BMI is Just a Number: Conflating Riskiness and Unhealthiness in Discourse on Body Size

Abstract

Despite the ubiquity of the body mass index (BMI) in discourse on health, there is ambiguity in its use as a biomarker of current abnormality versus future risk. This distinction is consequential for knowledge of the relationship between body size and health, as well as for individuals deemed to have abnormal and “unhealthy” bodies. Consequently, the purposes of this review are threefold. The first is to differentiate this ‘biomarker’ perspective from extant critiques of BMI as a proxy for health behaviors or as the defining characteristic of obesity as a disease. The second is to highlight the shift towards treating BMI as a measure of attained unhealthiness, rather than a probabilistic indicator of risk. Finally, rather than call for the abolition of BMI, this paper argues that its continued use as ‘just a number’ is in keeping with the push for weight neutrality in research and practice. The review concludes by demonstrating how the riskiness and unhealthiness of body size is conflated in public health messaging on COVID-19. BMI is a marker of risk, but its use as a surrogate for COVID-19 severity equates body size with health, shaping beliefs about vulnerability and personal responsibility amid an ongoing pandemic.

Introduction

Over the past 50 years, body size has become a focal dimension of how practitioners, researchers, and the public evaluate health (Fletcher 2014; Jutel 2001; Oliver 2006; Saguy 2012). The ability to quickly and easily measure body size using the Body Mass Index (BMI; body weight [kg] / height-squared [m²]), and then map these values onto substantive categories like “overweight” and “obesity,” has made body size pervasive in scientific and lay narratives about the healthiness of a given person or population (Nicholls 2013). Indeed, BMI is a progenitor to the development and growth of biosocial data in recent decades, wherein the collection of biomarker data allows sociologists and other researchers to obtain more direct and objective evidence of how social forces and environments contribute to bodily ‘wear and tear’ (Harris and Schorpp 2018; McDade 2008).

Indeed, the prevalence of BMI in research and clinical settings in the 21^st century is ever-increasing (Gutin 2018), attributable to its convenience of measurement (Burkhauser and Cawley 2008), ease of interpretation (Fletcher 2014), and an innate appeal to societal beliefs about how an individual’s appearance reflects underlying health (Jutel and Buetow 2007). BMI is an established and conventional measure; it is sufficiently widespread and accepted that its orthodoxy is largely unchallenged. Certainly, many have noted that BMI last many limitations as a measure of the harmful adiposity and unhealthy body composition that researchers want to identify (Rothman 2008). However, convenience outranks construct validity, as the tradeoff between expedience and accuracy is one many are willing to make in using BMI to study population health (Nicholls 2013).

Consequently, the purpose of this review is not to relitigate the measurement issues surrounding BMI, or call for replacement (Kragelund and Omland 2005). Rather, the goal is to discuss ontological issues in how BMI is thought about and used, wherein social theory is instructive. Undoubtedly, better measurement is the most direct route by which to improve research; however, a measurement-centered approach is untenable given methodological constraints in extant research and practice, where BMI will continue to be the primary diagnostic tool (Hu 2008). Nor does better measurement – ranging from waist and hip circumference to bioelectrical impedance – provide a full explanation of how a person’s physical appearance or body size and shape is associated with their health and wellbeing (Yates-Doerr 2013). BMI is an imperfect biomarker, but it is also inescapable, such that researchers are better served by improving understanding of how to incorporate and interpret BMI as the measure that is available rather than the measure that is desired. In turn, this review makes a case for better theory, emphasizing that measures of risk are not interchangeable with measures of health, especially when both biophysiological and psychosocial mechanisms are at play – as is true of BMI.

Pursuant of this goal, the paper begins by differentiating the ‘biomarker’ perspective from extant critiques of BMI as a proxy for individuals’ health behaviors or as the defining characteristic of obesity as a disease. Next, the paper demonstrates the shift toward treating BMI as a measure of attained unhealthiness, rather than a probabilistic indicator of risk. Finally, rather than call for the abolition of BMI, the paper advances a novel argument contending that its continued use as ‘just a number’ – with health-relevant attributes – is in keeping with the push for weight neutrality in research and practice. The review closes with an applied example, showing how the riskiness and unhealthiness associated with body size is conflated in public health messaging on COVID-19. BMI is labeled as a risk factor but in practice it is deployed as a surrogate measure of COVID-19 severity, shaping knowledge about who is most vulnerable to and responsible for severe illness in contemporary society.

