This study explores what happens in the body and wellbeing during and after a period of long-term fasting. At the Buchinger Wilhelmi Clinic, participants follow a medically supervised fasting program lasting five or more days, followed by a structured food reintroduction phase. The study observes participants over time to better understand how fasting and the return to eating affect weight, metabolism, nutrition habits, and overall well-being.

All participants will complete online questionnaires during the fasting phase and for up to three months afterward. These surveys help track changes in lifestyle, mood, and health. In addition during the first two weeks after fasting ends participants wear a continuous glucose monitor (CGM) to track blood sugar levels in real time, use a mobile app to document meals with photos, and perform simple at-home blood tests using dried blood spot (DBS) kits.

By combining digital tools with personal reports and at home tests, the study aims to gain a clearer picture of how the body adapts after a prolonged fast. The goal is to better understand the short- and medium-term effects of extended fasting on metabolism, diet, and well-being-and to provide practical guidance for those interested in fasting as a health practice.

Dear prospective study participants,

For optimal results of a fasting cure, the investigators would like to better understand the phase of food reintroduction, in which food is taken back after fasting.

The investigators would like to ask potential participants if they are willing to participate in the study of the Buchinger Wilhelmi Fasting Program. The study examines the post-fasting phase of food reintroduction. For this purpose, the first 14 days after the end of the fast are closely documented. This is done with the help of continuous glucose measurement and documentation of meals. In addition, various systemic biomarkers and the inflammatory status in blood are examined. In addition, further follow-up examinations take place after one and three months.

This study will include a total of more than 200 participants.

How will the study proceed and what investigations are planned?

The stay in the clinic begins with the admission examination by the study doctor, who explains the course of the study in detail and asks participants to sign the consent form. On the day of arrival, participants will receive an organic vegetarian dinner. This is followed by a "digestive rest day", a transitional day that is intended to prepare the body for the fasting phase. It consists of an easily digestible low-calorie mono-diet in three portions.

The fasting phase begins with a colon cleanse, e.g. with Glauber's salt. During fasting, the daily food intake consists of 1/4 l of freshly squeezed fruit or vegetable juice or 1/4 l of vegetable broth at noon, 1/4 l of vegetable broth in the evening and optionally 20 g of honey throughout the day. The end of fasting occurs individually after 7 or more (7+) days and is complemented by a phase of slow reintroduction of food with an ovo-lacto-vegetarian diet.

The study participants are offered the usual program of the Buchinger Wilhelmi Clinic. This includes fasting, physical exercise and individual treatments.

In addition to the admission examination, examinations take place during the clinic stay at the end of the fast and at the end of the food reintroduction.

The follow-up examinations 7, 14 and 21 days, one and three months after fasting take place independently from home. For study participants who live in the vicinity of the Buchinger Wilhelmi Clinic, the follow-up examinations take place after 14 days, one and three months on site at the Buchinger Wilhelmi Clinic in order to collect anthropometric data and take venous blood samples. If this applies to a participant, they will be contacted by the investigators.

If participants return to the clinic within 3 to 18 months after their initial stay, the investigators would carry out a follow-up examination including anthropometric measurements and a venous blood draw.

The following examinations will be carried out as part of the study:

Filling out questionnaires:

Participants will be asked about their lifestyle, their general health and their mental well-being before and at the end of the fast, at the end of the food reintroduction, 7, 14 and 21 days as well as one and three months after the fast. This will take about 15 minutes. During the fasting days and the first 14 days after fasting, participants will be asked daily about their well-being (approx. 5 minutes). All questionnaires are provided via the Buchinger Wilhelmi Amplius app and filled out in the app.

Documentation of anthropometric data:

During the stay, body weight, blood pressure and heart rate are measured daily in the morning by the nursing staff. The data is also recorded 7, 14 and 21 days, as well as one and three months after fasting through self-documentation. For this purpose, the body weight is measured at home on the scales. Blood pressure and heart rate are only measured at home if the appropriate devices are available. All values measured at home are entered into the Buchinger Wilhelmi Amplius app (there are fields in the questionnaires for this).

The waist circumference is measured at the beginning and end of fasting.

A bioelectrical impedance analysis (BIA) measurement takes place before fasting in the clinic. If participants come to the clinic for follow-up examinations, the measurement will take place again 14 days, one and three months after fasting. In BIA, body composition is measured by means of a weak current flow, which determines the percentage of muscle and fat mass in the body. The measurement must not be carried out in people with a pacemaker (tell the study doctor if this applies).

Blood analysis:

During the study, at least three, up to six, venous blood samples will be taken. On the first morning of the stay and at the end of the 7+ fasting days, as well as 3±2 days after breaking the fast, up to 30 ml of blood (amount of 2 tablespoons) is taken.

In addition, a total of 4 capillary blood samples are collected. On the first morning of the stay, as well as 14 days, one and three months after fasting. A small amount of blood is collected on blood collection swabs by a small prick in the fingertip. The capillary blood samples at the times after fasting must be taken by the participant themrselves with the equipment provided and sent by post to the research department of Buchinger Wilhelmi.

Blood sampling is a minimally invasive procedure in which a risk of side effects such as hematomas, infections, thrombosis or nerve damage cannot be ruled out.

Nutrition protocol:

From the day of breaking the fast, participants will log their meals for 14 days. To simplify this, the smartphone app "MyFoodRepo" is being used, developed by scientists for logging meals. This complies with the data protection guidelines of the GDPR. In the app, participants take a photo of all meals during the 14 days.

Continuous glucose measurement:

To measure the continuous glucose levels in blood, participants will wear a continuous glucose monitor for 14 days from the day of breaking the fast. To do this, a sensor with a fine filament similar to a needle is attached to the upper arm, which measures glucose during 24 hours. In study participants for whom the continuous measurement of the ketone bodies is also carried out, the continuous glucose measurement is already carried out from the digestive rest day (see next point).

There may be discomfort associated with wearing the sensor and there is a minimal risk of skin irritation or allergic reactions near the sensor site.

Documentation of ketone bodies in urine:

From the digestive rest day until 14 days after breaking the fast, participants will independently determine the ketone body content of the first morning urine with urine test strips every day and document the result in the Buchinger Wilhelmi Amplius app. The color change of the strip indicates the switch from energy production from glucose to energy production from fat reserves.

Collection of stool samples:

Three stool samples are collected, before the start of fasting, as soon as spontaneous bowel movements begin after fasting, and 14 days after fasting. The stool sample after 14 days must be taken independently with the equipment provided and sent by post to the research department of Buchinger Wilhelmi.

Inclusion Criteria:

• Men and women
• Fasting for 5+ days at the Buchinger Wilhelmi clinic in Überlingen or Marbella
• Age: 18-90 years old
• Signed informed consent

Exclusion Criteria:

• Not able to sign the informed consent
• Diagnosed with cachexia, anorexia nervosa, advanced kidney, liver or cerebrovascular insufficiency.
• Chronic manifest psychical and psychiatric diseases
• Participation in another study
• Pregnancy or breastfeeding

Patients of the Buchinger Wilhelmi clinic in Überlingen