What You Should Know About BMI

BMI is a popular, easy to use metric, but it’s probably not THAT valuable for you.

"My doctor said I was obese now. I didn't think I was but I guess I need to lose weight."

"I don't understand. I work out almost every day and eat well and I'm lean, but my BMI says I'm overweight! How is that possible?"


In last week's email, I talked about why I believe body weight is a fairly useless indicator of health, and most of us would be better off focusing on other metrics. [Check it out here if you missed it.]

Since BMI, or Body Mass Index, is literally an equation that relies on body weight to compute, it stands to reason that it's also an imperfect metric to use.

Unfortunately, along with weight, BMI is a pretty standard measure used in health and fitness settings today, so let's look at what it is and what it means.

What is BMI?

BMI stands for Body Mass Index. It's designed to estimate a person's body fat, based on their height and weight.

The exact calculation is BMI = kg/m².

The resulting number corresponds with a range that is categorized as "underweight," "normal weight," "overweight," or "obese," and that categorization is a total overgeneralization.

Let's remember that body weight is simply the measure of the force of gravity on the body. It does not, alone, indicate health, so any metric that relies on that number to make up half its equation is clearly also going to be limited in how well it assesses health.

What is not taken into consideration when computing BMI are things like: age, sex, race, fat distribution, activity level, family history of disease, bone density, etc.

Theoretically, higher BMI values correspond to higher risks of certain diseases, especially cardiovascular and metabolic diseases; however, there are a lot of other factors that are more valuable in determining health status and risk.

Where Did BMI Come From?

The BMI scale was originally known as the Quetelet Index after Adolphe Quetelet, the Belgian statistician whose work focused on studying characteristics of "the average man." The Quetelet Index was based on population data of (white, European) men, which indicated that weight increases as the square of height.

BMI wasn't used in healthcare until the 1970s, when researchers were looking for a simple way to estimate body fat. Though it was never intended to assess an individual's body fat, it was adopted by the National Institute of Health in 1985 as a way to identify obesity. In 1998, the NIH made a controversial move to adjust the index, which resulted in millions more people falling under the "overweight" or "obese" categories.

This is important, because many people have faced weight stigma and discrimination based on their BMI when the categorization is...almost arbitrary.

There were a number of tables and indices that were used in the past by insurance companies, and there were others being considered for standard use, but this is what they ultimately landed on regardless of its shortcomings.

Limitations of Relying on BMI

Since the BMI scale was designed based off the measurements of white, predominantly European men, women and people of non-European backgrounds may be miscategorized.

Women tend to have higher levels of body fat than men which is biologically necessary, but the BMI scale doesn't take biological sex into account.

Age is also not factored into the calculation, which results in older adults being more likely to fall into "overweight" or "obese" categories due to lower levels of lean muscle mass and lower bone density.

In fact, the BMI table is only recommended for use in adults from ages 18-65 or 20-60, depending on the organization.

Studies have also shown that race can be a factor in the chart's accuracy.

Black men and women are more likely to be incorrectly labeled as "overweight" or "obese" on the BMI chart.

On the flip side, Asian men and women are less likely to be categorized as "overweight" or "obese" but may face higher risk of heart disease and other cardio metabolic disorders at lower BMI thresholds.

Athletes, especially those with high percentages of lean muscle mass, may fall into the "overweight" or "obese" categories while being incredibly fit and healthy, because BMI does not differentiate between fat mass and lean mass -- it just uses weight.

BMI may also cause eating disorders to be overlooked or misdiagnosed, as according to the National Association of Anorexia Nervosa and Associated Disorders (ANAD), only 6% of people with eating disorders are medically considered "underweight."

Of course, people of any age, race, and sex may be metabolically healthy at higher BMI values or metabolically unhealthy while falling into the "normal" range. Relying on general numbers like BMI can prevent patients from getting the care and treatment they need.


Why Do We Even Use BMI Then?

It's cheap, quick, and easy.

Other more accurate means of identifying an individual's body fat may be more time-consuming or require more expensive equipment that not all facilities can access -- although recent studies have shown that using an individual's waist-to-height ratio is a better indicator of total body fat and disease risk than BMI, and that's also a fairly simple measurement.

The CDC, in its guidelines for healthcare practitioners, states: "Because BMI does not measure body fat directly, it should not be used as a diagnostic tool. Instead, BMI should be used as a measure to track weight status in populations and as a screening tool to identify potential weight problems in individuals."

Someone's BMI is not a diagnosis. Health is always more complex and nuanced than one number.

If a healthcare provider brings up BMI, ask them about their specific concerns. Have a discussion to ensure they're also considering factors like age, activity level, and actual test results before making blanket recommendations.


The Bottom Line...

Like weight, BMI is one number that doesn't, on its own, determine one's health.

As always, activity levels, nutrition & hydration, sleep status, stress levels, and health markers on blood work are far, far better indicators of health and well-being than weight or BMI could ever be.


Got questions? Get in touch and I'll be happy to help.