FastForward: A Prospective Analysis of Long-term Fasting and Subsequent Food Reintroduction in Humans

NCT07155993 | Enrolling By Invitation

• 1 other identifier: F-2024-032
• Study Type: observational
• Target: 300
• Locations: 1 country
• Sites: 1

Brief Summary

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.

Conditions

Diabetes (DM); Pre Diabetes; Overweight (BMI > 25)

Keywords

long-term fasting; fasting; continuous glucose monitoring; food reintroduction; meal documentation

Outcome Measures

Primary Outcomes (1)

• Body weight

  Changes in body weight in the three-month period after a 5+ day fasting period according to the Buchinger Wilhelmi fasting program

  From the fasting start until 3 month after fasting

Secondary Outcomes (11)

• Glycemic regulation

  From the fasting start until 3 months after fasting

• Change in systemic inflammation

  From the fasting start until 3 months after fasting

• Change in systemic inflammation

  From the fasting start until 3 months after fasting

• Eating behaviour

  During the first 14 days after fasting

• Eating behaviour

  From the fasting start until 3 months after fasting

Study Arms (1)

Food reintroduction observation group

Participants undergo a supervised long-term fast (≥5 days) followed by a structured food reintroduction at the Buchinger Wilhelmi Clinic. All participants complete online questionnaires assessing dietary habits, weight, and well-being during fasting and for up to 3 months afterward. Participants are monitored closely during the initial 14 days post-fasting with continuous glucose monitoring (CGM), photo-based meal tracking, and at-home dried blood spot sampling. Liquid venous blood and stool samples are collected at defined timepoints to analyze metabolic and physiological responses to fasting and food reintroduction.

Eligibility Criteria

• Age: 18 Years - 90 Years
• Sex: all
• Healthy Volunteers: Yes
• Age Groups: Adult (18-64), Older Adult (65+)
• Sampling Method: Non-Probability Sample

Study Population

Patients of the Buchinger Wilhelmi clinic in Überlingen

You may qualify if:

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

You may not qualify if:

• 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

Sponsors & Collaborators

• Buchinger Wilhelmi Development & Holding GmbH: lead
• ETH Zurich: collaborator

Study Sites (1)

Buchinger Wilhelmi Klinik

Überlingen, Baden-Wurttemberg, 88662, Germany

Location

Related Publications (13)

• Wilhelmi de Toledo F, Grundler F, Sirtori CR, Ruscica M. Unravelling the health effects of fasting: a long road from obesity treatment to healthy life span increase and improved cognition. Ann Med. 2020 Aug;52(5):147-161. doi: 10.1080/07853890.2020.1770849. Epub 2020 Jun 10.

  PMID: 32519900 BACKGROUND

• Wilhelmi de Toledo F, Grundler F, Goutzourelas N, Tekos F, Vassi E, Mesnage R, Kouretas D. Influence of Long-Term Fasting on Blood Redox Status in Humans. Antioxidants (Basel). 2020 Jun 6;9(6):496. doi: 10.3390/antiox9060496.

  PMID: 32517172 BACKGROUND

• Wilhelmi de Toledo F, Grundler F, Bergouignan A, Drinda S, Michalsen A. Safety, health improvement and well-being during a 4 to 21-day fasting period in an observational study including 1422 subjects. PLoS One. 2019 Jan 2;14(1):e0209353. doi: 10.1371/journal.pone.0209353. eCollection 2019.

  PMID: 30601864 BACKGROUND

• Scharf E, Zeiler E, Ncube M, Kolbe P, Hwang SY, Goldhamer A, Myers TR. The Effects of Prolonged Water-Only Fasting and Refeeding on Markers of Cardiometabolic Risk. Nutrients. 2022 Mar 11;14(6):1183. doi: 10.3390/nu14061183.

  PMID: 35334843 BACKGROUND

• Mesnage R, Grundler F, Schwiertz A, Le Maho Y, Wilhelmi de Toledo F. Changes in human gut microbiota composition are linked to the energy metabolic switch during 10 d of Buchinger fasting. J Nutr Sci. 2019 Nov 12;8:e36. doi: 10.1017/jns.2019.33. eCollection 2019.

  PMID: 31798864 BACKGROUND

• Laurens C, Grundler F, Damiot A, Chery I, Le Maho AL, Zahariev A, Le Maho Y, Bergouignan A, Gauquelin-Koch G, Simon C, Blanc S, Wilhelmi de Toledo F. Is muscle and protein loss relevant in long-term fasting in healthy men? A prospective trial on physiological adaptations. J Cachexia Sarcopenia Muscle. 2021 Dec;12(6):1690-1703. doi: 10.1002/jcsm.12766. Epub 2021 Oct 20.

