Maintenance Phase Reality Check: "The Body Mass Index"
This is going to be a shorter post than some of the others, because a lot of the errors in this episode are things we’ve already addressed in other posts, namely the fact check of Is Being Fat Bad For You? But there are some additional issues with this episode that make it worth reviewing.
To start, it’s important to acknowledge that there are many reasons to be critical about the way BMI is used, and especially how it is applied to individuals in a medical context. The widespread adoption of BMI as a health metric has done substantial harm, both by enabling the dehumanization of people based on a single number and by poorly measuring the actual physiological factors linking adipose tissue to increased health risks. BMI is not a medically nor scientifically appropriate way to diagnose health issues. As you read this, please understand that the goal of this article isn’t to defend the harmful way in which the BMI is applied to individuals: it is to fact check the podcast episode and also provide some factual information about the relationship between adipose tissue and cardiometabolic diseases.
Let’s start by acknowledging that all anthropometric tools, such as BMI, waist-to-hip ratio (WHR) and waist-to-height ratio (WHtR), are an imperfect attempt at estimating body fat. In most research settings, however, measuring body fat on an individual level is not feasible. And if we want to evaluate the existence or strength of a relationship between body composition and health outcomes, we need some proxy for body fat. One thing that I think we fail to teach people in science classes is that fat, or adipose tissue (AT), is not inert - there are tons of cellular and metabolic processes that occur in adipose tissue. It is actually really cool and complicated! AT stores and releases energy, but it also secretes hundreds of mediators that communicate with other tissues in the body. On one hand, having enough adipose tissue is crucial to well-being: research in male bodybuilders has shown that not having enough body fat comes with a host of health consequences. On the other hand, too much AT accumulating in certain places can also lead to problems including cardiovascular disease and type 2 diabetes. How much AT accumulates, whether it accumulates subcutaneously or viscerally, and the impact it has on health differs from person to person and depends on a host of variables, including genetics and lifestyle factors. Just another reason why BMI is imperfect!
I’m not a physiologist, and it’s important to me not to fall into the trap that MP does - pretending to be an expert instead of promoting the voices and content of actual experts. So, instead, I’m going to suggest that, if all those links above are overwhelming, a good place to start is to listen to Sigma Nutrition Episode #379: Obesity & Chronic Disease Risk with Dr. Spencer Nadolsky. It is a very accessible resource about the role of body composition in health and disease.
Ok, below are the things I think are the most egregious issues from this episode.
Aubrey: Again, talking about the nuance of this story in this history, the failings of the BMI are mostly failings of humans, which makes it really fucking challenging to talk about this stuff when it gets presented as like, it's just capital T, capital S, the science.
Not so much a fact check as additional context: the unfortunate reality is that BMI is not the only cut point tool that is a blunt instrument. Every single blood test that has a “normal” range is the same; all diagnostic tools and screening mechanisms require some sort of limit/cut-off. BMI is an imperfect tool, but the main criticism against it (i.e., “a single number doesn’t reflect health”) is true of MANY other medical phenomena (think blood sugar, cholesterol, etc.). It would be great if we could easily find more precise answers, but the truth is that a lot of medicine works this way. Even our social/economic support infrastructure systems use arbitrary cutoffs (income cut-offs for defining “poverty” and tax rates, for example). Anyway, the problem is not the tool, it's the inappropriate application of BMI as a holistic diagnostic device and the use of BMI to justify abhorrent treatment of other humans. But BMI is just a calculation. It cannot be racist or fat phobic. In fact, if you look at the actual origins of BMI - not the version that Aubrey and Mike made up, but the actual historical facts - you’ll see that it wasn’t developed for anything like what it is used for today.
Aubrey: Right. So, it's a 20-pound difference, just by the definitions that we use. These are really significant changes. We're in this moment where “ideal weights” are fully being invented. And there are people in a room going, “I think this is too fat.” “No, I think this is too fat.” Again, they're being defined not relative to health risks, but relative to other people.
Please see our post about the “Is Being Fat Bad For You?” episode, where I explain how BMI categories came into being.
(On how BMI categories for kids were created) Aubrey: Basically, what they decide to do, they track growth studies from Brazil, from Great Britain, from Hong Kong, from the Netherlands, Singapore, and the US. They decide that what they're going to do is just extend the curve of the BMI for adults.
This is not at all how this was done. In fact, there is a very extensive CDC report from 2000 that details the methods for developing the BMI indices for children and adolescents. All of the data sources for pediatric BMI were national surveys from the United States (NHES and NHANES). There are no studies from Brazil or Great Britain or Hong Kong. There are no longitudinal studies. There is nothing about the BMI curve for adults. I have absolutely no idea where Aubrey could have gotten this from. In actuality, statistical models were applied to nationally representative data on children and adolescents from the United States. For those who are interested in the technical details, page 10 of the report explains:
“Empirical percentiles from the national data were smoothed with LWR [locally weighted regression]. Ten empirical percentiles were calculated for the BMI-for-age charts because the additional 85th percentile was required for boys and girls to identify children and adolescents at risk for overweight. Each smoothed value was estimated by weighted linear regression on the five-neighborhood points adjacent to the value to be estimated from ages 2 to 12.5 years. From 13 to 20 years, a 25-point smoothing procedure was used for boys and a 27-point smoothing procedure was used for girls. At the lower end (that is, age 2 years), two additional points were needed in the smoothing window, so a neighborhood point of 1.75 years was used for BMI. This was calculated using unadjusted recumbent length, repeated at 1.75 and at 1.71 years for both sexes. At the upper end (that is, age 20 years), the maximum BMI values in each empirical percentile from age 19.75 through 25.25 years were chosen and repeated in 0.5-year intervals from 20.25 through 25.75 years for boys or from 20.25 through 26.25 years for girls. Taking maximum values as additional data in smoothing the windows ensured that the BMI curves did not increase beyond the maxima at the upper ends of the age ranges. The smoothed percentile curves obtained through LWR were then fit by a 4-degree polynomial regression to achieve parametric percentiles. (See section on weight-for-age, 2 to 20 years, above, for further description of LWR.) A set of 10, 4-degree polynomial regression equations, 1 for each of the major smoothed percentiles, was solved simultaneously to estimate the L [power in the Box-Cox transformation], M [median], and S [generalized coefficient of variation] parameters for boys and girls separately. The 10 final percentile curves for infants were predicted using the estimated L, M, and S values.”