Not a Behavior, Not Yet a Disease

This review makes a case for a more neutral framing and use of BMI, yet that this is far from the status quo in extant research. Behavioral/lifestyle and disease frameworks have served as the dominant perspective on body size, representing opposite ends of the pathogenic process by which individuals’ actions and exposures are manifest in diagnosable health outcomes (Kelly and Russo 2018). They reflect disciplinary distinctions in how BMI is conceptualized, and the epistemological concerns of those studying health. As will be shown, biomarkers do not represent actions or decisions, nor do they constitute definitive diagnoses. They instead represent a more ambiguous middle ground, linking behaviors to diseases – or actions to outcomes – via a process of mediation often couched in the language of ‘mechanisms’ and ‘pathways’ (Loucks et al. 2008). Beyond their mechanistic function, this review argues that a biomarker perspective allows for a form of ‘neutrality’ that circumvents the negative assumptions that arise from treating BMI as a proxy for harmful behaviors, or as a criterion by which individuals are labeled as ‘diseased.’

Health Behavior or Lifestyle

Treating BMI as a behavioral concern – commensurate with smoking, alcohol consumption, drug use, and other “actuarial,” health-relevant factors (Jones and Oppenheimer 2017) – is exemplary of the contemporary public health paradigm, which seeks to identify and intervene on the harmful behaviors accounting for the majority of morbidity and mortality (Dew 2012). Surveillance, or “surveillance medicine” (Armstrong 1995), is central to this framing; the individual- and population-level tracking of health behaviors becomes the primary way to minimize the uncertainty of risk and poor health in the modern world. In turn, the ongoing surveillance of BMI over past decades has made obesity inseparable from the label of “epidemic” (Fletcher 2014), emphasizing the prevalence of this epidemiologic perspective.

Crucially, this framework invokes BMI as a ‘summary’ measure of various changes in behaviors and lifestyles over past decades. The transition towards more sedentary labor and leisure activities and increasingly calorie-rich environment are hypothesized as key obesogenic lifestyle-relevant mechanisms afforded by modernization (Phillipson and Posner 2003). Thus, as public health and medical advances have largely eliminated many communicable diseases and other acute threats to health in many societies – and as other harmful behaviors like smoking continue to decline – obesity has been singled out as the primary lifestyle factor underlying future chronic morbidity and premature mortality (Stokes and Preston 2016).

However, using BMI as a measure of behaviors and lifestyles is laden with assumptions and stereotypes about individuals’ bodies as the literal embodiment of hedonism (Murray 2012; Saguy 2012). Disease prevention in public health is predicated on a shift towards the adoption and maintenance of healthy lifestyles, with weight ‘control’ as an illustrative case. Though well-intentioned, this framework advances the idea of people striving to be good “biocitizens,” who adhere to certain health standards that reflect their value as contributing members of society (Dew 2012). This reflexive and recursive link between health ideals and social ideals is unavoidable in the contemporary world, where societal concerns about reducing risk are located at the individual level, as individuals are held ‘responsible’ for minimizing their own risk and, by extension, risk in the population as a whole (Beck 1992; Lupton 2006). Health ideals and social ideals are thus comingled and co-constructed, influencing how individuals are judged as a function of whether their bodies are thought to reflect good decisions (Fox 2012).

Indeed, the conflation of epidemiologic and social norms about healthiness has made thinness a marker of one’s social status and morality. Managing one’s health – by means of having a thin, medically ‘appropriate’ body – is a hallmark of the “good” biocitizen (Greenhalgh 2015). Overweight or obesity, as a deviation from a “healthy” and “normal” BMI, represents a flawed identity premised on the inability for self-discipline (Ciciurkaite and Perry 2017; Oliver 2006; Shugart 2016). Thus, the linkage of body size to other lifestyle attributes like diet and exercise leads to the harmful and unfounded assumption that individuals with overweight and obesity are slothful, greedy, and selfish (Murray 2012; Saguy 2012; Shugart 2016), despite many adults with “unhealthy” BMIs engaging in the kinds of healthful behaviors that are valued under this paradigm of good biocitizenship (Bombak et al. 2019; Greenhalgh 2015; Tylka et al. 2014).

Disease

While the behavioral framework reflects the surveillance imperatives in epidemiology and public health, conceptualizing obesity as a disease reflects the drive for unambiguous assessments of health in the context of clinical practice. Contemporary biomedicine is premised on the mitigation of uncertainty in the understanding of health and, more importantly, threats to health. The biomedical model makes disease its focus, creating a standard by which normality and health are defined in relation to disease (Fox 2012), with the disease label delineating good and bad health (Jutel 2019). Describing obesity as a disease therefore reduces uncertainty in how much clinical significance health professionals, researchers, and laypersons accord to BMI.

