axSEND: Exploring Immune and Microbiota Effects of a Partial Enteral Nutrition Diet in Axial Spondyloarthritis

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.