Clinical Trial in Constitutional Thinness

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.