BODY MASS INDEX HAS BEEN AROUND SINCE THE 1800s. We are in 2022 — Is it time for a better approach to measuring health? I recently stumbled across a provocative article titled “When Do We Stop Using BMI to Diagnose Obesity?”
And then there is Dr. Marina Harris’s provocative tweet from August 2022: “BMI is trash. Full stop.” This controversial tweet received 26,500 likes and almost 3,000 retweets. The hundreds of comments from medical and non–health care personnel ranged from agreement to contrary to the offensive.
Here’s the simple response of @KarenPepperMD: “Yup.” Dr. Pepper then references MedPage’s kevinmd.com:
https://www.kevinmd.com/2022/01/why-you-should-not-use-bmi-for-your-new-years-resolution.html
I’ll tell you a little secret: I frequently focus on numbers reflecting various aspects of my health. How many steps did I get today? (15,238 if you must know.) How many hours did I sleep last night? And what is my body mass index (BMI)?
Today we explore the utility of body mass index measurements.
Defining body mass index (BMI)
We derive body mass index (BMI) from a person’s weight and height. To calculate your BMI, divide your body weight by the square of your height. Better yet, pull up one of the many online calculators. Here is one from the United States Centers for Disease Control:
https://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/english_bmi_calculator/bmi_calculator.html
The Body Mass Index is a handy and easy-to-use number that helps us to broadly categorize an individual as underweight, normal weight, overweight, or obese. The National Heart, Lung, and Blood Institute defines these categories:
- Underweight: BMI less than 18.5
- Normal weight: BMI 18.5 to 24.9
- Overweight: BMI 25 to 25.9
- Obese: BMI of 30 or greater
Please note that there is global variation in how we characterize underweight, normal weight, overweight, and obesity.
Body Mass Index and Early Mortality
Body mass indices under 20 and over 25 are associated with a higher probability of early death. The risk increases as we move away from the 20 to 25 range.
Let’s examine a meta-analysis to determine the relationship between BMI and all-cause mortality.
Researchers collected data on over 10.6 million participants in Asia, Australia, New Zealand, Europe, and North America from 239 prospective studies. The study had a median follow-up of just under 14 years. The investigators then examined early death risk relative to Body Mass Index.
Let’s look at the study findings. All-cause mortality during the follow-up period appeared minimal for those with a BMI of 20 to 25 kg/m2. The death risk increased significantly for those with a BMI below this range (1.13-times higher for those with a BMI of 18.5 to less than 20; 1.4 to 1.6-times higher for individuals with a BMI of 15 to less than 18.5).
The early death rate also increased for overweight subjects. For example, the risk, compared with someone in the BMI 20 to 25 range, increased by 1.07-times for those with a BMI of 25 to less than 27.5. Early death risk continued to increase by the level of weight:
- BMI 25 to under 27.5 — 1.1-times
- BMI 27.5 to under 30 — 1.2-times
- BMI 30 to less than 35 — 1.45-times
- BMI 35 to less than 40 — 1.9-times
- BMI 40 or over — 2.8-times
The risks appeared to be similar in studies with self-reported and measured BMI. The takeaway message? Being under- or overweight is associated with higher all-cause mortality.
The researchers concluded that “wherever overweight and obesity are common, their associations with higher all-cause mortality are broadly similar in different populations, supporting strategies to combat the entire spectrum of excessive adiposity worldwide.”
This landmark study received funding from the UK Medical Research Council, British Heart Foundation, National Institute for Health Research, and US National Institutes of Health.
BMI seems to have some usefulness, but how good is this tool?
Body Mass Index (BMI) — Remarkably flawed
Going back to that extraordinary tweet: “BMI is trash. Full stop.” Is Body Mass Index that flawed? Whether you think BMI is valuable or useless, this tweet stimulates discussion about body mass index calculation limits.
We begin with this framing observation: The progenitor of body mass index, Belgian Lambert Adolphe Jacques Quetelet, explicitly stated that we should not use BMI to indicate an individual’s fitness level.
Quetelet, a mathematician, developed the formula to provide a quick way to measure the population’s obesity levels to help the government allocate resources. The remarkably curious scientist appeared eager to unmask the secret face of the average man.
Listen to what the Atlantic Monthly has to say.
“Quetelet began to compute the average of every human attribute he could get data on. He calculated average stature, average weight, and an average complexion. He calculated the average age couples got married, and the average age people died. He calculated average annual births, the average number of people in poverty, average annual incidents of crime, average types of crimes, the average amount of education, and even annual suicide rates.
He invented the Quetelet Index — today known as the body mass index, or BMI — and calculated men’s and women’s average BMIs to identify average health. Each of these average values, claimed by Quetelet, represented the Average Man’s hidden qualities.”
Let’s get to three of the top reasons I consider BMI to be flawed. As a tool to predict an individual’s health (as opposed to a statistical measurement for groups), body mass index has limitations that can make it less valuable than some alternatives.
1. BMI doesn’t always match the physiology
Several years ago, I transitioned from the regular practice of Shito-Ryu karate to weight lifting regularly. As I bulked up, my body mass index swelled; at one point, I had a BMI of 26.5, rendering me overweight.
The problem? I had put on lean muscle mass while losing body fat, and muscle is heavier than fat.
BMI does not adjust for the relative proportions of your body’s muscle, bone, and fat. Here’s the thing: Bone is denser than muscle and twice as dense as fat. You will have a high body mass index if you have strong bones, good muscle tone, and low fat. Fit athletes may fall into the overweight (or even obese) category. However, BMI is moderately related to body fat measurements.
2. BMI — About those statistics
When Quetelet developed his BMI formula, folks led relatively sedentary lives, as do many individuals today. The calculation may have some value for such individuals. But does the calculation hold up well for those who are fit and healthy?
Quetelet came up with the idea of “the average man,” publishing in Paris’ Sur l’homme et le développement des facultés, ou essai de physique social.
While today, someone described as average is lacking, for Quetelet, the Average Man was perfection, an ideal to which Nature aspired, free from error. He declared that “the greatest men in history were closest to the Average Man of their place and time.”
3. Distinct categories
Body mass index suggests well-circumscribed categories of underweight, ideal, overweight, and obese categories based on height and weight. A decimal place creates sharp boundaries. Sounds a bit nonsensical to me.
On another note, Quetelet’s uncovering of hidden laws governing society had an outsized influence on numerous thought leaders. For example, Florence Nightingale used his ideas in nursing, while Karl Max turned to Quetelet’s ideas in developing his theory of Communism; to Marx, the Average Man proved the existence of historical determinism.
Physician John Snow used Quetelet’s ideas to fight cholera in London, marking the beginning of the field of public health. Quetelet’s mathematics inspired physicist James Maxwell to formulate the classical theory of gas mechanics.
My take — Body mass index
Body mass index is here, but I am not sure how long. When we use BMI, we should use an adjusted BMI chart that incorporates age, biological sex, ethnicity, and obesity-related conditions. And remember: BMI is by no means the sole determinant of health.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4672357/
An abnormal BMI should prompt a conversation and additional testing to determine an individual’s health. Is the person practicing a healthy lifestyle? Got metabolic diseases?
This observation is not theoretical posturing. Did you know that current studies show that a person with a high adjusted body mass index but practicing a healthy lifestyle and with no metabolic diseases is less at risk than a person with a “normal” BMI but a high waist circumference and an unhealthy lifestyle?
No matter how you feel about the 19th-century tool, there are better tools than BMI to assess your health status. We must not forget measures such as blood pressure, cholesterol and triglycerides, waist size, and fasting blood sugar.
Thank you for joining me in this look at body mass index (BMI).
