Relying only on data may fail to account for the effects of smoking.
The results are confounding: how can studies find that excess weight, typically associated with poor health, is associated with better survival among those with cardiovascular disease?
A study appearing in the current issue of the journal Obesity attempts to unravel this so-called “obesity paradox.” Researchers wanted to test whether the results of some studies were affected by 2 possible biases: could illnesses in some cases cause people to lose weight? And what about the effect of smoking, which is not associated with weight gain?
To test these theories, researchers looked at data from the National Health and Nutrition Examination Survey from 1988 to 2010 that linked weight status to mortality. They used a data set from people who had always maintained a normal weight throughout life as a reference point to limit the effects of bias, comparing the those who were overweight or obese (a body mass index or BMI of 25 kg/m2 or above) to those with a normal BMI (18.5 to 24.9 kg/m2 BMI).
To be sure, the obesity paradox appeared at first, with a hazard ratio (HR) of 0.89; 95% confidence interval (CI) 0.78-1.01. But paradox disappears once the reference category was limited to the always normal weight; HR 1.16; 95% CI 0.95-1.41. When limited to never smokers, the overweight/obesity group saw mortality risks climb further; HR 1.51; 95% CI 1.07-2.15.
Thus, the authors concluded that lower mortality from CVD among those who are overweight may overlook the risks associated with those who may weigh less but have other underlying illnesses, have weighed more in the past, but especially those who smoke.
Stokes A, Preston SH. Smoking and reverse causation create an obesity paradox in cardiovascular disease [published online September 30, 2015]. Obesity. DOI: 10.1002/oby.21239.