Inclusion Criteria:  One of the following diagnoses: rheumatoid arthritis (first diagnosis >6 weeks ago and <8 years), parkinson's disease OR healthy volunteer Control ("healthy") individuals must be without any evidence of active, known or treated RA, without any evidence of active, known or treated central nervous system disease, and without a known family history of idiopathic PD Arthritis in at least one joint Control individuals should match the RA or PD individuals as closely as possible, especially their age, sex, and education Present written declaration of consent Consent to specimen collection and specimen use Ability to understand the patient information and willingness to sign the consent form  Exclusion Criteria:  gout or proven bacterial arthritis Psychiatric illness that limits understanding of the examination protocol (unable to consent) BMI < 18.5 Pre-existing/current eating disorders (bulimia nervosa, anorexia nervosa) within the past 5 years. Severe internal diseases (e.g. renal insufficiency with creatinine > 2mg/dl) Participation in another study Existing vegan diet or fasting within the past 6 months Pregnancy or breastfeeding Chronic inflammatory bowel disease Use of antibiotics within the past 12 months Presence of anemia