Obesity is common in rheumatoid arthritis and associated with worse outcomes, but whether it reduces the effect of specific drugs has been hard to establish from trial reports alone. This individual patient data meta-analysis obtained participant-level data via the Vivli platform from 16 phase 3 trials of tofacitinib, baricitinib and upadacitinib, covering 11,883 participants, of whom 30.9% had class 1-3 obesity.
Higher BMI reduced efficacy across all endpoints, and the gradient is orderly. Against healthy weight, adjusted relative risk for ACR20 was 0.94 (95% CI 0.91-0.98) for overweight, 0.92 (0.89-0.96) for class 1 obesity, 0.88 (0.81-0.96) for class 2, and 0.78 (0.71-0.85) for class 3. DAS28-CRP differences rose in parallel, from 0.14 to 0.54. Heterogeneity was low to moderate and risk of bias low.
The finding that makes this credible is what happened in the placebo arms: no BMI gradient at all. That distinguishes a genuine reduction in drug effect from the alternative explanation, that obesity simply makes disease activity scores worse or harder to improve regardless of treatment. Practically, this is an argument for raising weight management as part of rheumatoid arthritis treatment rather than as general health advice, and for setting expectations honestly with a patient with class 3 obesity starting a JAK inhibitor.
- ACR20 relative risk fell stepwise: 0.94, 0.92, 0.88, 0.78 across BMI categories
- DAS28-CRP differences rose in parallel, up to 0.54 in class 3 obesity
- No BMI gradient in the placebo arms — the effect is on the drug, not the measure
- Individual patient data from 16 trials, low risk of bias
The statistics, in plain English
The stepwise pattern across four BMI categories is what elevates this above an association: a spurious finding would rarely line up in order of exposure. The absence of the same gradient under placebo is the second safeguard, ruling out the explanation that heavier patients simply improve less whatever they receive. Note the effect at overweight, relative risk 0.94, is small enough to matter little for an individual; it is at class 2 and 3 obesity that the difference becomes clinically consequential.
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