Correlation vs Causation Errors are Killing Fat Patients
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Correlation never ever, never ever, never ever implies causation.
This is burned into my brain because my first research methods professor made us say it out loud, repeatedly, every single class for the entire semester.
It is at the very root of how research is conducted, and what conclusions can (and can’t) be drawn. It is also an area that is ignored, overlooked, or straight up subverted when it comes to research and healthcare for fat patients.
Correlation means that things happen at the same time. Causation means that one thing makes the other thing happen. When we say that correlation does not imply causation, we mean that no matter how often things happen at the same time, we can never use that information to prove (or even assume) that one thing causes the other.
So, even if being higher weight is correlated with a higher incidence of healthcare conditions, we can’t assume that it’s the weight causing the health conditions. More on this in a minute.
The typical pushback I get at this point in the explanation is that within healthcare and the human body things get really complicated, so it’s not always possible to prove causation. For that reason, the argument goes, sometimes treatment protocols, best practices etc. are designed around strong correlation.
That’s true, but it does not mean that we get to throw due diligence out the window and use correlations willy nilly. We are still ethically obligated to examine the research to see if there are any methodological issues that could be at the root of the correlation, and we have to consider confounding variables (which is to say – other factors that may be impacting/causing the correlation.)
If we fail to do that, then we risk focusing “treatment” on the wrong thing, which will get you an “F” in Freshman Research Methods and, much more importantly, can actually harm or even kill fat patients.
Studying the Studies
So what happens when we examine the weight loss literature for methodological issues? We find them…by the truckload. The first is study length - it is well known that most people lose weight short-term and gain their weight back long-term (within 2-5 years) and yet studies that end after less than even 2 years of follow up will claim that they prove “long-term success.”
Many studies also simply ignore huge dropout rates among the study participants, and simply draw conclusions based on the small percentage of subjects who are still left at the end. And that’s just scratching the surface of the issues with research design and the conclusions drawn. I highly recommend Lucy Aprhamor’s paper Validity of claims made in weight management research: a narrative review of dietetic articles for a deep dive into the ways that this research often fails to meet the most basic research methods standards.
That brings us to confounding variables, and when it comes to weight and health there are three well-documented contenders: weight stigma, weight cycling, and healthcare inequality. I’ll get into these in more depth in later posts, but for now here is a quick look:
Muennig has two studies around weight stigma that are particularly salient, The body politic: the relationship between stigma and ob*sity*-associated disease examines the relationship between weight stigma and health issues, and finds that weight stigma may drive health issues that are typically blamed on body size, and I Think Therefore I Am: Perceived Ideal Weight as a Determinant of Health found that the difference between actual and desired body weight was a stronger predictor of physical and mental health than body mass index (BMI)
In their paper Weight Science: Evaluating the Evidence for a Paradigm Shift, Bacon and Aphramor found that the health impacts of weight cycling could explain all of the excess mortality that was attributed to “ob*sity” in both Framingham and the NHANES. In a recent paper, Gaesser and Angadi found that “The risks associated with weight cycling are very much the same as those associated with ob*sity.”
The many impacts of inequalities in healthcare on higher weight people are examined in Lee and Pausé’s Stigma in Practice: Barriers to Health for Fat Women.
Knowing all of this, the idea of assuming that higher weight causes health issues, and then developing “treatment” protocols (not to mention entire fields of medical practice) around trying to change body size as a “solution” (including interventions that risk fat people’s lives and quality of life for the smallest chance of becoming thin/ner) is not supportable by evidence or ethics.
First Do No Harm
Remember before when I mentioned that creating treatment guidelines based on assumptions made about causation from correlation could end up harming patients. That is exactly what has happened in modern medical practice.
The train of thought went – if higher weight people have health issues more often, then the weight must be the cause of the issue, and the solution must be to shrink the people.
And the weight loss industry – diets, “lifestyle changes,” pharmaceuticals, surgeries “ob*sity medicine” and all – was born.
When we look at the three confounding variables – weight stigma, weight cycling, and healthcare inequalities – we find the weight loss industry (including within healthcare) smack at the root of each of them.
The weight loss industry profits from weight stigma, and so it perpetuates weight stigma in its marketing and its application. Whether it’s a billboard in times square meant to help a weight loss peddler profit for body shame, or perpetuating stereotypes, or insisting that the solution to weight stigma is to fight our bodies rather than the stigma, the weight loss industry secures its $70+ Billion a year paycheck by making sure weight stigma is going strong.
The most common outcome of intentional weight loss attempts is weight regain. The most common outcome of more than one intentional weight loss attempt is weight cycling. Fat people are encouraged to continue dieting until they succeed which, for the majority of people, will be never. This leads to a lifetime of weight cycling.
The belief (despite the evidence that says otherwise) that body size is changeable has led to an attitude within healthcare that if “fatness” can be blamed for an inequality (like a lack of diagnostic tools, beds, chairs, etc.) then that inequality is acceptable. This drives additional healthcare inequalities.
This, then becomes a cycle. The healthcare and weight loss industries drive weight stigma, weight cycling, and healthcare inequalities by aggressively insisting that correlation implies causation. Then they blame the harms that come from those inequalities on fat bodies. Then they use those harms to justify additional weight stigma, weight cycling, and healthcare inequalities.
This cycle has to stop. It’s not evidence based, it’s not ethical, and not only is it failing to help, but it’s doing great harm, and at the root is a correlation vs causation error that a freshman research methods student should know better than to make.
Finally, while I think it’s worth having these discussions since so much of fat people’s treatment, including in healthcare, is driven by this, we can never lose sight of the fact that fat people have the right to live without shame, stigma, bullying or oppression no matter why we are fat, no matter what the “health impacts” might be, and whether or not we could, or even want to, become thin. That includes the rights to equal accommodation, including in healthcare.
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*Note on language: I use “fat” as a neutral descriptor as used by the fat activist community, I use “ob*se” and “overw*ight” to acknowledge that these are terms that were created to medicalize and pathologize fat bodies, with roots in racism and specifically anti-Blackness. Please read Sabrina Strings’ Fearing the Black Body – the Racial Origins of Fat Phobia and Da’Shaun Harrison’s Belly of the Beast: The Politics of Anti-Fatness as Anti-Blackness for more on this.