On the one hand, the disease label is a nosologic necessity in the context of medical practice, wherein the nosology – classification and naming – of various conditions and ailments is of the utmost concern (Jutel 2014). Disease categories serve as the gateway for legitimization, treatment, and reimbursement, which permits body size to be worthy of medical attention (Jutel 2014). The disease label opens avenues of treatment that might otherwise be closed due to an individual-level behavioral/lifestyle perspective (Rosenberg 2002). Pragmatism is a common justification for this framing, recognizing that medical care is as much a function of what is considered legitimate and billable, as it is a function of what individual practitioners and patients believe to be important. Many governing medical bodies are explicit about adopting the practice of labeling obesity as a disease in an effort to legitimate obesity as a serious and diagnosable health condition within the eyes of medical practitioners, their patients, and the population at large (Allison et al. 2008; Bray et al. 2017; Kyle et al. 2016).

On the other hand, the disease label is potentially unjustified and harmful in perpetuating weight-related stigmatization and bias (Kyle et al. 2016; Sharma and Campbell-Scherer 2017; Tomiyama et al. 2018). While not challenging the notion that excess adiposity may be harmful, labeling individuals with obesity as diseased on basis of BMI does not reflect practitioners’ knowledge of a person’s overall health and the extent to which it is directly affected by their weight (Sharma and Campbell-Scherer 2017). This conflation of measurement with a formal disease label undermines the conceptualization of disease as a distinct state of physiological impairment typically used in medical practice (Charrow and Yerramilli 2018)

Diagnoses and disease labels are descriptive but disruptive social categories, shaping how individuals view themselves and others, and having consequences that extend far beyond the purview of biomedicine (Jutel 2019). Though the disease label reflects an earnest effort to counteract narratives of personal culpability (Jastreboff et al. 2019), this decision cannot be disentangled from the consequences of labeling ‘abnormal’ bodies as diseased and unhealthy in a society where body size is seen as the product of harmful individual choices (Brownell et al. 2010). Just as the disease label legitimizes obesity in healthcare, it legitimizes stigma towards individuals whose bodies do not conform to the social norms about who is healthy and ‘good.’ Thus, there is a certain injustice in labeling a substantial proportion of the population as “diseased” on the basis of their BMI, when there is not only a lack of knowledge as to whether a person is truly in poor health, but also a lack of safe and sustainable means to make them “well” and “normal” (Greenhalgh 2015).

A Marker of Risk, Not Current Health

The behavioral/lifestyle and disease frameworks are subject to a similar epistemic fallacy surrounding BMI as a measure of individuals’ current health, broadly defined. Epistemologically sound measures should reflect the underlying ontological domains of interest; yet, BMI is rooted in an ontology of probability and risk (Nuttal 2015), or what a person’s body size suggests about future, rather than current, health. As this review contends, recognizing BMI as an imperfect biomarker speaks to this ontology in allowing for uncertainty in how and why body size is associated with specific health outcomes.

Biomarker for Abnormality

Though a universal definition of biomarkers is lacking, the National Institutes of Health’s Biomarkers Definitions Working Group (BDWG) identifies biomarkers as “objectively measured and evaluated” indicators of “normal biological processes [and] pathogenic processes,” encompassing numerous measures in recent years (BDWG 2001). Critically, the fact that biomarkers often stem from behaviors does not make them substitute measures. Blood pressure, lipids, and glucose are associated with similar behaviors as BMI, but they are not proxies for health lifestyles; indeed, these biomarkers are granted more latitude in what they suggest about a person’s choices – or lack thereof – to the extent elevated blood pressure or inflammatory markers reflect elevated stress from external stimuli (Harris and Schorpp 2018).

From a disease perspective, biomarkers represent pathways rather than definitive, diagnosable clinical endpoints (BDWG 2001). They are integral to disease diagnosis (Timmermans and Haas 2008), but a transition to an ‘abnormal’ value for a biomarker does not demarcate a latent transition from disease-free to diseased. Admittedly, diabetes and hypertension are near-exclusively defined on the basis of blood glucose and blood pressure, respectively, exceeding a predetermined threshold. However, BMI is distinctive given the pragmatic, rather than epistemic, choice to define obesity as a disease based on BMI (Bray et al. 2017). No biomarker is a perfect measure of the underlying biological or pathogenic process it represents (Loucks et al. 2008); indeed, very few biomarkers meet the criteria for being “surrogate endpoints,” in fully mediating the relationship between an exposure and a clinical endpoint (BDWG 2001; Loucks et al. 2008). But it is important to recognize that biomarkers exist on a spectrum of surrogacy, which should be acknowledged in their use. There is a tight conceptual linkage between the pathogenesis of diabetes and hypertension and their underlying biomarkers; by contrast, obesity is described as unhealthy or excess adiposity contributing to cardiometabolic abnormality, neither of which is perfectly measured or mediated by BMI. Per this logic, the proximity of a biomarker to its clinical endpoints (i.e., its degree of surrogacy), should dictate its use as a marker of risk as compared to a marker of current health.

