Part 1: Where the heck did BMI come from?
Body mass index (BMI) is the method of using an adult’s height and weight to place them into specific categories including underweight, normal weight, overweight, and obese. A person’s BMI has long been though to be important in the determination of potential future health issues and has been widely used as one of the factors of various public health policies.[1] This broad categorization has been proven time and time again to be nonspecific, particularly as it relates to variances in race, short stature, and even the differences in body types between biological men and women.
How did we get here in the first place? Lambert Adolphe Jacques Quetelet, a Belgian-born sociologist, astronomer, and mathematician/statistician, is responsible for developing the Body Mass Index, initially called the Quetelet index (QI). In 1819, at the age of 23, Quetelet received the first doctorate in mathematics ever awarded by the University of Ghent.[2] In the mid-19th century, Quetelet was searching for a way to relate an individual’s height to their ideal weight as a tool for studying populations. Using the heights of French soldiers and the chest measurements of Scottish soldiers to develop a bell curve, he published a book in 1846 defining the QI or BMI which is weight divided by height squared (QI = weight in kg/height in meters) kg/m2. If the QI/BMI was greater than 30, then the person was officially obese[3]. The quotient was first cited in 1972 in the Journal of Chronic Diseases. The article discussed BMI as a valuable tool in population studies, or “social physics,” as Quetelet called it, and mentioned that this measurement was not useful for studying an individual; however, due to the simplicity of the equation, it remains the most commonly used anthropometric analysis.
Other assessments of a person’s size, form, and functional capacity require measurements of an individual’s body. These numbers are less reliable than BMI because they can change depending on the measurement’s location and who is performing the measurements. BMI relies on the scale’s accuracy used to weigh an individual and can be comparatively referred to as an objective evaluation. For these reasons, a formula developed in the mid-1800s is still widely utilized in the 21st century. It seems like some smart 17-year-old at MIT would have figured out a new formulation by now to determine “fatness.”
[1]https://www.ncbi.nlm.nih.gov/books/NBK535456/
[2] https://www.theatlantic.com/business/archive/2016/02/the-invention-of-the-normal-person/463365/
[3] https://mnstats.morris.umn.edu/introstat/history/w98/Quetelet.html
To further discuss BMI and the issues, disease states, and clinical significance, it is important to understand the current cut-off points of each category as determined by the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC).
World Health Organization
- Severely Underweight: <16 kg/m2
- Underweight: 16.0 to 18.4 kg/m2
- Normal weight: 18.5 to 24.9 kg/m2
- Overweight: 25.0 to 29.9 kg/m2
- Moderately Obese: 30.0 to 34.9 kg/m2
- Severely Obese: 35.0 to 39.9 kg/m2
- Morbidly Obese: ≥40.0 kg/m2
Centers for Disease Control and Prevention
- Underweight: less than or equal to 18.4 kg/m2
- Normal weight: 18.5 to 24.9 kg/m2
- Overweight: 25.0 to 39.9 kg/m2
- Obese: ≥40.0 kg/m2
Centers for Disease Control and Prevention Child and Teen BMI (Utilized for those aged 2 to 20)
- Underweight: below the 5th percentile
- Healthy weight: between the 5th and 85th percentile
- Overweight: between the 85th and 95th percentile
- Obese: greater than or equal to the 95th percentile
World Health Organization Asian-Population Criteria:
- Underweight: <18.5 kg/m2
- Normal weight: 18.5 to 23.0 kg/m2
- Overweight: 23.0 to 27.5 kg/m2
- Obese: ≥27.5 kg/m2
In Part 2 of this series, we will discuss more of the limitations of the use of BMI in categorization of “fatness”, conflicting studies, and the medical risks and disease states associated with obesity or a BMI defined as >30 kg/m2.