Is BMI Useful?

Because much of our health system revolves around BMI (body mass index), our BMI is supposedly a big deal. I get asked questions about what exactly is BMI, what does it really measure, and is it valid. Today I want to tackle those questions.

First, let’s start by defining BMI, which is a person’s weight divided by their height squared. According to the National Institute of Health (NIH), “BMI is a measure of body fat based on height and weight that applies to adult men and women.” Ok, so that’s questionable, but we’ll get into that later. I think a brief history lesson on the origins of BMI and the campaign against “obesity” is useful in helping understand BMI’s usefulness for us today.

How BMI Was Developed

BMI was introduced in the early 19th century by a Belgian mathematician (not a physician!) named Adolphe Quetelet to define the statistical norm of human beings, i.e., “the average man.” Quetelet himself said that his index should not be used as a measure of individual health.

Nevertheless, later in the 19th century, U.S. life insurance companies began using it to classify individuals as a way to determine what to charge prospective policyholders. These Metropolitan Life Insurance Company height-weight tables were used to define “ideal” weights for almost a century and helped popularize the idea that weight is a determinant of health.

In 1972, researcher Ancel Keys and colleagues tested several indices of body fat and compared those results to actual measurements of body fat taken from skin calipers and underwater weighing. They concluded that Quetelet’s index was superior to other indices of body fat, and proposed the formula be termed body mass index (BMI). Even then, Keys et al. admitted that BMI does not accurately represent a person’s body fat percentage. Yet another warning that people largely ignored.

Changes in the BMI Categories

In 1985, the National Institutes of Health (NIH) published a paper summarizing their Panel’s definition of “obesity” and its health implications. The NIH grouped “overweight” and “obese” together as a BMIs ≥ 27.8 for men and ≥ 27.3 for women.

Then in 1998, the NIH once again changed the definitions, not only differentiating between “overweight” and obese”, but also significantly lowering the cutoff to be medically considered fat. Now, for both men and women, a BMI greater than 25.0 is considered “overweight” and a BMI over 30.0 is “obese.” An estimated 29 million people went to sleep “normal” one day and woke up “overweight” the next day.

CNN summarized it well when they reported that “Millions of Americans became ‘fat’ [that] Wednesday — even if they didn’t gain a pound — as the federal government adopted a controversial method for determining who is considered overweight.”

Flaws with Using BMI

It Doesn’t Really Tell Us About Fatness

One of the most obvious flaws with the BMI is that, as Keys et al. admitted (even while supporting its use), BMI doesn’t actually measure body fat. This where the NIH definition I mentioned earlier isn’t quite accurate. The calculations make no allowance for muscle mass, bone density, and other factors that can highly influence a person’s weight.

For example, a person with higher muscle mass, such as an athlete, will have a higher weight even compared to someone with the same amount of body fat who has a lower muscle mass. Same for bone density. Also, even some who support the use of BMI are calling for the formula to be changed, criticizing the current formula for what they believe is a tendency to exaggerate thinness in short people and fatness in tall people.

It Doesn’t Really Tell Us About Health

First, let’s keep in mind that the data used to create the BMI scale were based primarily on Caucasian men. Why does that matter?

Well, because people of different sexes, ethnicities, ages, etc. are… well, different. It’s overly simplistic and rather ignorant to assume that a white man is the same as an Asian women. I would think the failings of a one-size-fits-all approach would be pretty obvious, especially given our current concern about diversity, but apparently not.

For example, on average, women have a higher percentage of body fat in relation to total weight than men do, but this does not necessarily mean more health risks. Studies published by the Endocrine Society indicate that because black people tend to have heavier bones and therefore weigh more than white people, the BMI overestimates fatness and health risks for black individuals. And on the other end of the spectrum, according to the WHO, the BMI underestimates health risks for Asian communities, which may contribute to underdiagnosis of certain conditions.

So Does Being at a Higher BMI Mean You’re Unhealthy or At-Risk for Diseases?

Not necessarily. First, let’s take a look at the supposedly-indisputable evidence that being at a higher BMI is bad for your health.