Yet, in practice, BMI has effectively become a surrogate marker of present physiological abnormality or ‘unhealthiness’, as a nonspecific clinical endpoint (Jutel 2014). The notion of risk – and uncertainty – is replaced with a more definitive assessment of individuals’ proto-illness/disease status based on measured risk (Aronowitz 2009; Gillespie 2012; Gillespie 2015; Jauho 2019). One’s health is assumed to be impaired when exceeding a “normal” BMI range, as abnormality constitutes evidence of poor health, or the point at which BMI represents “a social condition of compromised health status” (Gillespie 2012: 195). There is strategic value to using BMI as a surrogate marker. Contemporary health is shaped by standards for how much significance is afforded to measures in terms of their ‘biomedical’ worth as evidence (Timmermans and Berg 2003). Yet, evidence is judged on its ability to provide “transparent, objective, and irrefutable information about the body” (Jutel 2014: 124); consequently, priority is accorded to measures that are quantifiable and standardized, and largely free of any uncertainty or ambiguity in what they imply about individuals’ health.

In the case of BMI, researchers and practitioners strive to impose certainty – and assume surrogacy – to compensate for the fact that BMI does not provide “transparent, objective, and irrefutable information” to the extent that these attributes are desired in biomarkers. In theory, biomarkers convey risk about the development of a given condition or disease. They are relationally defined, as a function of future morbidity, to the extent that there is confidence in their ability to provide evidence of a biophysiological mechanism. The central issue in using BMI is an inability to clearly identify poor health beyond tautologically describing BMI as a sign of impairment (Sharma and Campbell-Scherer 2017). Yet this has not curbed the reification of the association between body size and health in using BMI categories to sort individuals by their degree of healthiness, regardless of how they fare otherwise (Jutel 2011).

The conflation of future risk and present health with respect to BMI stands in contrast to the numerous other ‘risk’ factors strongly associated with varying degrees of elevated risk for morbidity and mortality – such as being male, over the age of 40, riding a motorcycle without a helmet, and not getting enough sleep (Allison et al. 2008). None of these risk factors are used as direct measures of health; nor are they used as surrogate markers to represent a pathology of poor health, even though many of these factors are more strongly associated with increased morbidity and mortality than BMI. For instance, smoking is so strongly associated with lung cancer that one could use this same crude logic to argue that smoking is a surrogate measure for the disease (Aronson 2005). Yet smoking remains a risk factor while BMI is used as a surrogate for obesity as a diseased state (Jutel 2006). Certainly, the preference is that individuals are free of these external risks, especially from modifiable factors, but such preferences for living in a ‘risk-free’ society should not serve as the basis of perceptions of, beliefs about, and interactions with individuals as being unhealthy and deviant, to the extent this is the true of BMI.

Just a Number

While BMI is a far from a perfect biomarker, from the perspective of both measurement and theory, one cannot ignore the wealth of evidence documenting strong associations between BMI and multiple dimensions of health. Yet, in explaining these associations, one also cannot ignore the discrepancy between the literal definition of BMI, based on height and weight, and the extent to which BMI is used as a surrogate for myriad health processes. These associations have become interchangeable with the values and categories of BMI that are used by researchers, practitioners, and individuals, ignoring that BMI is ‘just a number’ ascribed with meaning in different contexts that suit various stakeholders’ needs.

Namely, this ‘just a number’ framework is intended to downplay the power and specificity of meaning afforded to measures like BMI, in recognizing that conceptualizations and definitions of health are often a reflection of changing norms about what constitutes objective evidence, rather than changes in the objective reality of health (Timmermans and Berg 2003). The unchanging, objective reality of BMI is that – above all else – it is always the same number, calculated from the same equation, while the meanings it is imbued with are constantly in flux as a function of time, place, and the needs of individuals and institutions using it in various contexts. Its meaning and usage may be perfectly appropriate in one scenario, but not in another, despite the number remaining unchanged. Moreover, knowledge of how BMI is used in one context can be informative for the practice of its use in another, but these opportunities to broaden its interpretation are only possible when BMI is viewed as a number rather than as a surrogate for a specific domain of health.

The quantifiability of BMI has proved especially valuable given modern standards of objective and standardizable evidence; yet, numbers are rarely used in a ‘neutral’ manner. There is a societal propensity to “ordinalize” and “categorize” knowledge into heuristic rankings and groupings (Bowker and Star 1999; Fourcade 2016), using values like BMI to compare individuals and establish hierarchies of on the basis of weight and assumptions about health. Thus, in an effort to subvert the societal and scientific imperative to label and sort BMI, a biomarker-informed perspective on BMI as ‘just a number’ is arguably the most appropriate way of framing body size, recognizing that biomarkers can convey risk without being declarative about health.

