Is Body Mass Index on Its Way Out?

Lilian White, MD
Posted on September 2, 2025

Body mass index (BMI) has long been used as an indirect measurement of a patient’s body fat to classify obesity; BMI is widely accessible and inexpensive to obtain. A BMI of 18.5 to 24.9 kg/m² is considered healthy, whereas a BMI ³ 25 kg/m² indicates a person is overweight or has obesity. However, BMI has come under scrutiny for its inconsistent correlation with morbidity and mortality, as well as its inaccuracy in distinguishing between healthy vs overweight individuals.

Factors that influence BMI include a patient’s age, sex, ethnicity, muscle mass, and fluid status, making it an imperfect marker for diagnosing obesity. For example, people with a high muscle mass may be misclassified as obese. Conversely, patients with a BMI in the healthy range but who have a high percentage of body fat may benefit from interventions to reduce their risk of metabolic syndrome and related conditions.

In an article published in the July 2025 issue of Annals of Family Medicine, Body Mass Index vs Body Fat Percentage as a Predictor of Mortality in Adults Aged 20-49 Years, the authors linked data from the National Health and Nutrition Examination Survey to the National Death Index to estimate 15-year mortality risk based on body composition. The study compared BMI, body fat percentage (BF%), and waist circumference (WC) and concluded that BF% is a better predictor of 15-year mortality risk in young adults than BMI. This finding may prompt a future shift toward using BF% in place of BMI to better stratify patient risk. Limitations of the study include the limited age range of participants and lack of additional morbidity measures.

BF% is gaining traction as a more accurate measurement of body composition. BF% may be measured directly using bioimpedance analysis (BIA) devices or a dual-energy x-ray absorptiometry (DXA) scan. BF% provides a more nuanced understanding of a patient’s mortality risk than BMI alone. Although a higher BF% is associated with an increased mortality risk (similar to BMI), greater fat-free mass is associated with a lower risk of mortality. A limitation of using BF% is the lack of standardized thresholds for defining healthy levels.

DXA scans are considered the preferred standard for measuring BF%, but their clinical utility is limited by cost, time, and accessibility. BIA machines, although less accurate than DXA scans, are generally reliable for estimating BF% and fat-free mass. BIA machines work by passing a small electrical current through the body to measure the impedance or opposition to the flow of electricity. Tissues that contain more water (eg, muscle) tend to be more conductive than fat, providing an estimate of fat-free mass. The cost of BIA machines can vary considerably, but they are generally inexpensive ($300 for a consumer-grade product to thousands of dollars for a professional device) and safe to use. Caution is advised for patients with a pacemaker because of the theoretical risk of interference of  consumer-grade BIA machines with the device; however, several studies to date have demonstrated safety.

WC is another metric associated with obesity. Elevated WC is associated with an increased risk of cardiovascular disease that is independent of BMI. However, WC is somewhat limited in its use by technical challenges in measurement (eg, location of measurement, timing with breath) and significant inter-rater variability.

Given the significant health consequences of obesity, it is important to accurately measure body composition. At the same time, physicians must be aware of bias in caring for patients who are obese. When appropriate, a 2023 American Family Physician editorial highlights the preferred use of the term adiposity-based chronic disease (ABCD) in place of the term obesity for patients meeting specific criteria. An FPM article offers guidance on reducing the stigma of patients with obesity.