is going to be the first to focus on changes in body composition (fat mass and lean mass), bone mineral density and bone marrow adiposity under tofacitinib.

Inclusion Criteria:

• Adult patient's ≥18 years old with moderately to severely active Rheumatoid Arthritis (RA) (ACR/EULAR criteria )
• Previously untreated with Janus Kinase (JAK) inhibitors
• With an indication for tofacitinib will be eligible.
• All patients will have to be treated with tofacitinib either alone or with methotrexate. -Healthy volunteers should be ≥18 years old.

Exclusion Criteria:

• • treatment with more than three anti-Tumor Necrosis Factor alpha (TNFα). Patients who were receiving anti-TNFα will be required a washout period lasting at least five-half-lives before to start tofacitinib,

  • previously exposed to JAK inhibitors,
  • patients who were receiving non-anti-TNFα biologics (abatacept, tocilizumab, sarilumab or rituximab) will be required a washout period lasting at least five-half-lives before to start tofacitinib
  • Concomitant methotrexate (MTX) will be permitted if started ≥3 months prior to study start and at a stable dose (≤25 mg/week) for ≥4 weeks.
  • history or discovery of an osteoporotic fracture AND/OR T-score≤-3 if ≥50 years AND/OR Z-score ≤-3 if <50 years during the screening phase,
  • current treatment with oral corticosteroids higher than 10 mg prednisone/day,
  • pathologies or treatments that could affect the bone metabolism (breast cancer with aromatase inhibitors, gastrointestinal malabsorption, stomach cancer, primary hyperparathyroidism, uncontrolled hyperthyroidism…),
  • weight> 160 kg,
  • patients on restrictive diets or considering such a diet during the study period,
  • patients with an intense exercise program or planning to benefit from it during the study period,

The study population is composed of adult patients with moderately to severely active RA for whom tofacitinib is indicated and who are followed in the department of rheumatology at Lille University Hospital