In and of itself, BMI is neutral on the subject of an individual’s current health or the degree to which it conforms to ‘normal’ physiological functioning; imbuing the measure with additional assumptions creates linkages to abstract ideas about unhealthiness and abnormality. While there is guidance and helpful theory on how to use biomarkers (Harris and Schorpp 2018; McDade 2008; Timmermans and Haas 2008), there is less consideration of why some biomarkers become imbued with additional meanings outside their function as biophysiological mechanisms connecting exposures to outcomes. This mediational framework is contingent on biomarkers being reasonable measures of the physiological processes taking place, maximizing how much of the effect of the exposure on the outcome is accounted by the biomarker (Loucks et al. 2008). BMI, as ‘just a number,’ does not necessarily satisfy this framework, and additional explanations are sought out to prevent it from collapsing.

Consequently, researchers often only care about BMI inasmuch as it purports to convey information about other aspects of health, like the types of behaviors a person engages in or their level of physiological impairment, to the extent that having a scale or measurement like BMI facilitates comparisons and rankings (Bowker and Star 1999; Fourcade 2016; Jutel 2006). These behavioral and disease frameworks are inherently attractive as they convey definitive information about what a person is doing or how they are, at present, rather than describing health in probabilistic terms (e.g., an individual has a BMI in a range associated with a 20–30% higher risk of mortality, on average, compared to individuals with a BMI in a “normal” or “healthy” range). Avoiding, rather than acknowledging, uncertainty in describing body size pushes us to make more definitive pronouncements about individuals’ healthiness based on their BMI. But not all biomarkers allow us to make such clear assessments of underlying health as compared to underlying risk (Loucks et al. 2008). Health is a multifaceted, systems-level construct (Harris 2010; McDade 2008); individual biomarkers help identify its discrete components, but they do not serve as the basis for comprehensive judgements. Moreover, this perspective neglects the possibility that these measures associated with health through diverse mechanisms, some of which are primarily psychosocial rather than biophysiological. Thus, the challenge in working with BMI is preserving its neutrality by resisting the temptation to situate it in a specific domain of health.

Social Marker of Inequality

Mapping BMI onto tangible domains of health – whether behavior, disease, or as a biomarker of physiologic abnormality – represents the dominant ontological approach. However, there is increasing recognition of an alternative perspective that bridges clinical and epidemiologic research on the limitations of BMI as a health surrogate with a sociological and psychological understanding of body size as an axis of inequality. This ‘weight neutral’ framework does not downplay the importance of studying body size and health; rather, it downplays the need to directly and unambiguously equate body size with health in research, practice medicine, and the conceptualization of overweight and obesity. Body size is acknowledged as a neutral form of human variation (Saguy 2012), whereby BMI reflects both biophysiological and psychosocial mechanisms of risk. To the extent that BMI is surrogate marker of physical appearance, it is simultaneously marker of social abnormality and inequality, representing a distinct process by which individuals’ social interactions and experiences affect their health. Consequently, there is considerable heterogeneity in how and why researchers explain the relationship between BMI and health. In turn, treating BMI as ‘just a number’ provides a broader set of explanations and points of intervention.

Health at Every Size

More than just an abstract concept, weight neutrality is seen as a practical and sustainable alternative to the BMI-centric approaches used in assessing and intervening on individual and population health. The Health at Every Size (HAES) movement promotes weight neutrality in pushing researchers and practitioners to acknowledge the limitations of BMI as a defining characteristic of health, and recognize diversity in weight and health, in considering alternative explanations for why BMI confers higher risk. Promoting good health is a priority, rather than explicitly healthy weight, given that an overly-narrow focus on attaining the latter is often harmful, in and of itself, with respect to poor mental health, disordered eating and exercise, and how these psychosocial aspects of health affect physiological functioning (Bombak and Monaghan 2017; Bombak et al. 2019; Tylka et al. 2014).

Weight-targeted interventions are often ineffective, emphasizing dietary and exercise regimes for which success is measured solely by weight loss (Mann et al. 2007). Many adults can successfully and sustainably improve many other cardiometabolic indicators that allow for better overall health and longevity (Bacon and Aphramor 2011; Mann et al. 2007; Tylka et al. 2014). Indeed, discordance between individuals’ having an “unhealthy” BMI despite normal measures of cardiometabolic functioning (Greenhalgh 2015; Saguy 2012), and a general sense of robustness (Monaghan 2007), creates additional stress and anxiety surrounding what it means to be ‘healthy.’ In a society where body size is a source of stigma (Puhl and Heuer 2010), a focus on BMI can be limiting and distracting during a medical encounter (Phelan et al. 2015), leading to skewed assessments of individuals’ health which are magnified when they serve as the basis for population-level guidance and policies on what constitutes good health and a healthy body.

