Body Mass Index

Body Mass Index

Body mass index (BMI) is a common body measure: your weight in kilograms divided by the square of your height in meters. The result places you in a weight category: underweight (below 18.5), normal weight (18.5-24.9), overweight (25.0-29.9) or obese (30.0 and above). Obesity is further split into Class I (30-34.9), Class II (35-39.9) and Class III (40 and above).

BMI is a main entry criterion in obesity trials. The Phase 2 retatrutide obesity trial enrolled people with a BMI of 30 kg/m2 or higher (or 27 kg/m2 or higher with at least one weight-related condition). That threshold follows standard regulatory guidance for obesity drug development. BMI is also used in efficacy endpoints, such as the share of participants who move from an obese to a non-obese BMI category during treatment.

BMI is practical and easy to repeat, but it has well-known limits. It cannot tell fat from muscle, ignores where fat sits (around the organs or under the skin) and can label muscular people as overweight. So modern obesity trials, including retatrutide studies, increasingly add other measures, such as waist circumference, body composition scans (DEXA) and visceral fat assessments, to get a fuller picture of how treatment changes body fat.