Can we trust the BMI?
By Amber Charles, MSPH, RDN
October 25, 2020
The body mass index (BMI) number often rules over so many of our healthcare decisions; do I need to lose weight?...what is my risk for a heart attack? Just how accurate is this number?
History of the BMI
The body mass index (BMI), formerly called the Quetelet index, was developed by Lambert Adolphe Jacques Quetelet in the 1830's....yes, you read that right...over a century ago!
The Belgian astronomer, mathematician, statistician and sociologist devised a formula that compared weight to height to determine health risk.
The BMI formula: weight in kilograms divided by height in meters squared (Kg/m2) - you can calculate your BMI here (NIH).
The BMI Chart
For adults over the age of 20 years, BMI falls into one of the following categories:
Underweight: less than 18.5 Kg/m2
Normal weight: 18.5 to 24.9 Kg/m2
Overweight: 24.9 to 29.9 Kg/m2
Obese category 1: 30.0 to 34.9 Kg/m2
Obese category 2: 35.0 to 39.9 Kg/m2
Obese category 3: 40.0 kg/m2 or above
What the BMI is good at...
The BMI is great for characterizing the risk for developing chronic diseases in a population/group - It is an inexpensive screening tool that enables health professionals to measure numerous individuals in a day.
As BMI increases, the risk for developing some health conditions also increases, such as heart disease, diabetes, some cancers, arthritis, among many others.
What the BMI is not good at...
It is not very good at assessing health risk at the individual level. This is because the BMI only considers weight and height, and not body composition. As such, the BMI overestimates fat mass in some people and underestimates it in others.
We can think of body composition as the body having two major compartments - one is fat mass (measured as body fat percent)and the other fat-free mass (muscle, water, organ weight etc.).
The health risks associated with a high BMI are based on having a high body fat percent and not simply on body weight.
What this means is that someone can be "overweight" because they are muscular or they are tall, and not simply because their body fat percent is high. Therefore, overweight does not always mean "unhealthy".
Likewise, someone can be within the normal weight range and have a high body fat percent and be at an increased risk for chronic disease.
Alternatively, having a body fat percent that is above 'normal', does not automatically make you unhealthy - where you store the fat is important - "in your hips" versus in your abdomen surrounding your organs (visceral fat).
Quick BMI Facts:
Women generally have more body fat than men
"At the same BMI, Blacks have less body fat than do Whites, and Asians have more body fat than do Whites" (CDC)
Older individuals, on average, tend to have more body fat than younger adults as body fat typically increases with age and muscle mass decreases
Athletes have less body fat than non-athletes
Other body measurements to use (along with BMI):
Waist circumference: aim for under 35'' in women and 40 '' in men (CDC)
Waist to height ratio (WHtR)
Body fat percent (from Bioelectrical Impedance Analysis, skinfolds, DEXA): checkout this chart
What is the verdict?
The BMI is associated with adiposity and is a good indicator of health risk at the population level.
However, body composition measurements should accompany a BMI interpretation to accurately assess health risk - the BMI is not a stand-alone health assessment tool.