Q&A: BMI Link to Disease Activity in Axial Spondyloarthritis

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Patients with axial spondyloarthritis may be more susceptible to higher disease activity if they are overweight or obese, according to a systematic review and metaanalysis recently published in RMD Open.

While a higher body mass index (BMI) in patients with axial spondyloarthritis is associated with worse outcomes, including response to biologics, there is a knowledge gap regarding whether higher BMI is associated with higher disease activity in axial spondyloarthritis. Researchers conducted a systematic literature review and metaanalysis of observational studies and randomized clinical trials in patients with axial spondyloarthritis assessing the association of BMI on disease activity as measured by the Bath AS Disease Activity Index (BASDAI) or AS Disease Activity Score (ASDAS). Twelve studies were included in the metaanalysis.

In this Q&A with study author Jean Liew, M.D., senior fellow at the University of Washington in Seattle, we discuss the study and its significance.

What were the main findings?

In these studies a higher BMI (e.g., overweight/obese BMI) was associated with higher disease activity compared to a normal BMI. When we restricted our analyses to specific subgroups of studies (e.g., studies that compared obese BMI to normal BMI; studies conducted in Europe only; studies including people with ankylosing spondylitis only), the same held true.

The pooled standardized mean difference of the BASDAI for those with an obese or overweight BMI compared with a normal BMI was 0.38 (95% CI 0.21 to 0.55, I2 =75.2%). The pooled standardized mean differencesof the ASDAS for those with an obese or overweight BMI compared with a normal BMI was 0.40 (95% CI 0.27 to 0.54, I2=0%). Higher BMI was significantly associated with higher disease activity as measured by both BASDAI and ASDAS.

Why do you think a higher BMI may contribute to disease burden in patients with axial spondyloarthritis?

One hypothesis is that adipose tissue is involved in inflammation, through the release of signals that propagate further inflammation. A higher BMI is just a proxy for adiposity, though. A higher BMI may contribute to higher disease burden in other ways, such as through mechanical stress.

Previous reviews and metaanalyses have found a link between BMI and clinical outcome measures of axial spondyloarthritis. What do your findings add?

Outcome measures explored in these previous reviews and meta-analyses were imaging findings of inflammation, and response to TNF inhibitors. We looked at disease activity across the board. It was a more general question.

Can the results be party explained by age, gender or comorbidities contributing to the increased BMI?

Age and gender would have been accounted for in adjusted analyses in some of the studies that we included that still showed this association. Prior studies may not have accounted fully for the effect from comorbidities, so that remains a possibility.

How can your findings help clinicians and benefit patients?

There is a lot more research to be done to sort out what components of a higher BMI are driving higher disease activity. That would help with identifying targeted recommendations for clinical practice. In the meantime, exercise and physical therapy should be recommended as these are in the current axial spondyloarthritis guidelines.

In psoriatic arthritis, weight loss has been found to be associated with lower disease activity. The same may be true in axial spondyloarthritis, though we would need more direct evidence.

Do you have anything else to add?

Remember to interpret the metaanalyses and the studies included in the meta-analyses as studies of association and not causation. These were all observational studies. We cannot say for sure that a higher BMI leads to higher disease activity. Other factors must be accounted for, which we weren't able to do here.

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REFERENCE

Jean W Liew, Irvin J Huang, Diana N Louden,et al. “Association of body mass index on disease activity in axial spondyloarthritis: systematic review and meta-analysis.”RMD Open. May 20, 2020. doi: 10.1136/rmdopen-2020-001225