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 obese.

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 guide providers around care recommendations including additional tests or screenings. 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. In both these scenarios, BMI is an important piece of the puzzle - but only one piece. Providers will consider BMI in the bigger picture of a patient's overall health. 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. So someone with a 'normal' BMI but who has a higher distribution of fat at the waist may face more health risk than someone with a higher BMI but evenly distributed body fat.

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.

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.

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 grasp if individual health concerns may be present. Physical examinations and lab tests are also used to get a clearer picture of a patient's overall health status to create an effective plan of care.

If you are interested in clinical research opportunities around weight management, it is important to know your current height and weight. This will make sure 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, be sure to ask your healthcare provider.