The Limitations of BMI

Health is multidimensional, and so is your body. While body mass index (BMI) is a helpful metric for evaluating obesity in populations, there are better ways to measure your health.  

I could write for days about the confusion my patients have experienced when focusing on BMI as a measure of health—even more so the unhealthy compensatory reactions that followed.

In this article, we’ll explore what BMI is, how insurance capitalizes on BMI, why your doctor tracks this number in your chart, and what alternatives we have to measure your health.

Cue you, entering your doctor’s office, stepping onto the scale. You unpack your pockets—wallet, keys, phone—and elect to remove your shoes to save a quarter of a pound. You stand tall and still—holding your breath—to avoid accidentally increasing your weight by one to two pounds.

Or perhaps the opposite rings true; you hope you ate enough this week to keep on that extra five pounds that the chemo/radiation, wrestling team, or marathon training prevents you from gaining. 

Our culture places an enormous pressure on your weight. The good news is, weight is not always a direct measure of health.

How Is BMI Calculated?

According to the CDC, BMI is measured by dividing your weight in kilograms (kg) by your height in meters (m)¹. There are many calculators online that can do the math for you, depending on what metrics you use (meters or inches).

BMI = kg/m2 

BMI provides a measure of body fat based on weight and height for adults. Historically, this indicated your “correct” weight and subsequent cardiovascular health².

BMI Categories

BMI is grouped into four main categories³:

• Underweight = <18.5
• Normal weight = 18.5 – 24.9
• Overweight = 25 – 29.9
• Obese = >30

Brief History of BMI

Origins of BMI

A Belgium mathematician named Lambert Adolphe Jacques Quetelet first developed the BMI equation in the 1800s⁴. He was not a physician, nor did he claim the number would be any direct measurement of health. Squaring the height of an irregularly shaped object, let alone a human, is not the way to accurately determine anyone’s… anything. He developed this metric as a way to gauge rates of obesity in large populations.

Adoption by Life Insurance Companies

Life insurance companies adopted BMI as a way to determine a person’s risk and coverage⁵.

1970s

BMI later appeared in medical headlines in 1972 in an article written by Ancel Keys and published in the Journal of Chronic Diseases⁶. He argued that even from the beginning, BMI was not “fully satisfactory,” but merely a good rule of thumb in helping to determine our body mass and measurement, while still searching for its meaning in health, disease, and survival. Despite clearly stating the usefulness of BMI in population studies, but not in individual evaluation, medical business took BMI to be the north star to measure an individual’s cardiovascular health.

Standardization of BMI Categories

In the 1990s, the National Institute of Health (NIH) and the World Health Organization (WHO) reconciled their differences in BMI ranges, and both agreed that a BMI of 25–30 should be considered overweight, with >30 meeting criteria for obese. These ranges are used for both men and women. You don’t need to be a doctor to know that different rulers ought to be used for the two sexes; men and women don’t even share the same life expectancy⁷.

Use of BMI to Determine Health Risks

In 2005, the National Center for Health Statistics compiled BMI data from 1971–2000 that showed those who are “overweight” died earlier than those of “normal” weight. This did not take into account adipose tissue percentage or distribution, which we now know plays largely into health outcomes⁸.

ADA and Financial Incentives for Employees

A final ruling to Title I of the Americans with Disabilities Act (ADA) in 2016 offered guidance to employers on the financial incentives—or insurance premium discounts—they can offer employees to participate in wellness programs and biometric screenings⁹. 

The biometric screenings can include measuring things like blood pressure and BMI. Unfortunately, using BMI as a quick screening method can have negative financial consequences when it comes to insurance premiums.

While the widespread claims made by BMI remain largely over generalized, they continue to be upheld by the insurance industry. 

What happens if your BMI is over 25?

If you register over 25 on the scale, your doctor will likely have a conversation with you about losing weight. And perhaps this is warranted, but BMI itself is not something to cause serious alarm.

Why is your BMI tracked by your doctor?

Healthcare providers record your BMI on every chart for population health metrics, insurance, and billing information. If someone has an abnormal BMI, this adds an additional ICD-10 code that can be billed for, and alerts your provider to screen you for other health issues if warranted.

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Limitations of BMI

Numerous studies have illustrated how BMI alone tends to misclassify your overall health^10,11.

BMI does not measure body fat directly, nor does it account for sex, ethnicity, muscle mass, or age. A BMI measurement cannot be used on children, as they grow tall during puberty at rates that may not directly match their weights¹². 

