Precision and Accuracy of Defining Obesity

The fifth item on the  disease definition modification checklist developed by the Guidelines International Network (G-I-N) Preventing Overdiagnosis Working Group published in JAMA Internal Medicine,  deals with issue of precision, accuracy, and reproducibility. Obviously, any definition of obesity that requires clinical assessment and clinical judgement will not have the precision, accuracy, or reproducibility of simply measuring height and weight. Thus, if we define obesity as the presence of abnormal or excess body fat that impairs health, we will necessarily have to deal with the issue of assessing health, which is not something that you can simply measure by stepping on a scale. Rather, because abnormal or excess fat can affect virtually every organ system as well as psycho-social well-being, we are going to be faced with a rather complex system of diagnosing who has obesity and who hasn’t. In fact, as the authors of the checklist point out, “…an appropriate gold standard will rarely be available and therefore, traditional measures of diagnostic test accuracy, such as sensitivity and specificity, will generally not be appropriate.” Both repeatability (agreement in identical conditions) as well as reproducibility (agreement across comparable conditions) may result from biological variability, analytical variability, and clinical judgement. The only way to test the reproducibility and precision will be to evaluate the use of the new definition in clinical practice and ultimately determining whether or not clinicians can reasonably agree on who has the condition and who doesn’t. While this may seem daunting to non-clinicians, let us remember that in clinical practice many diagnoses are dependent on clinical evaluations and clinical judgement, whereby experienced clinicians or specialists may perform better than the novice or the non-specialist (a good example is psychiatry, but there are countless other examples). Moreover, there will always be grey areas in “borderline” cases, where examiners may disagree on the exact result and only time will tell, who is right. Welcome to the messy world of clinical practice. Just because BMI is simpler, more precisely measured, and more reproducible, does not make it a better measure of diagnosing whether or not someone actually has a disease. After all it only makes sense that it will take a complex definition to diagnose a complex disease. @DrSharma Edmonton, AB

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Obesity Prognosis

Continuing in my discussion of the disease definition modification checklist developed by the Guidelines International Network (G-I-N) Preventing Overdiagnosis Working Group published in JAMA Internal Medicine,  I turn to the fourth item, which deals with issue of prognostic ability of changing the obesity definition? Indeed, as pointed out by the authors of the checklist, “The most important feature of a disease definition is its ability to accurately predict clinically meaningful outcomes.” This, of course is, where BMI fails hands down. As we have demonstrated using several large data sets, the ability of BMI to predict mortality is rather limited with almost no relationship between outcomes and BMIs, at least over a rather wide range of BMI levels. Contrast this to the power of predicting outcomes when using a definition that actually looks at the presence of health impairments, such as the Edmonton Obesity Staging System.  When applying this system, which takes into account the impact of excess weight on mental, physical, and functional health, the prognostic power ranges from virtually no risk (Stage 0) to halving your chances of being alive in 20 years (Stage 3). Thus, a definition of obesity that actually considers impairments in health have a far greater prognostic power than simply knowing someone’s BMI. If nothing else, this alone should be a reason to abandon BMI for a more meaningful definition of obesity, that actually considers health and not just size. @DrSharma Edmonton, AB

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Obesity Definition Then And Now

The first item on the disease definition modification checklist developed by the Guidelines International Network (G-I-N) Preventing Overdiagnosis Working Group published in JAMA Internal Medicine,  pertains to the issue of how a proposed new definition would differ from the existing definition. As authors are well aware, the current definition that is widely used to define obesity is based on BMI, a simple anthropometric measure calculated from body height and weight – a great measure of size, not such a great measure of health. In contrast, the proposed definition of obesity, where obesity is defined as the presence of abnormal or excess fat that impairs health, would require the actual assessment and demonstration of the presence of health impairments attributable to a given subject’s body fat. Thus, while anyone can currently “diagnose” obesity simply by entering height and weight into a BMI calculator and looking up the value on a BMI chart, the new definition would in fact require a full clinical assessment of an individual’s health. Such an assessment would need to look at both mental and physical health as well as overall well-being for issues that may be directly caused (or aggravated by) the presence of abnormal of excess body fat. This does in fact bring up the issue of how exactly you would define “abnormal” or “excess” body fat and, even more importantly, how you would establish a relationship between body fat and any health impairments in a given individual. While these issues would clearly need to be worked out, the face value of this approach should be evident in that it focusses on the issue of actual health impairments rather than an arbitrary BMI cut-off, above which everyone would be considered as having obesity. This of course raises a number of issues around definition precision and accuracy, which is another item on the checklist and will be discussed in a future post. @DrSharma Edmonton, AB    

