The aim of the study is to find out the effects a specific religious fast (i.e. Bahá'í fast) has on certain metabolic parameters, hydration, psyche and circadian clock. In a follow-up questionnaire series in 2019 we want to additionally validate a specific questionnaire for Bahai fasting, which was developed in 2018.

Followers of the Bahá'í Faith worldwide follow a yearly fasting tradition, where they fast intermittently for nineteen days. The intermittent fast is defined as abstinence from any food, drink and smoking from sunrise until sunset. These nineteen days are always in March and so do not coincide with climatic extremes in any country worldwide. This makes this kind of fasting a good model to study the psychological and medical effects of intermittent fasting in humans.

Inclusion Criteria:

• member of the Bahá'í religious community, Age between 18-69 years, the study participant must be able to understand the instructions given to him by the study personnel, the performance of the religious fast is planned in 2018 (and for the validation of the questionnaire also in 2019)

Exclusion Criteria:

• An interruption of the religious fast is planned for more than five days, pregnant and nursing women, severe internistic condition, eating disorders (anorexia nervosa, bulimia), terminal or severe disease with marked impairments in mobility and vitality, non-existence of email address and Internet Access (because of online questionnaires), severe psychiatric disorder, simultaneous participation in another Trial

Exclusion Criteria for subsample (energy metabolism measurements and microdialysis)

• Body Mass Index <18,0 und >30,9 kg/m2, claustrophobia, clinically relevant haemostaseological conditions or medication, vegan diet, special diet out of medical reasons, current dieting for weight loss, weight loss of more than 2 kg in the month before the study commenced, postsurgical conditions, acute and chronic infections, known drug or alcohol abuse
• additional exclusion criteria for microdialysis (subsample of energy metabolism measurements): allergy to local anaesthetics

Members of the Bahá'í community residing in Germany