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Obesity paradox

Study finds obesity paradox is a product of biases

Credit: Walter Siegmund
The researchers said these results could improve disease treatment, since some clinicians may use the obesity paradox in patient care decisions.

Demographers Samuel Preston of the University of Pennsylvania and Andrew Stokes of Boston University have found that studies reporting that obese or overweight people with cardiovascular disease are outliving their normal weight counterparts – the so-called obesity paradox – are a product of biases involving reverse causation and confounding by smoking. According to their research, ‘Smoking and reverse causation create an obesity paradox in cardiovascular disease’, published in the journal Obesity, when accounting for weight history in addition to weight at the time of survey and when adding in smoking as a factor, obesity is harmful, not helpful, to someone with cardiovascular disease.

Samuel Preston

"There are claims that … it's good to be obese when you have cardiovascular disease, that if you have fat stores, maybe you'll live longer," said Preston, a sociology professor in Penn's School of Arts & Sciences. "It's conceivable that there are health advantages. But we show they are overwhelmed by the disadvantages of being obese, once you control for these two sources of bias."

Stokes and Preston started with data from more than 30,400 participants of the National Health and Nutrition Examination Survey between 1988 and 2011 - a nationally representative sample considered the gold standard in the United StUSates. Of those participants, 3,388 had cardiovascular disease.

Most research of this type looks only at weight at time of survey. For example, if a participant who long weighed 300lbs lost one-third of his mass by the time he weighed in, he would be counted at 200lbs. Incorporating the second factor, smoking, also contributed to resolving the paradox. Smokers are less likely to be obese, and those who are obese are less likely to smoke. This correlation is much stronger for those with cardiovascular disease, so the researchers limited their pool to lifelong non-smokers.

Stokes and Preston opted to include weight history as not including it would be like classifying a lifelong smoker who quit the day before the survey as a non-smoker, even though as a lifelong smoker they carry those risks over even if the person stops smoking.

This allowed the researchers to identify a control group of people who had been normal weight their whole lives, those considered low-risk for disease. The found that the paradox was present among those with overweight/obesity at the time of survey (hazard ratio (HR) = 0.89; 95% confidence interval (CI) 0.78-1.01). However, when the reference category was limited to the always-normal-weight, the paradox disappeared (HR = 1.16; 95% CI 0.95-1.41). When analysis was additionally confined to never-smokers, mortality risks were significantly higher in the overweight/obesity group (HR = 1.51; 95% CI 1.07-2.15; p=0.021).

Andrew Stokes

“It turns out to have a profound effect on the findings," said Stokes. “Eliminating the mortality advantage for those who are overweight or obese.”

The researchers said these results could improve disease treatment, since some clinicians may use the obesity paradox in patient care decisions.

"There's every reason to imagine that clinicians are at least confused," said Preston.  "And in some cases, are believing that being overweight or obese is a good thing among people with cardiovascular disease, diabetes and other conditions for which a paradox has been demonstrated.”

“This may be trickling down into clinical decision making," added Stokes. “Which is concerning because we don't think it's a real finding." 

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