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Is BMI Accurate? BMI is a Poor Measure of Obesity...But Not for the Reason You Think

Something I have seen a lot in articles published online is the rebuttal to obesity based on BMI. Many posit that BMI is not an accurate measure. There are plenty important and valid concerns with using BMI as a measure of obesity. First, we have to understand what is obesity.

To define obesity simply, it is excess body fat. Excess body fat has been shown repeatedly in research to be strongly associated with a higher risk of having type 2 diabetes, strokes, heart disease, and some cancers, along with a higher risk of dying from these diseases. So if BMI is a poor measure of obesity, people might be told they are at increased risk of disease when in fact they are not...right?

body weight scale

Concerns about the Use of BMI as a Measure of Obesity

The first concern often mentioned is that BMI does not take into account how much of the weight is muscle. The example often mentioned is that football players can have very high weights and BMIs, but they also have a lot of muscle. Muscle does weight more than an equal volume of fat tissue, so a very muscular person will have a higher BMI without increased body fat. For this muscular person, BMI is not accurate.

Another concern is based on how old the BMI is. The BMI calculation was developed in 1832 by a mathematician named Adolphe Quetelet. The fact that it is nearly 200 years old is used by some as evidence to its obsolescence.

A third concern relates to how it started gaining popularity. In the 1980's, researchers started to use BMI as a population measure, not as a measure to be used with individuals to diagnose obesity. The CDC even states that BMI is a screening tool, not a diagnostic tool.

Some cite the fact that it is used by the life insurance industry to infer that it is not even a medical assessment. If it is used by a non-medical industry, perhaps it is not meant to be used by the medical world.

My Concerns about Using BMI to Measure Obesity

My own concerns about BMI is that it might miss some people with obesity. I have seen studies that certain populations are at increased risk of obesity-related diseases at a lower BMI than the average in the United States. For example, Asians see higher rates of obesity-related conditions before reaching a BMI of 25 than other groups. The data used to define the categories of BMI was based on non-Hispanic white people, so other groups are not accounted for. For some ethnic groups, BMI is not accurate.

Is BMI Accurate: What Research Says about BMI and Obesity?

The most recent data agree that BMI is not an ideal measure of obesity. Not because it overestimates obesity, but because it underestimated obesity. A research abstract presented in June 2023 showed that obesity rates are twice as high as BMI would show when more accurate measures of body fat are used. This study looked at body fat data assessed by DEXA, which is a direct measure of body fat. While 36% of participants had obesity based on BMI, 74% had obesity based on excess body fat shown on DEXA measures. Not surprisingly, the subgroup of Asian participants had one of the highest rates of missed obesity based on BMI.

Addressing Some of the Concerns about Using BMI

Regarding the concern that BMI does not take into account muscle, here is what I say as a practicing dietitian. A full nutrition assessment is not just a bunch of numbers. It is a comprehensive interview. I'll find out if my patient is an athlete, and I will be able to put the BMI number into perspective. If my patient has average physical activity, I can put aside the concern that he or she has a high BMI due to high muscle mass.

Additionally, there are some fairly simple clinical measures that can help differentiate between high BMI from muscle mass and high BMI from fat tissue. Waist circumference measures and waist to hip ratio are additional measures of body fat that can help point to or away from obesity as the cause of higher weight.

I am not concerned by the fact that the equation is old. Old itself is not bad. Understanding that BMI was originally intended as a population measure is important. It is for this reason why BMI alone is not meant to be diagnostic of obesity--it is only a screening tool. Healthcare providers can use the positive screen on the BMI measure to ask more questions and gather more data to determine if a patient has excess body fat. Its use by the life insurance industry as a risk factor for death is telling. Their statistical methods aim to accurately assess risk, so their use of it reflects its accuracy as a risk factor for death.

My thoughts as a dietitian about BMI and Obesity

When I have a patient in front of me, the only person I am focused on is that person. I don't care about society's opinion about fatness, and I don't care about anti-fat bias activists' views on weight loss efforts.

Even if a patient is referred to see me for obesity, I need to understand what they want out of the appointment. Perhaps their weight is not a concern. Maybe they are afraid that I am another healthcare provider who will shame them for their weight.

The fact that a lower BMI correlates with a lower risk of disease does not mean that the person in front of me wants to lose weight. They might even understand their risk of disease--perhaps they also have prediabetes or elevated blood pressure. They still are not required to want to lose weight or engage in behaviors that might affect their weight.

Add to this the difficulty with losing weight. Research does not show promising results with dieting to lose weight. If a person has excess body fat, is at a higher risk of disease, and is interested in addressing this medical concern, they should be educated on their entire array of options. Some people opt to use nutrition to improve their health and disease risk even if it might not change their weight. Others are interested in medical or surgical options.

Whether or not any of those options are right for an individual should be determined with a thoughtful discussion between the person and their healthcare provider. This conversation should include information about health history, current medical concerns, family medical history, limits weight places on quality of life, and most importantly, if the person wants to try any of the options.

BMI is not Accurate: It is an Inadequate Measure of Obesity

What we have been hearing for a while, that BMI is a poor measure of obesity, seems true. A knowledgeable healthcare provider will be able to use BMI as a screener, and along with other clinical data, they can make a more accurate assessment. Hopefully this new research will remind providers to also consider obesity in those who screen negative with the BMI measure. A good clinical interview can uncover other factors that increase likelihood of obesity, such as ethnicity, medical history, family history, and lifestyle. Yes, BMI is an inadequate measure of obesity--it underestimates obesity.

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