Studies have been conducted that show a significant increase in the body mass index of people on the three anti-TNFs, Remicade, Enbrel, and Humira. No reason has been determined yet but they do know that the meds can cause weight gain. The largest look/study was done on people with PsA. I think they are reluctant to run the test on those with bowel issues since their nutritional status can be compromised from the disease.
Besides, AS, dealing with Crohn's, peripheral neuropathy, avascular necrosis, pulmonary hypertension, PAD, and a host of other issues.