Is a Heavier America Healthier? Weighing the Evidence
Pub Date: Monday, June 8, 2009
Author: Mercedes R. Carnethon, PhD, FAHA
Lewis CE, McTigue KM, Burke LE, et al. Mortality, health outcomes, and body mass index in the overweight range: a science advisory from the American Heart Association. Circulation 2009. Published online before print June 8, 2009. 10.1161/CIRCULATIONAHA.109.192574.
With one-third of US adults classified as overweight [body mass index (BMI) = 25 to 30 kg/m2] in 2003-2004 , increased attention to the health consequences of overweight is warranted. In response, Lewis and colleagues discuss the association of overweight with mortality and other health outcomes in an American Heart Association (AHA) Science Advisory published in Circulation. A unique feature of the advisory is the focus on overweight adults, a group whose mortality risks are unexpectedly lower than those of the surrounding weight categories in many, but not all, studies. If we are to answer the question of whether a heavier America is healthier, we must consider whether existing studies have addressed all alternate explanations for such an association.
Lewis and colleagues present a thorough discussion of potential confounders and biases that could lead to the observation of a U- or J-shaped relationship between BMI and mortality. One, the observation that weight loss can be a consequence of illness that precedes death, reverse causality, can lead to the misattribution that leanness causes mortality. Two, statistically adjusting for risk factors such as diabetes and hypertension that are consequences of overweight and that independently raise mortality risk, are obscuring the causal pathway leading from overweight to morbidity (e.g., diabetes and hypertension) and on to mortality. Three, at a similar BMI, adults may express considerable heterogeneity in the distribution of lean to fat mass, thus resulting in measurement error and the potential for misclassification of exposure. Previous studies have used a variety of strategies to limit the influence of these factors on their observations, but whether the association is believable comes down to the biologic plausibility of such an association.
The most convincing evidence that overweight could lead to lower mortality arises from the final point above, measurement error associated with using BMI as an estimate of body composition. Clinicians and researchers are estimating the relative distribution of lean mass to fat mass by using BMI, and waist and hip circumference measures. It is too costly and impractical to use direct measures of body composition (e.g., dual x-ray absorptiometry, bioimpedance analysis, computed tomography) in routine clinical practice and in large population studies. Consequently, most population studies describing the association between body weight and health outcomes use surrogate measures that are correlated to a varying degree with direct measures. However, there is considerable variability in the strength of the correlations that differ across population subgroups defined by demographic characteristics or health behaviors.
An example of this variability cited in the advisory  is the high prevalence of sarcopenic obesity in the elderly. Although the BMI of an elderly person may fall in the normal weight range, a relatively greater proportion of total body weight may be fat mass due to sarcopenia. Attempting to capture fat distribution using measures of waist circumference may misclassify risk in subgroups defined by race and ethnicity. Research demonstrates that African Americans have less of the metabolically active visceral fat relative to subcutaneous fat at a similar waist circumference than whites , whereas total and subcutaneous fat is higher in South Asians as compared with whites.
Health behaviors are equally important confounders of the association between body weight and mortality. Although the leaner body habitus of cigarette smokers as compared with nonsmokers is typically taken into account when investigating the association of body weight with mortality, physical activity and fitness levels are equally important confounders that, because they are less well measured, are not regularly accounted for in analyses. Persons who engage in regular aerobic or resistance physical activity may have more lean mass, which weighs more than fat mass , thus resulting in a heavier body weight, but without the attendant increased risks associated with fat mass. Adults with more fat mass have higher prevalence of metabolic syndrome risk factors including dyslipidemia, glucose disorders, and higher blood pressure than men and women with less fat mass.[3,8] Because body composition is the result of training (i.e., physical activity) and genetic influences, it may be the act of acquiring lean mass through resistance or aerobic activities that leads to more favorable cardiovascular risk factors. Physical activity is inversely associated with cardiovascular risk factors , and higher levels of cardiorespiratory fitness are inversely associated with mortality, even in the presence of overweight, (i.e., the "fit-fat" individual).
Although such an argument is biologically plausible, it probably only applies to a small proportion of the population; too few to drive morbidity and mortality down in the large segment of the population who are overweight. Population surveys describe that the proportion of persons who are physically inactive is going up in tandem with the obesity epidemic , and poor aerobic fitness (one consequence of inactivity) is prevalent and closely associated with cardiovascular and metabolic disorders in adolescents and adults. Rather, what the evidence for biologic plausibility highlights is a target for intervention. The advisory describes a progressive danger, which is that the large number of overweight children and adults are likely to cross into the obese category where health risks are clearly elevated. The trajectory can be halted and slowed if physical activity and lifestyle recommendations are adopted.[12,13]
In conclusion, the evidence that a heavier America is healthier is not convincing, particularly because we know that the majority of overweight in the country does not reflect healthy lifestyles, the only biologically plausible explanation. Even more important, we know that more often than not, overweight children and adults eventually become obese children and adults. Sending the message that a little extra weight is all right may discourage these individuals from adopting lifestyle changes that can thwart the development of chronic diseases and extend years of healthy life.
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-- The opinions expressed in this commentary are not necessarily those of the editors or of the American Heart Association --