This past century witnessed a significant increase in the prevalence of obesity. Successful weight loss maintenance is defined as losing at least 10% of the initial body weight and maintaining it for at least one year. However, keeping a low body weight is rarely maintained, as 80% of people who lost 10% of their body weight will return to their initial weight within a year. Although there is no agreement as to what contributes to the recurrent weight regain phenomenon (also known as 'weight cycling' or 'yo-yo diet'), it is strongly associated with the risk of developing metabolic risk factors and their complications, including heart disease and all-cause mortality. Altering the gut microbiota is one method to treat disease states associated with gut bacteria. Antibiotics consumption is known to influence host glycemic response through changes induced in microbiome composition and function. Therefore, it may be a possible pathway to jumpstart changes in the gut microbiota.

This study will determine whether microbiome modulation might be a possible future target against recurrent obesity in humans and whether orally administered antibiotic treatment post-weight loss might be an effective intervention to prevent weight regain.

This is a placebo-controlled, double blinded interventional study evaluating the effect of antibiotic treatment in the prevention of weight regain after a successful weight loss intervention.

Volunteers will be recruited in the following ways: e-mail, and social media. Volunteers wishing to participate will be asked to complete a questionnaire regarding the aforementioned inclusion and exclusion criteria. Volunteers who qualify for the study will be invited to an introductory meeting at the Weizmann Institute of Science. The details of the experiment and potential risks or discomforts involved in it will be presented, after which the volunteers will sign an informed consent form. Volunteers will start the study and become active participants for a maximal duration of one year.

Samples collected for the study will be used for microbiota profiling (stool and oral), metabolomics (stool, blood) and for bomb calorimeter (stool).

The study will comprise of four parts:

1. Baseline - one week of profiling and screening.
2. Nutritional intervention - weight loss intervention which will be based on personalized hypocaloric diet restriction. Meetings will occur twice a month starting from the second week of the study for a three-month period.
3. Antibiotic treatment - 10 days of either antibiotics- Augmentin (amoxicillin+clauvonate), 875mg, 2 /day, or 10 days of placebo pills, 2/day.
4. Follow-up meetings - total follow up of 9 months after antibiotic/placebo treatment. Meetings will occur once a month for the first 3 months of the follow up period, and once every 3 months, afterwards.

Inclusion Criteria:

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