Obesity Paradox Should Not Interfere with Public Health Efforts

DEBATE Obesity Paradox should not interfere with public health efforts

J Stevens1,2, PT Bradshaw1, KP Truesdale1 and MD Jensen3

The Obesity Paradox could result in confusing messages that derail beneﬁcial environmental changes and lead to reduced efforts by physicians to provide healthy lifestyle treatment plans to their obese patients. The Obesity Paradox applies in the main to individuals who have a disease, and therefore observed associations with mortality illustrating the Paradox may be more susceptible to certain types of bias than would be found in healthy individuals. Although individualization of weight loss advice for patients with serious disease is appropriate in medical settings, this does not supplant the need for general efforts to prevent and treat obesity.

International Journal of Obesity (2015) 39, 80–81; doi:10.1038/ijo.2014.60 Keywords: public health; adults; cardiometabolic risk; BMI

In their commentary on the Obesity Paradox, Dixon et al.1 support serious disease or in individuals with diseases for which obesity is the notion that a body mass index (BMI) in the obese range (over a cause. Much of the evidence for the apparent obesity paradox – 30 kg m 2) often provides a survival advantage over having a BMI comes from just those types of studies.5–9 Reverse causation can – in the normal range (18.5–24.9 kg m 2). There are numerous meta- be pervasive in samples with prevalent disease if subjects analyses that have shown the opposite—that mortality is increased experience disease-induced weight loss prior to the assessment 2 in adults in the obese compared with normal weight range. Results of body weight as a study exposure. This weight loss can make it –2 on the association between overweight (25.0–29.9 kg m ) and appear that those who are normal or underweight are more likely all-cause mortality are more mixed, but not really germane to die than those with greater BMI if the disease caused both to arguments about the Obesity Paradox, which is about obesity. weight loss and increased mortality rates. Although not shown in We are concerned by claims published by Dixon et al. that obesity all studies, some have shown that accounting for reverse provides mortality advantage and normal weight may have a causation can attenuate or eliminate an observed inverse causal negative effect on mortality. Cause is recognized by association between BMI and mortality.3,10 Selective survival epidemiologists as well as other types of scientists as a powerful biases the BMI–mortality association if those who have more ﬁ term, not to be bandied about without strong justi cation. disease or a more advanced level of a disease and are obese were It assumes that were one able to assign the cause randomly to less likely to survive to participate in a study than individuals with individuals in a sample, the expected incidence of the outcome normal weight. So the study sample is enriched with the type of would be different according to assignment. In a situation in obese subjects who have less susceptibility to die from the disease which an attribute causes something as monumental as a under study, as the susceptible are already deceased. mortality advantage, it would follow that efforts would be made By its nature, selective survival is more of a problem in studies by clinicians, public health workers and policy makers to increase of the elderly, whereas studies limited to subjects with prevalent that attribute, that is obesity, in the population. disease are particularly prone to a different type of selection bias These authors do not advocate obesity as a public health goal, that can impact the BMI–mortality association.4,11 This latter type nevertheless, their message may interfere with public health 11 efforts to prevent obesity. They seem to imply that if obesity can of bias is perhaps best illustrated by Lajous et al. who posit a be shown to provide a survival advantage under any circum- scenario in which there are only two causes of diabetes: one is stances, then concern about avoiding obesity is misplaced. They obesity and the other a genetic factor unrelated to body weight. provide a list of diseases and conditions for which there are in This genetic factor, independent of the diabetes effect, is deadly. their words ‘consistent reports’ to indicate an advantage of (class I) In this scenario, all persons with diabetes are either normal weight obesity over normal weight. We will not review the copious, often with the genetic factor that results in higher mortality or obese mixed and controversial literature surrounding this claim, but we (with or without the genetic factor). Restricting an analysis to do point out that decades of literature on the topic point to the diabetic patients creates an inverse association between obesity problem of complex confounding obscuring mechanistic or causal and the genetic factor that is not causal. This association associations between obesity and all-cause mortality. attenuates, or potentially reverses the direction of a positive Recognized sources of bias include reverse causality, selective causal association between BMI and mortality. In this example, the survival and bias caused by conditioning on a variable affected by factor causing both disease and death could be environmental as BMI.3,4 These biases may be magniﬁed in circumstances in which well as genetic, the distinguishing characteristic being that it is an the sample under study is restricted to individuals suffering from alternative cause of a disease that can also be caused by obesity.

1Department of Nutrition, School of Public Health, University of North Carolina, Chapel Hill, NC, USA; 2Department of Epidemiology, School of Public Health, University of North Carolina, Chapel Hill, NC, USA and 3Mayo Clinic, Endocrine Research Unit, Rochester, MN, USA. Correspondence: Dr J Stevens, Departments of Nutrition and Epidemiology, School of Public Health, CB 7461, University of North Carolina, Chapel Hill, NC 27599, USA. E-mail: [email protected] Received 11 February 2014; revised 1 April 2014; accepted 2 April 2014; accepted article preview online 15 April 2014; advance online publication, 13 May 2014 Obesity Paradox and public health efforts J Stevens et al 81 These three types of bias (reverse causation, selective survival causes. The fact that weight reduction is difficult does not bias and selection bias) cause tricky methodological problems that change that. need to be considered carefully before concluding that obesity is protective. All are very difficult to study, so it is not surprising that the pertinent literature is mixed. Furthermore, there are situations CONFLICT OF INTEREST — in which survival is likely increased by obesity being stranded for The authors declare no conflict of interest. an extended period of time without access to food is an obvious, though extreme example. More relevant, having a potentially life- threatening disease that interferes with appetite and causes REFERENCES severe weight loss is a scenario in which obesity might be 1 Dixon JB, Egger GJ, Finkelstein EA, Kral JG, Lambert GW. ‘Obesity Paradox’ protective. This protection would be manifest during the weight misunderstands the biology of optimal weight throughout the life cycle. Int J Obes loss phase of the disease, preventing or prolonging starvation. (Lond) 2014; e-pub ahead of print 15 April 2014; doi:10.1038/ijo.2014.59. 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