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Jun 25, 2024 · What Is and Isn’t Accurate About BMI (Body Mass Index) By Heather Jones. Updated on June 25, 2024. Medically reviewed by Suzanne Fisher, RD. Print. Table of Contents. BMI Accuracy. BMI Inaccuracy. Other Ways to Assess Health. A Healthy BMI.
- Overview
- BMI exaggerates thinness in short people and fatness in tall people
- Waist size linked to diabetes risk, regardless of BMI
- Waist-to-height ratio better than BMI
- An example of the biggest flaw in using BMI
- Authorities still promote BMI
BMI (body mass index), which is based on the height and weight of a person, is an inaccurate measure of body fat content and does not take into account muscle mass, bone density, overall body composition, and racial and sex differences, say researchers from the Perelman School of Medicine, University of Pennsylvania.
Every few months the same comment is made by experts “BMI is flawed”. The news hits the headlines, everybody agrees, and then all goes quiet for a while.
You are of normal weight if your BMI is between 18.5 and 25, overweight if it is between 25 and 30. Anybody with a BMI of 30 or more has obesity.
Mitchell Lazar, MD, PhD, Professor of Medicine and Genetics and Director of the Institute of Diabetes, Obesity, and Metabolism, and Rexford Ahima, MD, PhD, Professor of Medicine and Director of the Obesity Unit in the Institute for Diabetes, Obesity and Metabolism, discuss the challenges health professionals face when studying the mortality risks and health of people with obesity in the journal Science1.
We all know that obesity increases the risk of developing heart diseases, type 2 diabetes, cancer, sleep apnea and other diseases and conditions.
However, according to recent studies, obesity may also protect against death from all causes, as well as death due to stroke, heart failure and diabetes.
Nick Trefethen, Professor of Numerical Analysis at Oxford University’s Mathematical Institute, in a letter to The Economist explained that BMI leads to confusion and misinformation.
BMI = weight in kilograms divided by height in meters squared.
Professor Trefethen believes that the BMI height2/weight term divides the weight by too much in short people and too little in tall individuals. This results in tall people believing they are fatter than they really are4, and short people thinking they are thinner.
BMI was devised in the 1830s by Lambert Adolphe Jacques Quetelet (1796-1874), a Belgian mathematician, sociologist, statistician and astronomer.
Trefethen explained that during Quetelet’s time there were no calculators, computers or electronic devices – which is probably why he opted for a super-simple system. Trefethen wonders why institutions today on both sides of the Atlantic continue using the same flawed-BMI formula.
“Perhaps nobody wants to rock the boat”, Trefethen added.
Researchers from the Medical Research Council (MRC) Epidemiology Unit, UK, reported in PLoS Medicine that waist circumference is strongly and independently associated with type two diabetes risk5, even after accounting for BMI.
Study leader, Dr Claudia Langenberg and team suggested that waist circumference should be measured more widely for estimating type 2 diabetes risk.
They pointed out that a male without obesity who is overweight with a waist circumference of at least 40.2 inches (102cm) has the same or higher risk of type 2 diabetes as a male with obesity. The same applies for females with a waist of 34.6 inches (88cm) or more.
A study published by the RAND Corporation showed that waist size explained the higher type 2 diabetes rate in the USA than UK, not BMI6. Co-author, James P. Smith said “Americans carry more fat around their middle sections than the English, and that was the single factor that explained most of the higher rate of diabetes seen in the United States, especially among American women. Waist size is the missing new risk factor we should be studying.”
Dr Margaret Ashwell, an independent consultant and former science director of the British Nutrition Foundation, explained at the 19th Congress on Obesity in Lyon, France, May 2012, that waist-to-height ratio is a superior predictor than BMI7 of type 2 diabetes and cardiovascular diseases.
Dr. Ashwell said “Keeping your waist circumference to less than half your height can help increase life expectancy for every person in the world.”
Thus a 6ft-tall man should have a waist circumference of 36 inches or less, while a 5ft 4in woman’s waist should not exceed 32 inches.
The waist-to-height ratio should be considered as a screening tool, Ashwell added.
Ashwell explained that BMI does not take into account the distribution of fat around the body. Abdominal fat affects organs like the kidney, liver and heart more severely than fat around the bottom or hips. Waist circumference gives an indication of abdominal fat levels.
Dr. Ashwell and colleagues believe that the thought “keep your waist circumference to less half your height” is an easier one to hold on to that BMI.
Body Mass Index’ biggest flaw is that it does not take into account the person’s body fat versus muscle (lean tissue) content.
Muscle weighs more than fat (it is denser, a cubic inch of muscle weighs more than a cubic inch of fat). Therefore, BMI will inevitably class muscly, athletic people as fatter than they really are.
A 6ft-tall Olympic 100 meter sprinter weighing 90kg (200lbs) may have the same BMI (26) as a couch potato of the same height and weight.
A BMI calculation would class both of them as overweight.
That calculation is probably right for the sedentary couch potato, but not for the athlete.
The athlete’s waist circumference, at 34ins, is well within “healthy weight” – if his height is 72 inches, his waist is less than half his height.
The Centers for Disease Control and Prevention (CDC) says “BMI is a fairly reliable indicator of body fatness for most people.”8
The National Institutes of Health (NIH) says “A good way to decide if your weight is healthy for your height is to figure out your body mass index.”9
Jan 6, 2022 · Is BMI an accurate way to measure your health? Yes and no. Learn how to measure your BMI, why it’s still a useful tool, its limitations and when to talk to your doctor.
Aug 23, 2021 · Despite concerns that BMI doesn’t accurately identify whether a person is healthy, most studies show that a person’s risk of chronic disease and premature death does increase with a BMI...
Jan 8, 2024 · If your BMI number is 25 or higher, you fall into the overweight or obese category. So, does that mean that a BMI over 25 is unhealthy? Although many of us cling to the belief that a thin body is a healthy body, recent evidence suggests that’s not necessarily true.
Sep 18, 2024 · So when it comes to the individual, “this metric doesn’t really accurately predict health,” says Janet Tomiyama, a professor of psychology at UCLA. Why do we still rely on BMI, and...
May 24, 2024 · Body mass index (BMI) uses your height and weight measurements to estimate how much body fat you have. It’s a quick screening tool that can help find out if you are in a healthy weight range....
