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Letters to the Editor |
R1 Stanford University, School of Medicine, Cardiovascular Medicine. Stanford, CA 94305, Fax 650-725-1599
E-mail greaven{at}cvmed.stanford.edu
To the Editor:
Given the level of heresy in my recent critique (1) of the notion of the metabolic syndrome as defined by the Adult Treatment Panel III (ATP III), I am surprised that the only concern expressed by Dr. Lear related to my skepticism concerning the clinical utility of using measurements of waist circumference (WC) to "diagnose" the metabolic syndrome. In his zeal to champion the virtues of measuring WC, Dr. Lear seems to have overlooked the overall thrust of my comments concerning the relationship between obesity, insulin resistance, and the ATP III criteria for identifying the metabolic syndrome. The major point I tried to make was that obesity (no matter how it is measured) is not a consequence of insulin resistance, but simply makes it more likely that a person will be insulin resistant and that a substantial number of obese individuals are neither insulin resistant nor at increased risk of cardiovascular disease (2)(3)(4)(5). Indeed, it is likely that differences in degree of fitness are as powerful as variations in adiposity in modulation of insulin action (2).
Turning now to the specific issues raised by Dr. Lear, I agree that measurement of body mass index (BMI) is not a perfect way to quantify adiposity. However, in a recent analysis (6) of data from 
20 000 participants in the National Health and Nutrition Survey, measurements of BMI and WC were shown to be highly correlated (r >0.9) irrespective of age, gender, and ethnicity. Furthermore, although the ATP III proposed specific numbers to identify an individual as being abdominally obese, the panel did not offer any instruction on how to quantify WC. In this context, the authors of a recent report (7) pointed out that at least 14 different anatomical sites have been used to measure WC in studies aimed at relating differences in abdominal obesity to adverse outcomes, and even the 4 most commonly used sites yield quite different absolute values for WC. Furthermore, as pointed out by Dr. Lear, the relationship between excess adiposity and insulin resistance (and its consequences) will vary as a function of ethnicity, generating the need, as stated by Dr. Lear, to establish "appropriate ethnic-specific targets" to quantify adiposity. In the absence of agreement on how to quantify WC, questions concerning the likelihood that different healthcare facilities will perform the measurement with the identical method and/or degree of seriousness, and the need to have different cut points for every ethnic group, it seemed reasonable to question the notion of some specific value of WC with which to decide whether an individual was at increased risk of an adverse outcome.
Viewing all of this information in the context of the ATP III criteria for diagnosing the metabolic syndrome, I suggested that it might be simpler to measure height and weight, and calculate BMI, with the understanding that the higher the BMI, the more likely an individual is to be insulin resistant (1). Implicit in this statement is the belief that there is no magic value of WC that possesses unique clinical utility, a totally different point of view than the proposal by the ATP III of specific criteria for WC that enable a clinician to decide whether an individual is, or is not, at increased risk of cardiovascular disease. If the focus is on excess adiposity as increasing the risk of insulin resistance and its consequences, rather than on a specific value of WC as having some unique clinical significance, it is not clear to me whether it makes a great deal of difference if you measure BMI or WC. Although Dr. Lear feels that there is greater educational virtue in overweight/obese persons knowing their WC compared with their BMI, I am not sure that either would mean as much as just knowing their weight and how much they should lose. More importantly, Dr. Lear also suggests that "WC is superior to BMI as a risk marker". This issue is an important one that cannot be discussed in detail in this context, but it may not be as simple as implied. For example, in Pima Indians, BMI was the estimate of adiposity with the highest hazard ratio in the prediction of type 2 diabetes (8). Furthermore, adding WC to that study’s model did not improve its predictive ability. In a prospective study of Mexican Americans, Haffner et al. (9) reported that individuals with the highest baseline plasma glucose and insulin values were most likely to develop type 2 diabetes independently of differences in age, BMI, or central obesity. It has also been shown in studies of several different ethnic groups that BMI is more strongly associated with blood pressure than is abdominal obesity (10)(11). A similar conclusion was reached concerning the presence of carotid atherosclerosis in Japanese men (12), and it was concluded that insulin concentration, abdominal girth, and BMI all contributed to the clustering of dyslipidemia, hyperuricemia, diabetes, and hypertension described in both whites and African Americans. Thus, although Dr. Lear may be correct in stating that WC is the more powerful predictor of clinical outcomes linked to insulin resistance, there is also considerable evidence that overall obesity, as estimated by BMI, not only significantly predicts insulin resistance, but also increases the likelihood that an individual will develop the clinical syndromes associated with the defect in insulin action (2)(3)(4)(5)(8)(9)(10)(11)(12)(13). Taken in conjunction with the fact that measurements of BMI and WC are almost perfectly correlated (6), I think it is possible to question the unique clinical advantages of measuring WC.
Finally, Dr Lear points out that a tape measure costs no more than $5.00 and can be "readily included in the clinical tools of health professionals worldwide". In light of this information, it could be argued that once there is worldwide agreement on how to measure WC and agreement on ethnic-specific values for what represents abdominal obesity, it will be possible, as suggested by Dr. Lear, that " appropriate ethnic-specific targets, whether BMI or WC, will have a major impact on prevention of diabetes and cardiovascular disease on a population level." I have no reason to dispute the cost of a tape measure, but I do believe there are more important issues to be addressed than the need to gain worldwide agreement on ethnic-specific targets for abdominal obesity, based on a standardized method of quantifying WC.
References
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