How the Obesity Paradox Works

Obesity Paradox Explanations

Photo courtesy FDA

The medical community has had mixed reactions to the idea of the obesity paradox. Many physicians and scientists are skeptical because the findings go against what is expected from the normal population. Researchers at the University of Texas School of Public Health and Baylor Medical College reviewed many published reports of the obesity paradox and came up with six reasons to explain the findings -- and possibly to be skeptical of them [source: Habbu].

  1. The numbers of people studied in the reports of the obesity paradox were generally small. Therefore, do these results apply or hold in much larger populations?
  2. The statistical techniques show associations between factors, but not cause-and-effect conclusions. So, the results may or may not be real.
  3. In many studies, congestive heart failure was diagnosed from clinical symptoms (difficulty breathing, swelling in extremities) instead of from laboratory tests (like echocardiography, cardiopulmonary testing, cardiac catheterization).
  • These clinical criteria to diagnose congestive heart failure have not been validated in obese populations and may not be applicable.
  • In some of the studies where lab tests were done, the obese patients had slightly better heart functions (pumping ability, oxygen delivery) than normal or underweight patients.
  • Therefore, the obese patients may have been either slightly "healthier" with respect to CHF or in earlier stages of CHF than their normal/underweight counterparts. So obese survival rates were better.
  1. Congestive heart failure (and chronic kidney disease) is a wasting disease. Patients are so sick that they lose weight (fat, muscle mass) over the course of the disease. This could lead to two conclusions:
  • Again, obese patients could be "healthier" or in earlier stages of these chronic diseases. Chronic disease patients with low BMIs do not have low BMIs intentionally, but because of the nature of the wasting disease. None of the studies discriminated between intentional weight loss (from diet and exercise) and unintentional weight loss (from disease).
  • Obese patients may have a better metabolic reserve than their normal/underweight counterparts.
  1. Few of the studies looked at extreme obesity (BMI more than 35). In some studies that did, the extremely obese did not have a greater chance of survival than the underweight. Therefore, the survival curves may be U-shaped. Normal and overweight patients would have better survival probabilities than those at the extremes -- underweight and extremely obese.
  2. Some recent studies have questioned whether BMI is the best way to categorize obesity. Maybe waist circumference or waist-to-hip circumference ratios may be better indicators of obesity. These criteria are based on observations that body fat stored in the waist is worse with respect to the risks of obesity than fat stored elsewhere. [source: Romero-Coral].

On the next page we'll learn about more explanations for and criticisms of the obesity paradox theory.