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.

Eligibility Criteria for Participants with axSpA

Inclusion criteria. Participants will fulfil ALL of the following:

• Diagnosis of axSpA (fulfilling the ASAS criteria)
• Active disease (BASDAI score ≥4) on day of study visit
• Stable on treatment [defined as (1) no major change to therapy (change in treatment type) in the preceding 3 months AND (2) no minor change in therapy (adjustment of treatment dosage) in the preceding 1 month]
• Age ≥ 16 years
• Willing and able to give informed written consent.

Exclusion. Participants will have NONE of the following:

• Pregnancy or breastfeeding
• Prior diagnosis of IBD
• Recipient of chemotherapy/immunotherapy/radiotherapy in prior 3 months
• Recent systemic antibiotic use (within last 1 month)
• Current eating disorder
• Food allergy incompatible with diet (e.g. cow's milk allergy)
• Following a vegan diet
• Major surgery in prior 3 months or planned in forthcoming 3 months.
• Unable or unwilling to give informed consent
• Unable or unwilling to try the PEN diet.

Eligibility Criteria for Healthy Volunteer Participants

Inclusion criteria. Participants will fulfil ALL of the following:

• Age ≥ 16 years
• Current student and/or staff member at the University of Glasgow
• Willing and able to give informed written consent

Exclusion. Participants will have NONE of the following:

• Pregnancy or breastfeeding
• Prior diagnosis of an immune-mediated inflammatory condition
• Recipient of chemotherapy/immunotherapy/radiotherapy in prior 3 months
• Recent systemic antibiotic use (within last 1 month)
• Current eating disorder
• Food allergy incompatible with diet (e.g. cow's milk allergy)
• Following a vegan diet
• Major surgery in prior 3 months or planned in forthcoming 3 months.
• Unable or unwilling to give informed consent
• Unable or unwilling to try the PEN diet.