BMI doesn’t accurately predict cardiometabolic health

In 2016, the International Journal of Obesity published a study¹¹ aimed at disrupting the new U.S. Equal Employment Opportunity Commission rule that allows employers to penalize employees up to 30% in health insurance costs for failing to have normal health criteria, including a normal BMI⁹.

Researchers evaluated over 40,000 adults in the 2005–2012 National Health and Nutrition Examination Survey. They looked at blood pressure, triglycerides, cholesterol, glucose, insulin resistance, and many other labs that are used to evaluate a person’s health and compared it to the participants’ BMI. The study found:

• 45% of the participants were overweight or obese but cardiometabolically healthy
• 30% of the participants in the normal weight category were cardiometabolically unhealthy
• The cardiometabolic health of almost 75 million U.S. adults is misclassified when using BMI 

Professional weight lifters and athletes have much more muscle than fat (adipose) tissue and can have a high body weight due to a lot of lean body mass. This can make their BMIs register as overweight or obese¹³ in some cases.  

BMI measures quantity, but percent body fat evaluates quality 

Having a high BMI indicates that you could be overweight, or just muscular. The original studies that evaluated BMI in the 1800s largely studied white men; these metrics weren’t considered for women or other ethnicities⁴.

BMI vs. Death Rates

A study published in JAMA in 2005 showed that overweight people—according to BMI measurements—have a death rate similar to that of normal weight people⁸.

Another population study published in 2009 in The Lancet showed that over and underweight persons both have a higher mortality rate than most normal weight people¹⁰.

Alternatives to BMI

Instead of using BMI, I highly recommend you use the following:

Waist-to-Hip Ratio

Here’s how to measure your waist-to-hip ratio:

1. Measure the circumference at the level of your belly button and again at the thickest circumference of your thighs. 
2. Divide the waist/hip numbers (the units will not affect the result). 

Women should have a ratio less than 0.8 and men less than 0.95. 

The World Health Organization classifies abdominal obesity as¹⁴:

• >0.85 waist-to-hip ratio for women
• >0.9 for men

Many studies illustrate that BMI underestimates the rate of true obesity, and that measuring the waist-to-hip ratio more accurately predicts health risks, like heart disease and stroke^{15, 16}. BMI tends to overestimate fat percentage on those with generally lean body mass (athletes) and underestimates excess fat on those with less lean body mass.

Percent Body Fat

Percent body fat has been found to be more accurate than BMI in identifying obesity¹⁷. 

While there aren’t universally accepted healthy ranges for percent body fat, a recent study suggests that cardio-metobolic risks increase when body fat percentages exceed 25.8% for men and 37.1% for women^{18, 19}.

Is having some body fat healthy?

Yes. External and internal (visceral) fat are absolutely necessary to support and protect our internal organs. It provides a buffer in the event we become gravely sick and cannot eat for some time, or if you experience a serious high-impact accident³.

Focus on Nutrition, Diet, and Exercise, Not BMI

I have never made a serious medical decision for a patient based on their BMI, including medication changes. While I’ve had my fair share of nutrition, diet, exercise, and weight conversations, I don’t bring BMI into the mix. When my patients ask me for their BMI, I’m happy to share, but I encourage you to spend as little time focusing on your BMI as I do. 

References

1. https://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/english_bmi_calculator/bmi_calculator.html

2. https://www.nhlbi.nih.gov/heart-truth/know-and-control-your-heart-health-numbers 

3. https://apps.who.int/iris/bitstream/handle/10665/43190/9241593024_eng.pdf 

4. https://pubmed.ncbi.nlm.nih.gov/17890752/ 

5. https://www.jstor.org/stable/40267940 

6. https://academic.oup.com/ije/article/43/3/665/2949550 

7. https://www.cdc.gov/nchs/data/vsrr/VSRR10-508.pdf 

8. https://jamanetwork.com/journals/jama/fullarticle/200731 

9. https://www.federalregister.gov/documents/2016/05/17/2016-11558/regulations-under-the-americans-with-disabilities-act 

10. https://pubmed.ncbi.nlm.nih.gov/19299006/ 

11. https://www.nature.com/articles/ijo201617 

12. https://www.cdc.gov/healthyweight/assessing/bmi/childrens_bmi/about_childrens_bmi.html#normalWeightRanges 

13. https://pubmed.ncbi.nlm.nih.gov/15231223/

14. https://apps.who.int/iris/bitstream/handle/10665/44583/9789241501491_eng.pdf 

15. https://pubmed.ncbi.nlm.nih.gov/31930817/ 

16. https://pubmed.ncbi.nlm.nih.gov/15314626/ 

17. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3445161/ 

18. https://pubmed.ncbi.nlm.nih.gov/10966886/ 

19. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7229792/