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What is the obesity paradox?

09/24/2021

The so-called obesity paradox seems to suggest that obesity can in fact have a protective effect leading to better medical results in certain conditions. Is this actually the case, or is it a potential misreading of data?

In essence, in some studies involving heart conditions, tumors, and other medical issues, heavier patients were observed to be statistically less likely to die.

Possible arguments in favor of the idea that obesity may have a protective effect might be that overweight people have higher energy reserves from fat, which is useful when fighting certain debilitating illnesses like tumors. Or, fat can serve to help cushion older people against bone fractures when they fall. Another interesting hypothesis is that obese patients are more likely to require medical care and monitoring from an earlier age, thus making it easier to detect diagnosis of other potentially severe medical conditions at the same time.

Opposing arguments note that there is a key difference between causation and correlation. Weight loss is common in many diseases particularly in the advanced stages. When a patient is enrolled in an observational study, it is possible that by the time they are considered part of the observation group, they've already lost weight due to disease, and as such, compared to those that have yet to lose weight, they may be in a more advanced stage of the disease. Such a direct comparison is not possible.

In addition, most definitions of obesity used in studies rely upon Body Mass Index, which is calculated using weight and height. However, it notably does not distinguish between muscle and fat. An athletic, healthy individual with significant muscle would be categorized as obese under BMI standards, whereas someone suffering from sarcopenia but has significant fat remaining might be considered normal.

When studies utilize body fat percentage rather than BMI to study health outcomes, the relationship between obesity and risk of disease correlates strongly, showing that obese patients (as defined by body fat, not body weight) do not in fact have better survival outcomes, but in fact tend to have worse results.

In summary, the obesity paradox is likely to be caused by conflating weight with fat (which isn't the case, and done generally due to limited measurement equipment), and a potential misreading of data. In general, we recommend maintaining a normal body fat percentage, as no protection has been demonstrated from high body fat level.