Worldwide, 13% of the population had obesity in 2016 and overweight and obesity are recognized as the fifth leading risk factor for death (roughly 5 million deaths per year). In the United States alone, a recent study predicts that over half of the population will have obesity in 2030. At the global level, overweight and obesity are also estimated to account for 44% of diabetes, 23% of heart disease and between 7% to 41% of cancer cases, in addition to numerous other pathologies, including neurological disorders. While obesity and overweight are classified as a general disease (i.e. a body mass index (BMI) above 25 kg/m2 or 30 kg/m2, respectively), there are large variabilities between classifications of obesity observed. For example, sub-populations of obesity present either a rapid or delayed onset of other chronic diseases, such as diabetes or cardiovascular disease.

Many studies show that lifestyle interventions are effective in improving overweight and obesity through weight loss, but with very large inter-individual variability, especially in the long-term. These interventions and the respective observed weight loss are also shown to reduce the risk of other cardiovascular or metabolic diseases, demonstrating the importance of weight loss for future quality of life Interestingly, there is a large variation in weight loss when implementing the same dietary or lifestyle changes, even when many factors are accounted for in clinical studies. Similar variable weight loss or metabolic responses are also observed for other obesity treatments, such as pharmaceutical or surgical interventions. Therefore, in order to prevent and treat overweight and obesity, it is critical to progress in the understanding of individual variations in responses (trajectories) to weight loss programs.

While biological, environmental, and behavioral factors indeed drive personal responses, recent advances have allowed more insight into how the human body processes these stimuli, namely through microorganisms inhabiting the gastrointestinal tract. Over the last 10 years, the gut microbiota, the 100 billion bacterial cells inhabiting our intestines, has emerged as a recognized factor contributing to our health. Given its access to the food and medicine consumed by an individual, the gut microbiota can be seen as a "super integrator" highly sensitive to our environmental and lifestyle changes. Accumulating evidence has highlighted that the gut microbiota translates these environmental changes by altering its diversity of bacteria or functions and producing molecules that interact with organs and the brain.

As part of a weight loss program conducted within the standard of care in a network of clinical centers across France, the investigators set out to establish a cohort to examine the relative contribution of clinical, nutritional, and lifestyle factors related to individual's weight loss success with an emphasis on evaluating the gut microbiome of individuals.

Within this context, the investigators are testing whether an individuals' microbiota profile before the real-life dietary intervention influences weight loss responses and changes in metabolic health parameters to a standardized weight loss diet.

Inclusion Criteria:

• Subject having signed the informed consent
• Male or female, 18 to 65 years of age
• Body Mass Index (BMI) greater than or equal to 25 kg/m²

Exclusion Criteria:

• Pregnant woman (positive serum pregnancy test for selection) or breastfeeding,
• Infected subject under anti-retroviral treatment
• Subject with severe hepatic and / or renal insufficiency (awaiting transplant)
• Subject with anemia <10 g / dl
• Subject with known gastrointestinal illness
• Subject having undergone bariatric surgery
• Subject with a weight loss > 10% of body weight during the last 3 months (special diet: low calorie diet (slimming diet), special diet (vegetarians, vegans, nutritional supplements) before treatment),
• Subject taking pro or prebiotics before treatment
• Subject having taken antibiotics in the 2 months preceding inclusion
• Subject participating in another clinical study,
• Subject not enrolled in the French national healthcare system
• Subject not compliing with the exclusion period from the study in which he/she has previously participated

All individuals who meet the various inclusion criteria as part of the routine care activity of French RNPC centers