Wednesday, August 22, 2007

Obesity paradox: More risk, better outcome?

myocardial infarct
A heart infarct (dark spotted area in this microscopic image) is likely to be smaller in obese than in normal-weight patients. This outcome of an Italian study adds further evidence to the so-called obesity paradox.

The term obesity paradox describes a puzzling conflict of observations on body weight and the risk of death from a heart attack: When we look at the population as a whole, obese persons have a higher risk of dying from a heart attack. But when we look at patients suffering from a heart disease or having survived a heart attack, the obese are less likely to die than the normal-weight. In short, the obese have a higher risk but a better outcome.

For this paradox I see three possible explanations that are not mutually exclusive:

  • Scenario 1: Bad and good body fat. A higher mass of body fat is the cause of sickness in general and heart disease in particular. Once the heart is attacked, body fat has a protective effect. But this effect does not outweigh the negative impact of body fat: Obese persons are still more likely than normal-weight to die from a heart attack. The protective effect of body fat only can be seen in normal- or underweight heart patients who have a worse outcome than the obese.
  • Scenario 2: Bad metabolism and good body fat. A higher mass of body fat is caused by a bad metabolism (unhealthy nutrition, lack of physical activity, genetic influences) that is also the cause of sickness in general and heart disease in particular. Body fat as such has no negative influence but a protective effect once the disease has developed.
  • Scenario 3: Bad metabolism and self-repair. There is no causal relationship of body fat, neither with the risk of disease nor with the outcome. The accumulation of body fat and the development of a heart disease have the same underlying causes in the metabolism. Later, when the disease has developed, the body fights it with self-repair mechanisms. Again, body fat is not a cause of such mechanisms but only a concomitant outcome.
At the moment, scenario 2 makes most sense to me. Firstly, there is little evidence that body fat as such causes heart disease. And secondly we should remember why the human body accumulates fat: As a reserve for bad times when energy input is low and energy demand is high. A sickness, certainly, is a condition of higher demand. Thus, it makes sense to me that a reserve of fat helps the body to cope with disease.

Photo credit: Wellcome Photo Library

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