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Obese people average higher mortality rates than normal weight people, but mortality among obese heart failure patients is lower than mortality among normal weight heart failure patients. This combination of statistical results is consistent with (at least) two very different causal structures. One possibility—call it the “opposite effects” hypothesis— is that heart failure qualitatively transforms the consequences of obesity: that is, obesity among people without heart failure increases mortality risk but is protective among people with heart failure. Because heart failure patients are only a small fraction of the whole population, under the “opposite-effects” hypothesis the population average effect of obesity would be harmful. As Curtis et al note, however, we often wish to know the effect of obesity on patients with heart failure because “recommendations for weight management derived from the general population may not be appropriate for patients with heart failure.”
An alternative causal structure is that obesity harms everyone, even heart failure patients, and the lower mortality observed among obese patients with heart failure is an artifact of selecting the study sample from this subset of the population, in combination with unmeasured confounders of the relationship between heart failure and death. This is a special form of selection bias4 and could arise as follows. Obesity approximately doubles the risk of heart failure5 but is only one of many harmful factors that increase the risk of heart failure; therefore, normal weight people who develop heart failure are more likely than their obese counterparts to have one of these other harmful risk factors. If the effects of these other risk factors on mortality among heart failure patients are larger than the effect of obesity, obesity can appear protective in the selected heart failure population in an analysis not controlling for the other risk factors. Distinguishing between these two possibilities has obvious clinical importance: in the first scenario, heart failure patients need not diet, whereas in the second scenario, dieting might be beneficial
Den här effekten finns i flera andra områden inom medicinen. En viss faktor ökar risken för att du ska utveckla en sjukdom men när du väl har det så verkar istället faktorn vara skyddande. Det här kan dock, som du kan läsa i det långa citatet, bero på andra riskfaktorer som inte är identifierade.