The basics of BMI

BMI, or Body Mass Index, is used to assess if a person has a healthy weight based on their height. BMI is calculated by dividing weight (in kilograms) by height (in meters, squared). A BMI calculated to be between 18.5 to 24.9 is categorized as “normal” weight. A person with a BMI score under 18.5 is considered underweight. Someone with a score over 25 is considered overweight; a BMI score greater than 30 is categorized as having obesity.

Though BMI is a common measurement used medically today in areas such as primary care and clinical research, it has a long history. It was first developed almost 200 years ago by a Belgian mathematician who was doing population-based research to define characteristics of the “average man.” It was not until the 1970s that the calculation was given its current name and the purpose by which it is largely applied today: individual screening for obesity.

In a primary care setting, BMI can help guide providers around care recommendations, including whether additional tests or screenings may be needed. For example, a low BMI may point to an eating disorder or digestive problem. A higher BMI may lead to counseling around lifestyle changes in order to reduce the risk of serious chronic illness.

BMI is ideally considered as a screening starting point and not a standalone measure of health. Providers now commonly interpret BMI alongside other indicators, such as waist circumference, body composition, and overall health history, to build a more complete picture. This reflects a growing understanding that a single number cannot fully capture a person’s health risk.

For example, an athlete may have a higher BMI due to the presence of muscle mass, not body fat. In addition, BMI does not take into account where body fat is stored. Recent research has shown that where fat is distributed in the body (especially around the abdomen) can be an important predictor of health risks such as heart disease and diabetes. Because of this, simple measures like waist circumference or waist-to-height ratio are increasingly used alongside BMI to better assess overall risk.

BMI and pharmaceutical research

In the clinical research field, the BMI calculation serves several purposes. First, since the measurement is standardized, it can be easily compared across different studies and groups. This helps in data analysis and interpretation. Because of this, BMI is often referenced in study eligibility criteria. It is vital that clinical trials ensure that participant populations are representative of those who would ultimately use the potential treatments. This is especially true in trials related to obesity and weight management/weight loss.

More recently, research has also expanded how obesity itself is defined. Experts now recognize that excess body fat exists on a spectrum. Some individuals may have increased body fat without current health complications, while others experience direct impacts on organ function or daily life. These distinctions may be referred to as “preclinical” and “clinical” obesity and they help providers better understand who may benefit most from early intervention versus more immediate treatment.

Since a person’s amount of body fat can affect how medicines are distributed and absorbed, BMI also plays a role in understanding and determining appropriate dosing of a new therapy. In this area, researchers look at how a drug concentrates in the blood and tissue, and any impact on effectiveness. Some drugs may concentrate in fatty tissue, altering the action of the therapy. This is also why, even with approved medicines, physicians often doublecheck patient weight or BMI when writing a prescription.

One of many factors

BMI is a simple but valuable tool in healthcare delivery, but it is only one data point. BMI can change over time and, as such, should not define a person. The measurement is intended to help providers and researchers quickly identify whether individual health concerns may be present. Physical examinations, lab tests, and additional measurements are also used to create a clearer, more complete understanding of a person’s overall health.

If you are interested in clinical research opportunities around weight management, it is important to know your current height and weight. This will help ensure that your calculated BMI is accurate during initial study screening steps, which may happen virtually and be based on what you self-report. If you have questions about BMI or your overall health risk, be sure to discuss them with your healthcare provider.