This study tests the effectiveness of dietary interventions that have the possibility to improve markers of gut health and improve general well-being. This study will allow healthcare professionals to learn how dietary interventions involving fasting can affect health. Food is increasingly recognized as a core component of preventive and ameliorative health care. Juice fasting has quickly become one of the most popular self-prescribed dietary interventions in the United States. A wide variety of juice fasts are available in the popular market; a popular variation is the three-day juice fast. The purpose of this study is to assess the effects of a three-day juice fast on certain markers of age-related disease and bio-markers of longevity. In particular, this study will assess certain epigenetic markers, which measure how the environment (including diet) can change the way that genes are expressed without changing the genes themselves. The study will also assess the microbiome, and inflammatory and glycemic markers.

Up to 30 participants will be enrolled in the study and will be randomized into one of the three arms: the "juice fasting" arm, the "juice plus ad hoc" arm, and the "caloric restriction" arm. Participants will come into the lab for an initial visit, during which they will complete questionnaires on their usual diet and demographics and will be informed about their assignment to one of the three arms.

In all three arms, the three-day diet intervention will be preceded and followed by three days of self-conducted elimination diet. The pre-intervention diet will encourage participants to eat a healthy, light diet for three days (basic instructions: eat only fresh raw or cooked fruits and vegetables (preferably organic), whole grains and eggs; drink 8 glasses of water a day; avoid or eliminate alcohol, caffeine, sugar, processed foods, dairy, red meat, and gluten). The post- intervention diet will instruct participants to gradually return to eating solid foods, and limit strenuous exercise (basic instructions: follow a diet similar to the pre-intervention elimination diet; day 1 after the intervention: eat fruits & green vegetables, and drink 8 glasses of water; day 2 add in: nuts, gluten-free grains like whole oats, quinoa, or brown rice; day 3 add in: organic meats, fish, & gluten-containing grains).

Biological samples and self-reported outcomes will be collected at four time points: baseline (prior to the first elimination diet period), pre-intervention (after the elimination diet and before the three-day intervention), post-intervention (after the intervention), and 14-day post- intervention. At each time point, participants will complete questionnaire on quality of life (PROMIS - Global Health Scale, which is an NIH measure on an individual's' overall physical and psychological well-being; see appendix) and collect their stool samples. Participants will be provided self-administered tool kits and be taught how to collect the stool sample at their first lab visit. At each of the four time points, the research assistant will coordinate the collection of cheek swabs, saliva samples, and blood spot samples. Biometrics will also be collected at each time point (weight, waist circumference, and blood pressure).

Over the course of the study, participants will be instructed to keep a diet diary days one through nine to record their daily dietary intake. During the intervention period, the research team will also contact the participants for daily check-ins. Participants will not need to record diet data for the remaining 11 days of the post-intervention period.

At the conclusion of the study, participants will bring back the final biological samples (stool wipe and saliva), provide the set of samples, and be debriefed about the purposes of the study.

The biological samples collected will then be processed and assayed for microbiome, methylation, inflammatory markers, and glycemic markers.

Inclusion Criteria:

• Subjects considered as healthy by the investigator based on medical history and completion of the screening questionnaire.
• Subjects who, according to the investigator, can and will comply with the requirements of the protocol and are available for all scheduled visits at the investigational site.
• Healthy male or female aged between 18 and 35 (included) years
• 18.5 ≤BMI ≤ 30 kg/m²
• Ability to give their informed consent in writing

Exclusion Criteria:

• Documented history of previous cardiovascular disease, including coronary heart disease (CHD) (angina, myocardial infarction, coronary revascularization procedures or existence of abnormal Q waves in the electrocardiogram (EKG)), stroke, syncope, and clinical peripheral artery disease with symptoms of intermittent claudication.
• Severe medical condition that may impair the ability of the person to participate in a nutrition intervention study (e.g. digestive disease with fat intolerance, advanced malignancy, or major neurological, psychiatric or endocrine disease including diabetes).
• Daily use of any prescription or non-prescription medication that has a high likelihood of impacting systemic inflammation (e.g. non-steroidal anti inflammatories or steroids), blood sugar control (e.g. medication for diabetes) or the human microbiome (e.g. antibiotics).
• Any other medical condition thought to limit survival to less than 1 year.
• Known immunodeficiency disorder
• Illegal drug use or chronic alcoholism or total daily alcohol intake >80 g/d.
• Difficulties or major inconvenience to change dietary habits
• Impossibility to follow an elimination or juice fast diet, for religious reasons or due to the presence of disorders of chewing or swallowing (e.g., difficulties to consume nuts)
• A low predicted likelihood to change dietary habits according to the Prochaska and DiClemente stages of change model (Nigg, 1999).
• History of food allergy with hypersensitivity to any of the components of the juice or diet
• Patients with an acute infection or inflammation (e.g., pneumonia) are allowed to participate in the study 3 months after the resolution of their condition.
• Dietary restrictions due to medical (including allergies), religious, or other concerns
• Any diagnosis of allergic rhinitis, eczema, asthma, or inflammatory bowel disease (e.g. ulcerative colitis or Crohn's) from a health professional