People with axial spondyloarthritis (axSpA) often have intestinal inflammation and intestinal microbiome dysbiosis, with some similarities to Crohn's-like inflammatory bowel disease (IBD) gut inflammation. However, research has not addressed whether Partial Enteral Nutrition (PEN), a diet formed of a liquid formula and some solid whole foods, which is effective at inducing remission in IBD, may influence the dysbiotic microbiome and inflamed, hyperpermeable intestine of axSpA patients, and whether these changes may be accompanied by alterations in systemic markers of inflammation. Thus, there is a need to determine the effects of PEN on these aspects in axSpA patients. In this study, the investigators intend to trial a 2-week course (with optional additional 2-week extension) of a PEN diet in people with active axSpA disease. A group of healthy volunteers following the same diet will act as a control.