The purpose of this study is to evaluate the molecular differences between a group of Constitutionally Thin (CT) subjects and a group of controls.

Inclusion Criteria:

• Inclusion criteria specific to subjects with constitutional thinness
• Women: BMI less or equal to 17.5
• Men: BMI less or equal to 18.5
• Stable weight for at least 3 months

Inclusion criteria specific to subjects with normal weight

• Men and women: BMI above or equal to 20 but not more than 25
• Stable weight for at least 3 months
• No previous family history of first or second-degree obesity

Inclusion criteria common to both groups

• Age: between 18 and 35 at the inclusion visit
• Normal blood sugar, liver function, lipid and coagulation profiles
• Signed consent form to participate in the study
• Signed consent form for genetic analysis
• Acceptance of a moderate weight gain of 2 kg (less than 10% of body weight)
• Beneficiary or member of a social security system.

Exclusion Criteria:

• Adults with a legal guardian, or subjects unable to act freely due to a decision by a legal or administrative authority,
• Pregnancy in the case of women
• Women without contraception
• Vegetarian subjects or those with lactose intolerance
• Subjects with an eating disorder (DSM IV)
• Significant alcohol consumption equivalent to more than 10 glasses of wine per week
• Severe progressive disorder (diabetes, for example)
• Subjects who undertake intensive physical activity (more than 3 sessions of physical activity per week)
• Significant tobacco consumption equivalent to more than 10 cigarettes per day
• Depression or psychiatric condition (treated with antidepressant or psychotropic medication), depression questionnaire/score
• Medical or surgical history considered by the investigator to be incompatible with this study (stomach or intestinal surgery, for example)
• Subjects receiving treatment that may interfere with the parameters measured: antihypertensives (blockers, centrally acting antihypertensives), anti-hyperlipidemic agents or corticosteroids for more than 8 days
• Inclusion in a different clinical study within the previous 12 weeks.