Mike: Also, from my years ago research into BMI stuff, there also is very different data based on race. The correlation between disease and BMI starts to show up at lower BMIs for Asian people, although there's huge diversity within Asia, obviously. But then, for black people, black people can be larger before those correlations start to show up. Again, it's all like there's other things that are more correlated and there's no way to talk about this without sounding like a eugenicist and shitty. But humans are diverse on every dimension, including the size that they're kind of best suited to be. It's not an academic issue that this was only done on white people. It's like an actual epidemiological issue. We're getting shitty data by applying this to everybody.
BMI cut-offs are different for Asian individuals. In Asian populations, the lower BMI cut-off is important, because it does a better job of identifying those at risk of cardiovascular disease and diabetes. This is based on well-established research that body fat percent and build differ substantially between Caucasians and Asians. Specifically, Asians are more likely to develop visceral adiposity (vs peripheral adiposity), which is associated with insulin resistance and type 2 diabetes. Research also suggests that Asian individuals have higher body fat percentages than non-Hispanic White individuals with the same BMI. Adjusting BMI cut-offs by ethnicity is actually a GOOD thing in terms of health equity because it is the recognition that the association between BMI and risk of metabolic disease is different in different populations; this is an example of BMI becoming a more robust tool with respect to research and medical screening.
This doesn’t mean the BMI is a perfect tool, or that it is a single number that reflects an individual’s “healthiness”, but it isn’t different from any other reference ranges in health and medicine that are intended to be a guide. It’s also worth noting that the updating and reassessment of BMI cut-offs is a GOOD thing, too, because new information absolutely should be incorporated when available. For a deeper dive into this topic, Sigma Nutrition Radio discusses ethnicity and cardiometabolic diseases in their episodes Role of Ethnicity in Cardio-Metabolic Disease Risk and Inequalities in Diabetes Outcomes for African & Caribbean Communities.
Aubrey: Yeah. The top 10 is, the Cook Islands, the Marshall Islands, Palau, Tuvalu, Tonga, Samoa. In addition to all of the garbage science stuff, we also now have this handwringing about the health of populations that have always been in this size range. Not only that, but in the United States, the way that we categorize race and ethnicity lumps together Asian and Pacific Islander communities.
It is incorrect and ultimately harmful to suggest that Pacific Islanders have ‘always been in this size range’; the reality is that body fat in these populations has increased rapidly since the middle of the 20th century, for a variety of complicated reasons. This isn’t an issue because larger bodies are morally inferior; it is an issue because the interaction between the AT and the rest of the body results in serious health complications. We help no one by obfuscating the link between the western colonial exploitation and suppression of indigenous cultures around the world and the subsequent health consequences that those populations face. The reality is that great harm was and is caused by breaking down the lifestyles and cultures of indigenous populations and replacing them with alternatives that economically benefit western nations, and anyone positing that these populations have ‘always been this way’ are perpetuating the problem and removing accountability from the colonial systems as well as the current capitalist economy.
Some closing thoughts:
This episode follows MP’s typical trope, which is that they put forth a question (i.e., “is being fat bad for you?”) and then present a single study or a group of studies that were not designed to answer that question to support the answer that they want to provide. They completely ignore entire fields of established science and research and continue to argue that fatphobia (which is very very real and very very harmful) is the true explanation for all of the evidence connecting excess adipose tissue and disease. This is anti-scientific to the core. Humans are made up of complex systems, some of which scientists have a solid understanding of and some of which we don’t. This is an issue of biology, not morality. We should be seeking to empower people by educating them about how bodies work. Continuing to perpetuate misinformation, as Aubrey and Mike do, is dishonest and dangerous.
Hobbs' statement about eugenics seems to be the ur example of what's wrong with his way of thinking/arguing. He wants to make to discredit BMI, so he argues that it's racist by (correctly) pointing out that body fat has different health impacts for different groups, but he has to stop himself short of actually making that argument, not because it isn't correct, but because he doesn't like the implications that there are meaningfully genetic differences (on average) between human populatios. Of course, he also overlooks the fact that you can't argue that body fat has different health implications for different groups without acknowledging that body fat has health implications, which he insists on denying.
For everything, he starts with his conclusions and then looks for evidence thst can be massaged to reach that conclusion, rather than letting the evidence take him where it leads (and then calls you racist if you reach a different conclusion).
The big problem with the BMI is that it's a very easy to administer screening tool. The big advantage of the BMI is that it's a very easy to administer screening tool. In either case, it is just supposed to be a screening tool to help inform clinical judgement.