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A Checklist For Modifying Disease Defnitions

At the recent European Congress on Obesity in Porto, Portugal, there was much support for calling obesity a chronic progressive relapsing disease. However, there was also much agreement that the current criteria for diagnosing this disease, based on BMI criteria alone, has important limitations in that it may over-diagnose a significant number of individuals at no or very little imminent risk from their body fat and (even more importantly) under-diagnose a substantial number of individuals, who may well stand to benefit from anti-obesity treatments. Thus, as my readers are well aware, I have long called for a redefinition of obesity based on the actual presence of health impairments attributable to abnormal or excess body fat. It is thus timely that JAMA Internal Medicine has just published a seminal article by Jenny Doust and colleagues on behalf of the Guidelines International Network (G-I-N) Preventing Overdiagnosis Working Group, that provides a framework for anyone proposing changes to disease definitions. Using a 5-step process that included (1) a literature review of issues, (2) a draft outline document, (3) a Delphi process of feedback on the list of issues, (4) a 1-day face-to-face meeting, and (5) further refinement, the group developed an 8-item checklist of items to consider when changing disease definitions. The checklist specifically deals with the issues of definition changes, number of people affected, trigger, prognostic ability, disease definition precision and accuracy, potential benefits, potential harms, and the balance between potential harms and benefits. The authors propose that, “…the checklist be piloted and validated by groups developing new guidelines. We anticipate that the use of the checklist will be a first step to guidance and better documentation of definition changes prior to introducing modified disease definitions.” No doubt it would be prudent to consider all of the identified aspects in the checklist, when considering changing the definition of obesity from one based simply on BMI to a more clinical definition, based on actual impairments in health. In coming posts, I will consider each of the proposed checklist items and how they may apply to such a change in the definition of obesity. @DrSharma Edmonton, AB Hat tip to Dr. Marcela Flores for drawing my attention to this paper

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World Obesity Federation Recognises Obesity As a Chronic Relapsing Progressive Disease

Following in the footsteps of other organisations like the American and Canadian Medical Associations, the Obesity Society, the Obesity Medical Association, and the Canadian Obesity Network, this month, the World Obesity Federation put out an official position statement on recognising obesity as a chronic relapsing progressive disease. The position statement, published in Obesity Reviews, outlines the rationale for recognising obesity as a chronic disease and is very much in line with the thinking of the other organisations that have long supported this notion. In an accompanying commentary, Tim Lobstein, the Director of Policy at the World Obesity Federation notes, that recognising obesity as a disease can have the following important benefits for people living with this disease: 1) A medical diagnosis can act to help people to cope with their weight concerns by reducing their internalized stigma or the belief that their problems are self-inflicted and shameful. 2) A classification of obesity as a disease, or disease process, may help to change both the public and professional discourse about blame for the condition, the latter hopefully encouraging greater empathy with patients and raising the patient’s expectations of unbiased care. 3) Recognition of obesity as a disease may have benefits in countries where health service costs are funded from insurance schemes that limit payments for non-disease conditions or risk factors. While all of this is great, and I am truly delighted to see the World Obesity Federation come around to this statement, I do feel that the policy statement seems rather tightly locked into the notion that obesity (or at least most of it) is a disease “caused” primarily by eating too much, with the blame placed squarely on the “toxic obesogenic environment”. Personally, I would rather see obesity as a far more etiologically heterogenous condition, where a wide range of mental, biological and societal factors (e.g. genetics, epigenetics, stress, trauma, lack of sleep, chronic pain, medications, to name a few) can promote weight gain in a given individual. Although these factors may well operate through an overall increase in caloric consumption (or rather, a net increase in energy balance), they, and not the act of overeating per se  must be seen as the underlying “root causes” of obesity. Thus, I tend to see “overeating” (even if promoted by an obesogenic food environment) as a symptom of the underlying drivers rather than the “root cause”. Thus, saying that obesity is primarily caused by “overeating” is… Read More »

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