A Closer Look at the Research that Says Being “Overweight” is Bad

As I covered in a previous post, Can You Be Fat and Healthy?, while there is research that associates higher BMIs with certain health conditions, we don’t really know that higher weight causes those conditions. I’ll say it again and again: correlation does not equal causation.

Even the previously-mentioned NIH paper (1985) defining and vilifying “obesity” states that: “The distribution of fat deposits may be a better predictor of mortality than BMI.” And my personal favorite: “The mortality and morbidity related risks of obesity are influenced by concurrent risk factors such as smoking.”

Well yeah, of course it is. That sounds like a complete ‘duh’ statement, but I don’t think it is considering the disproportionate amount of attention that’s placed on weight. And smoking as a confounding factor is just the tip of the iceberg. There are a lot of factors that are considered determinants of health. Some of those factors include:

• Genetics
• Age
• Sex
• Ethnicity
• Activity level
• Stress levels
• Mental health
• Sleep quantity and quality
• Personal behavior, such as sexual activity and even things like handwashing and oral hygiene
• Amount of social support and social interactions
• Socioeconomic conditions, such as poverty
• Access to medical care
• Availability of resources to meet daily needs, such as educational and job opportunities, living wages, or healthful foods
• Alcohol, cigarette, and other drug use
• Traumatic experiences
• Social norms and attitudes, such as experiencing discrimination
• Exposure to crime, violence, and social disorder
• Environment, such as the presence of trash
• Exposure to toxic substances and other physical hazards

This isn’t an exhaustive list, but hopefully you can see even considering just these factors, it’s incredibly difficult in research to account for all the possible variables. Much of the research about “obesity” doesn’t level the playing field by making sure all these things aren’t influencing the data negatively toward people with higher BMIs.

For example, a study found that people in the U.S. who live in the most poverty-dense counties are those most prone to being at an “obese” BMI—it showed “obesity” rates to be 145% greater than affluent counties. Now, we know that poverty reduces life expectancy and quality of life, and increases the risk for a number of health issues. Other studies have indicated that socioeconomic factors were found to be the most important predictors of death by all causes.

And yet, we’ve downplayed the poverty issue (along with all the other confounding factors) and have decided weight is the main issue when it comes to health. Based on the U.S. Census Bureau’s 2017 estimates, there are an estimated 39.7 million Americans living in poverty. If we truly care about health, perhaps our efforts would be much better spent on the “war on poverty” instead of “obesity.”

There is another issue with some of the studies that seemingly indicate being “obese” is bad for your health. Let’s consider what researchers often consider “healthy” from a metabolic standpoint (because there’s no standard definition, it can be a bit subjective). Typically, it would be individuals who exhibit good health markers such as appropriate levels of blood sugar, triglycerides, high-density lipoprotein (HDL) cholesterol and blood pressure without the use of medications. So the question is… can thin people be “unhealthy” and fat people be “healthy” according to these types of metabolic markers?

Yes, absolutely.

But too often in studies that (supposedly) show “healthy obese” people are still at higher risk for disease and death, the “healthy obese” people are defined as individuals who have no more than 1 metabolic risk factor—hypertension (high blood pressure), for instance. That is problematic because hypertension alone increases the risk of death.

Do you see how misleading that is? The results seem compelling until you look closer and realize the “healthy obese” people weren’t actually healthy to begin with so their conclusion is inherently flawed.

A Look at the Research that Says Being “Overweight” Can be Okay, Sometimes Even Healthy

Contrary to popular belief, the studies associating higher BMIs with negative outcomes aren’t the only research on the topic. There are, in fact, a number of studies indicating being “overweight” is a protective factor for many health issues, meaning that for who people do get a certain condition, they are less likely to die from it if they are at a higher BMI. This phenomenon is referred to as “reverse epidemiology” or the “obesity paradox”.

For example, “obesity” has been associated with improved survival in patients with certain types of cancers as well as some chronic diseases, including congestive heart failure (CHF), chronic obstructive pulmonary disease (COPD), chronic kidney disease, and more.