Essentializing BMI

Weight neutrality is premised on the fact that conflating BMI, risk, and health is damaging and unjust, in reinforcing stereotypes about individuals who do not adhere to social norms for physical appearance and health, and how the two are equated on the basis of social norms for beauty and fitness (Jutel and Buetow 2007). The notion that phenotypic attributes become imbued with social meaning – and thus become health-relevant traits – is not a novel concept (Link and Phelan 2001). Directly equating body size with race is too strong a comparison, to the extent that race is tied to endemic legacies and systems of oppression (Phelan and Link 2015), but one should not ignore how BMI and race exemplify how one’s phenotype affects health though non-biophysiological pathways.

The issues surrounding race as an essentialized concept provide a clear illustration of how phenotypic traits are conflated with their social consequences (Frank 2007; Gutin 2019; Morning 2011), wherein race, itself, is assumed to be the innate, causal mechanism underlying poor health. Yet, decades of research prove that the relationship between race and health is attributable to race being a proxy for the many social ills inflicted upon non-White persons via interpersonal and institutional forms of discrimination and disenfranchisement (Phelan and Link 2015). Unfortunately, this message fails to resonate in a society where health is actively used to gauge individuals’ social standing (Scambler 2009); the moral judgment attached to healthiness substantiates the belief that those who are unhealthy are ‘bad’ members of society. This gives rise to a vicious cycle by which the poor health of a marginalized group is used to justify their marginalization, likely leading to worse health in the future.

A comparable process of essentializing BMI has been at work for decades, legitimizing BMI as a surrogate marker of biophysiological health, while ignoring the psychosocial implications of its being a marker of appearance and status. Once again, tautological reasoning is partially to blame; a person becomes unhealthy upon attaining an unhealthy BMI, implying some kind of transition in their latent health. Body size has been problematized and stigmatized as an abnormal form of human variation just as other forms of human variation have been considered ‘undesirable.’ In a highly weight-conscious society where morality is linked to one’s appearance (Jutel and Buetow 2007; Shugart 2016), body size represents another form of phenotypic stratification that influences health. Stigma is a fundamental mechanism underlying health disparities (Hatzenbuehler et al. 2013), with body weight being one of the earliest forms of stigma examined by sociologists (Cahnman 1968; Maddox et al. 1968). Yet, nearly 50 years on, it continues to be a “socially acceptable form of bias” (Puhl and Heuer 2010: 1019), and the association between having an abnormal body and poor health as a given (Greenhalgh 2015).

There is strong evidence to suggest that stigma underlies mechanisms linking obesity to numerous health conditions, independent of the physiological consequences of BMI. Institutional biases in the workplace, educational settings, healthcare, interpersonal relationships, and media lead to worse treatment and fewer rewards for individuals with overweight and obesity (Puhl and Heuer 2009), directly impacting their socioeconomic prospects, quality of life, and health. Moreover, the omnipresent stigmatization of body size is manifest as discrimination, ostracism, harassment towards those with higher BMIs, and the internalization of negative self-imagery among individuals whose bodies do not conform to social and medical ‘norms’ (Puhl and Heuer 2010; Puhl et al. 2008). Weight-based stigma is associated with numerous mental and physical health outcomes such as depression, anxiety, psychiatric disorders, impaired cardiovascular health, and many others (Hatzenbuehler et al. 2009; Papadopoulos and Brennan 2015; Puhl and Heuer 2010; Puhl and Suh 2015; Schafer and Ferraro 2011). The cumulative impact of these chronic insults is implicated in the association between weight-related stigma and premature mortality (Sutin et al. 2015). In their totality, there is compelling evidence that psychosocial mechanisms constitute some of the primary pathways through which individuals’ body size negatively impacts their health (Tomiyama et al. 2018). Researchers often lack direct measures of weight-based stigma, but this does not excuse ignoring such explanations when interpreting BMI as a ‘surrogate’ measure.

More broadly, BMI is a marker for health in the same way phenotypic attributes like race and gender are determinants of health; they gauge future risk rather than serve as measures of current health. Certainly, BMI is distinctive – and challenging – as it is not exclusively a marker of appearance and has real biophysiological consequences. Studying the relationship between body size and health is important, but there is a need to better acknowledge uncertainty in what BMI serves a marker of. The conceptualization of race in health research continues to serve as a useful parallel; there are legitimate concerns about how race being used to perpetuate biogenetic explanations (Bliss 2012; Shim 2002; Smart and Weiner), but recognition that race is a socially-meaningful category is vital for advancing justice and equity (Borrell et al. 2021; Epstein 2008). Thus, rather than limit discussion to biophysiological explanations for why BMI is associated with adverse health, the inequality framework allows for a broader set of psychosocial pathways and interventions.

