Experimental Approach to Test Predictions of Body Weight Regulation Models

Inclusion Criteria

• Body mass index lower than 20 kg/m2 for the low body weight group, or 30 kg/m2 or greater for the obesity group
• 18-40 years old
• Nulliparous and regular menstrual cycle (25-35 days) during the last six months (in women)
• Normal thyroid function, blood count, and chemistry 15 panel (normal plasma glucose will be considered at \<100 mg/dL; normal serum HDL cholesterol at >=50 mg/dL for women and >=40 mg/dL for men; and normal serum triglycerides at \<150 mg/dL).
• Self-reported weight stability during the last six months (±3 kg)
• Rate the liking of at least one of the flavored liquid meals (Ensure Plus, Abbot Nutrition) between 5 and 8 on a 9-point Likert scale
• Willing to only drink up to two assigned flavors of Ensure Plus for two consecutive days

Exclusion Criteria

• Eating disorders as indicated by a global score ≥2.80 in the Eating Disorder Examination Questionnaire, or a previous diagnosis of an eating disorder
• For the low body weight group, having food insecurity (with or without hunger) as assessed by the USDA
• Recreational moderate-intensity physical activity ≥150 min/week, recreational vigorous-intensity physical activity ≥75 min/week, or a combination of recreational moderate-intensity and vigorous-intensity physical activity (moderate time + [2 × vigorous time]) ≥150 min/week as assessed by the Global Physical Activity Questionnaire, or being a professional athlete
• Cigarette or vape smoking
• Intake of more than 14 alcoholic drinks per week
• Use of medications that may affect energy intake and/or expenditure, for example, semaglutide, liraglutide, exenatide, other GLP-1 receptor agonists, phentermine-topiramate, naltrexone-bupropion, orlistat, metformin, SGLT2 inhibitors, pramlintide, levocarnitine, amphetamines, and amphetamine-like drugs
• Human immunodeficiency virus, galactosemia, and lactose intolerance
• Diseases that affect energy homeostasis including endocrine such as hypo/hyperthyroidism, or type 1 or 2 diabetes; cancer; chronic pulmonary diseases; cardiovascular disease; and renal disease
• History of inflammatory bowel disease (ulcerative colitis, Crohn's), malabsorption syndromes (intestinal, pancreatic), and sprue or gluten intolerance
• Having moderate to severe sleep apnea defined as an oxygen desaturation index \&gt;10 times/hour assessed by overnight oximetry to be conducted on the night following the screening visit
• Use of oral hormonal contraceptives or less than 6 months using a hormonal intrauterine device (in women)
• Adults who are unable to consent
• Prisoners
• Currently pregnant or breastfeeding (in women)