  PMID: 34668663 BACKGROUND

• Muller H, de Toledo FW, Resch KL. Fasting followed by vegetarian diet in patients with rheumatoid arthritis: a systematic review. Scand J Rheumatol. 2001;30(1):1-10. doi: 10.1080/030097401750065256.

  PMID: 11252685 BACKGROUND

• Grundler F, Plonne D, Mesnage R, Muller D, Sirtori CR, Ruscica M, Wilhelmi de Toledo F. Long-term fasting improves lipoprotein-associated atherogenic risk in humans. Eur J Nutr. 2021 Oct;60(7):4031-4044. doi: 10.1007/s00394-021-02578-0. Epub 2021 May 7.

  PMID: 33963431 BACKGROUND

• Grundler F, Mesnage R, Michalsen A, Wilhelmi de Toledo F. Blood Pressure Changes in 1610 Subjects With and Without Antihypertensive Medication During Long-Term Fasting. J Am Heart Assoc. 2020 Dec;9(23):e018649. doi: 10.1161/JAHA.120.018649. Epub 2020 Nov 23.

  PMID: 33222606 BACKGROUND

• Gabriel S, Ncube M, Zeiler E, Thompson N, Karlsen MC, Goldman DM, Glavas Z, Beauchesne A, Scharf E, Goldhamer AC, Myers TR. A Six-Week Follow-Up Study on the Sustained Effects of Prolonged Water-Only Fasting and Refeeding on Markers of Cardiometabolic Risk. Nutrients. 2022 Oct 15;14(20):4313. doi: 10.3390/nu14204313.

  PMID: 36296997 BACKGROUND

• Drinda S, Grundler F, Neumann T, Lehmann T, Steckhan N, Michalsen A, Wilhelmi de Toledo F. Effects of Periodic Fasting on Fatty Liver Index-A Prospective Observational Study. Nutrients. 2019 Oct 30;11(11):2601. doi: 10.3390/nu11112601.

  PMID: 31671589 BACKGROUND

• Buono R, Longo VD. When Fasting Gets Tough, the Tough Immune Cells Get Going-or Die. Cell. 2019 Aug 22;178(5):1038-1040. doi: 10.1016/j.cell.2019.07.052.

  PMID: 31442398 BACKGROUND

• Beck RW, Calhoun P, Kollman C. Use of continuous glucose monitoring as an outcome measure in clinical trials. Diabetes Technol Ther. 2012 Oct;14(10):877-82. doi: 10.1089/dia.2012.0079.

  PMID: 23013201 BACKGROUND

Biospecimen

Retention: SAMPLES WITH DNA

This study involves the collection and retention of the following biospecimens: Venous whole blood (liquid samples): Collected via standard venipuncture into during fasting and follow-up timepoints to assess metabolic, hormonal, and inflammatory biomarkers. Capillary blood (dried blood spot samples): Self-collected by participants in the clinic and at home using finger-prick methods and spotted onto a collection fdevice. These samples are used for remote monitoring of selected biomarkers including HbA1c. Stool samples: Self-collected by participants in the clinic and at home using standardized kits. All biospecimens are retained for future analysis of physiological responses to long-term fasting and the food reintroduction phase.

MeSH Terms

Conditions

Diabetes Mellitus; Glucose Intolerance; Overweight; Fasting

Condition Hierarchy (Ancestors)

Glucose Metabolism Disorders; Metabolic Diseases; Nutritional and Metabolic Diseases; Endocrine System Diseases; Hyperglycemia; Overnutrition; Nutrition Disorders; Body Weight; Signs and Symptoms; Pathological Conditions, Signs and Symptoms; Feeding Behavior; Behavior

Study Design

• Study Type: observational
• Observational Model: COHORT
• Time Perspective: PROSPECTIVE
• Target Duration: 7 Months
• Sponsor Type: OTHER
• Responsible Party: PRINCIPAL INVESTIGATOR
• PI Title: Principal Investigator

Study Record Dates

• First Submitted: July 23, 2025
• First Posted: September 4, 2025
• Study Start: August 1, 2024
• Primary Completion (Estimated): August 1, 2026
• Study Completion (Estimated): December 1, 2026
• Last Updated: September 10, 2025

Record last verified: 2025-09

Locations

DE (1)