BMI in the Time of COVID-19

The need for conceptual clarity in what BMI measures and how it becomes associated with health is not an abstract concern, given that many clinicians, epidemiologists, and public health officials rely on the measure to make decisions about individual and population health. It is beyond the scope of this review to consider the myriad diagnoses and interventions where more careful use of BMI may alter experts’ interpretations and resulting course of action. However, recent discourse on COVID-19 in the United States serves as an illustrative and timely example of how BMI has effectively become a surrogate for severity of illness. The fact that BMI reflects adiposity and biophysiological abnormality that puts one at elevated risk for severe infection and mortality is undeniable (Popkin et al. 2020); yet, non-biophysiological factors cannot be ignored (Hill et al. 2021; Townsend et al. 2020). As with numerous other conditions, in treating BMI as a marker of health, rather than risk, the scope of plausible explanations is unnecessarily limited.

Conflating risk and health in discourse on BMI creates confusion about vulnerability amid existing uncertainty about what it means to be a safe and responsible member of society, especially when this messaging plays a key role in shaping public knowledge of healthiness, morality, and even social status (Monaghan 2021). The risk factors identified by the Centers for Disease Control (CDC) in the United States represent an ontological mélange of health behaviors, biomarkers, and conditions or diseases, such as smoking, high blood pressure, and diabetes (CDC 2021). There is a ‘hierarchy’ of risk, in using these factors to separate individuals who “might be” at risk from those who already “are.” Yet, the only risk factor that appears throughout this hierarchy is BMI – in distinguishing between overweight, obese, and morbidly obese – whereby the graded relationship between body size and COVID severity mirrors the narrative of BMI as a surrogate for latent biophysiological health vis-à-vis cardiometabolic and immune functioning (Kwok et al. 2020). Decisions about how to categorize risk are difficult, reflecting empirical evidence linking BMI to COVID-19 outcomes. However, it is important to recall that this evidence is associational, and far from neutral in reinforcing a clinical, biophysiological conceptualization of increased risk as indicative of worsening health (Gillespie 2012; Jutel 2011). In other words, emerging narratives actively contributed to individuals’ current obesity as effectively being a surrogate marker of their future COVID-19 risk, and severity therein, adding to a long list of health conditions for which BMI is used as a proxy.

How risk is conceptualized and conveyed reflects different narratives about vulnerability and underlying mechanisms. Deploying the neutral perspective on BMI as ‘just a number’ can be instructive in communicating that the association between BMI and COVID-19 is a multidimensional process of increased physiological risk compounded by the stigma of medicalized identities, as well as individuals’ social environments and contexts (Puhl et al. 2020). There is concern about BMI being a marker of social status amid the pandemic, reflected health professionals’ assumptions about individuals with obesity and emerging media narratives about who is to blame. These messages reinforce social beliefs about individual responsibility for worse health among people with higher BMIs, which may exacerbate COVID risk by increasing their propensity for unhealthy behaviors or avoiding needed medical care (Hill et al. 2021; Flint 2020; Le Brocq et al. 2020; Puhl et al. 2020; Townsend et al. 2020; Wu 2020).

Race and ethnicity are also ‘risk factors’ given the disproportionate toll of COVID-19 on minority individuals and communities (CDC 2020). However, it is apparent that greater vulnerability and exposure owing to a multiplicity of sociostructural mechanisms – such as systemic disinvestment, material deprivation, and institutional racism (Phelan and Link 2015) – shapes worse COVID-19 outcomes for non-White persons, rather than race representing a direct causal link between race and severe illness (Hooper et al. 2020; Krieger 2020; Yancy 2020). Many of these mechanisms of discrimination, bias, and ignorance are well-known explanations linking body size to health, and serve as plausible mediators for the association between BMI and COVID-19 severity (Hill et al. 2021; Townsend et al. 2020), independent of biophysiological pathways. Yet these sociostructural explanations are ignored when the neutrality of BMI is confounded by pre-existing assumptions about body size, biocitizenship and healthiness.

Ultimately, the issue of delineating biophysiological from sociostructural risk had direct bearing on determining who was prioritized for COVID-19 vaccinations. Having a BMI greater than 30 made one ‘eligible,’ entirely from a biophysiological risk perspective, in representing a comorbidity (CDC 2021). As noted, these decisions are difficult, and all efforts should be made to prioritize care for those at the highest risk of severe illness, as seen in data on BMI. It is also understandable that definitions of biophysiological risk and unhealthiness err on the side being liberal, with the goal of unintentionally excluding risk factors because the exact mechanisms of action are unclear. Despite the strong, ethical case that a sociostructural definition of risk would allow for a more inclusive definition of eligibility – especially in the case of high-risk minority populations (Schmidt et al. 2020) – the current emphasis on biophysiological risk reflects extant norms about using BMI as a flawed, but acceptable, surrogate of health.

