Body Mass Index

Body Mass Index

Body mass index (BMI) is a widely used anthropometric measure calculated by dividing a person’s weight in kilograms by the square of their height in meters. The resulting value is used to classify individuals into weight categories: underweight (below 18.5), normal weight (18.5-24.9), overweight (25.0-29.9), and obese (30.0 and above). Obesity is further subdivided into Class I (30-34.9), Class II (35-39.9), and Class III (40 and above).

BMI serves as a primary inclusion criterion in obesity clinical trials. The Phase 2 retatrutide obesity trial enrolled participants with a BMI of 30 kg/m2 or higher (or 27 kg/m2 or higher with at least one weight-related comorbidity). This threshold aligns with standard regulatory guidelines for obesity drug development. BMI is also used to define key efficacy endpoints, such as the proportion of participants who transition from an obese to a non-obese BMI category during treatment.

While BMI is a practical and reproducible screening tool, it has well-known limitations. It does not distinguish between fat mass and lean mass, does not account for fat distribution (visceral versus subcutaneous), and may misclassify muscular individuals as overweight. For this reason, modern obesity trials, including retatrutide studies, increasingly incorporate additional measures such as waist circumference, body composition analysis (via DEXA scanning), and visceral fat assessments to provide a more complete picture of treatment effects on adiposity.