A landmark study by Flegal et al. revealed that compared to being at a “normal” weight, “overweight” is associated with lower death rates by all causes. Unless you’re already into the Health At Every Size® (HAES) movement, you likely have never heard about that research. It doesn’t exactly fit into our normal fear-mongering paradigm about weight.

Though this study did account for some confounding variables such as smoking and pre-existing illness, this research has been criticized by some as not accounting for other variables. I think they did a pretty good job on that level, but again, we have to acknowledge how complex all the variables are when it comes to studies on health and the difficulty in isolating the one variable being studied. So perhaps there’s validity to that argument. But you have to admit the irony of such criticism. These critics are applying a standard of scientific rigor only to the research they want to discredit, while blatantly ignoring the same issues in the research supporting their preconceived ideas.

In 2018, researchers at York University’s Faculty of Health published a study comparing several groups, including individuals who were:

• “obese” with no metabolic risk factors
• “obese” with a metabolic risk factor
• “normal” weight with no metabolic risk factors
• “normal” weight with a metabolic risk factor

This is a more fair comparison than other research with “healthy obese” people who weren’t actually healthy. The conclusion of this study was that “obese” people with no metabolic risk factors do not have an increased rate of mortality over “normal” weight people without risk factors:

“We found that a person of normal weight with no other metabolic risk factors is just as likely to die as the person with obesity and no other risk factors…This means that hundreds of thousands of people in North America alone with metabolically healthy obesity will be told to lose weight when it’s questionable how much benefit they’ll actually receive.”

Final Thoughts…

Sadly, though our culture prides itself on being science-based, a whole segment of the research on weight and health is completely ignored while the other segment of the research is misinterpreted. Then those misinterpretations are treated as indisputable.

That’s how powerful and destructive fat bias is. It’s bad enough that we’ve marginalized fat people in the beauty industry, but now we also do it in the LAST places it should ever happen—in science and medicine.

This is not okay. Weight stigma is making people sick and even killing people through physical and mental illnesses. Taking the time to educate yourself about the Health at Every Size and body acceptance movements is a great place to start if you haven’t already.

Please reach out to us if there’s anything I can do to help you in your journey. May we all be better people tomorrow than we are today.

Sources

https://www.psychologytoday.com/us/blog/the-gravity-weight/201603/adolphe-quetelet-and-the-evolution-body-mass-index-bmi

https://elemental.medium.com/the-bizarre-and-racist-history-of-the-bmi-7d8dc2aa33bb

https://frac.org/obesity-health

Semega, J; Fontenot, KR; Kollar, MA. Income and Poverty in the United States: 2017. Census Bureau, September 2018.

Levine J. A. (2011). Poverty and obesity in the U.S. Diabetes, 60(11), 2667–2668. https://doi.org/10.2337/db11-1118

Health Implications of Obesity. NIH Consens Statement Online 1985 Feb 11-13; 5(9):1-7.

Robert J Kuczmarski, Katherine M Flegal, Criteria for definition of overweight in transition: background and recommendations for the United States, The American Journal of Clinical Nutrition, Volume 72, Issue 5, November 2000, Pages 1074–1081, https://doi.org/10.1093/ajcn/72.5.1074

Wang Z, Liu M, Pan T, et al. . Lower mortality associated with overweight in the U.S. National health interview survey. Medicine 2016;95:e2424–e24. 10.1097/MD.0000000000002424

Melaku, Y. A., Gill, T. K., Appleton, S. L., Hill, C., Boyd, M. A., & Adams, R. J. (2019). Sociodemographic, lifestyle and metabolic predictors of all-cause mortality in a cohort of community-dwelling population: an 18-year follow-up of the North West Adelaide Health Study. BMJ open, 9(8), e030079. https://doi.org/10.1136/bmjopen-2019-030079

Hu FB. Obesity Epidemiology. Oxford University Press; New York: 2008.

York University. (2018, July 12). Obesity alone does not increase risk of death: New study could change the way we think about obesity and health. ScienceDaily. Retrieved June 18, 2020 from http://www.sciencedaily.com/releases/2018/07/180712114440.htm