Given limited initial supply, and far greater demand, the debate over who was sufficiently ‘at risk’ to be vaccinated intersects with social norms about personal responsibility and health. BMI categorizes over 40% of the U.S. adult population as obese and ‘unhealthy’ (Hales et al. 2020); it is the most prevalent risk factor and source of eligibility. Certainly, many adults for whom body weight is a health issue stand to benefit, just as they have from the legitimization of obesity as a medical issue in other contexts. Yet, given the inadequacy of BMI as a measure of individual health, it is likely that not all vaccines will go to individuals who are most in need. The implications this may have in perpetuating weight bias and stigmatization are unclear, especially if society’s propensity to assign personal responsibility for higher BMIs is conflated with the misperception that irresponsibility and poor health is being ‘rewarded’ with vaccination. Health is a well-established source of stigma in society (Scambler 2009), and the imprecise use of BMI as a measure underlying health has potential to do harm. Thus, just as there is concern with the long-term consequences of COVID-19 as a disease, it is important to consider the ramifications of how conflating obesity and COVID-19 as intersecting pandemics perpetuates a narrative of individual culpability and social burden that continues after COVID-19 is no longer a threat.

Conclusion

Sociologists, public health scientists, and numerous other researchers engaged in the study of population health are in a difficult position when it comes to understanding body size and health. The work is important and necessary; even if the conceptualization and measurement of an “unhealthy” body size has limitations, one cannot ignore the totality of evidence identifying increased body size as a key factor underlying morbidity and mortality (Stokes and Preston 2016). Yet, studying this relationship taps into an enormity of biophysiological, psychological, social, and cultural mechanisms that are beyond the reach of extant approaches to defining a “healthy” body. BMI is, and will continue to be, the dominant metric, short of unforeseen advances that allow researchers and practitioners to easily measure the many indicators associated with poor cardiometabolic health. Given the unlikelihood of this occurring, the key motivation for this review asks how research can better use BMI to improve knowledge of the relationship between body size and health.

Drawing on contemporary critiques of BMI, obesity, and the dominant ontological frameworks used to describe health, this paper highlights how pre-existing assumptions and beliefs bias interpretation of BMI. The health behavior/lifestyle and disease frameworks are consequential, in assigning specific meaning to BMI as a measure of what one does with their body to maintain their health, or how their body is with respect to latent health (Greenhalgh 2015). Situated between the two, biomarkers are perhaps less definitive in suggesting a mediated process by which individuals may be at future risk for poor health without necessarily being unhealthy at present (Loucks et al. 2008). Yet, in practice, treating biomarkers as “surrogate markers” undermines this emphasis on risk, to the extent that biomarkers are substituted for the future concerns they may be associated with (Jutel 2014). Consequently, the risk attached to BMI is a surrogate for current biophysiological abnormality, despite the epistemological inadequacy of equating BMI with “obesity” as a state of health – or whichever clinical endpoint BMI is alleged to represent.

This does not mean that a biomarker approach to conceptualizing BMI is flawed; rather, it is limited. All health measures are ‘just numbers’ until they are imbued with clinical meaning. BMI is laden with many assumptions and beliefs, many extending beyond the realm of biomedicine. It is a valid measure of body size that captures individual’s physical characteristics, and how these qualities come to be associated with health through numerous mechanisms – both biophysiological and psychosocial. This holistic interpretation of BMI not only advances weight neutrality in research (Gutin 2018), but also facilitates the discussion of body size as a source of stigma and axis of inequality – and BMI as a measure of both – in weight-conscious societies (Puhl and Heuer 2010; Saguy 2012).

The ubiquity of BMI is attributable to its simplicity as a measure of health; yet, this understates its complexity as a surrogate for numerous biological and social processes. This complexity critical for sustaining a more holistic understanding of the relationship between body size and health. In turn, a balanced framing of BMI in research – accepting of uncertainty and risk, and less declarative about normality and health – can inform broader social norms about diversity in bodies and wellbeing, and how individuals with ‘deviant’ bodies are perceived and treated. The present confusion surrounding BMI as surrogate marker of COVID-19 severity highlights the consequences of how inadequacies in conceptualization create difficulty in using health measures to make difficult policy decisions. Conflating riskiness and unhealthiness – and how this equivalence translates into ideas about who deserves help, or who is responsible for creating a social and health burden – is not an abstract concern. The conceptualization and understanding of BMI has tangible implications for prioritizing care, evaluating the appropriate level of concern, and passing judgment on who is considered to be a contributing member of society.