What is BMI and Why is it Measured in Clinical Trials?

Trial Design

During a clinical trial, you will have several types of assessments, such as blood draws, questionnaires about your health, and other measurements such as body mass index (BMI). BMI is used as a screening tool to check if your weight may put you at risk for certain diseases such as high blood pressure, high cholesterol and type 2 diabetes. Understanding that this risk can be reduced with weight loss has led to clinical research in weight management. BMI is one measure used in clinical trials to determine if you qualify to be included in a trial and is measured throughout the trial.

What is BMI?

Your BMI will fall into one of the following groups when compared to your height:

  • Underweight
  • Healthy weight
  • Overweight
  • Obese

BMI is calculated using height and weight and it does not measure body fat level directly. However, for most people, a BMI in the overweight or obese group suggests the level of body fat might be more that what is considered ‘healthy'.

What is BMI and Why is it Measured in Clinical Trials | Image

What is your BMI?

If you want to calculate your BMI, try our BMI calculator.

How is BMI interpreted?

BMI results are usually classified into 4 categories and all BMI measurements are in kg/m²:

  • Underweight: BMI below 18.5
  • Healthy weight: BMI between 18.5–24.9
  • Overweight: BMI between 25.0–29.9
  • Obese: BMI 30 or above

What is BMI and Why is it Measured in Clinical Trials | Infographic

At any BMI level, Asians and Asian Americans tend to have a greater amount of body fat and a higher risk of obesity-related complications when compared to white people of the same age and sex. This is based on data from clinical research studies. As a result, the World Health Organization recommends using lower BMI cut-off points to identify people of Asian ancestry who may be at risk:

  • Underweight: BMI below 18.5
  • Normal weight / acceptable risk: BMI between 18.5-22.9
  • Increased risk: BMI between 23-26.9
  • Further increased risk: BMI 27 or above

Limitations of BMI

BMI doesn't take into account if your weight comes from lean muscle or fat. This means a person might be in the healthy weight category based on their BMI, but still face health risks, like heart disease, due to increased:

  • Body fat
  • Cholesterol
  • Blood pressure levels

BMI also doesn't tell the difference between two different types of fat in the body which are:

  • Subcutaneous fat: the fat located just under the skin
  • Visceral fat: the "hidden" fat around organs which has a higher health risk.

People with the same BMI can look different. If you ask 10 people of the same weight, height and BMI to stand in a line, will they all look the same? No, it is likely they will all be slightly different body shapes. They will look different because BMI doesn’t give us information about muscle and fat tissue which can affect the way you look.

Advancing Science Together

We at Lilly are working hard to support people living with a higher BMI.

If you are interested in learning more about weight management clinical trials, visit our Weight Management Site.


  • Body Mass Index Advantages and Disadvantages | livestrong
  • Asian BMI Calculator | joslin.org
  • About Adult BMI | Healthy Weight, Nutrition, and Physical Activity | CDC
  • Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies | The Lancet

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