Over 15 years ago, a strange counter-intuitive bit of data was identified in patients undergoing hemodialysis for chronic kidney disease. Across several studies, overweight or mildly obese patients were displaying greater survival rates than those with healthy weights. The phenomenon was dubbed the "obesity paradox" and for well over a decade scientists have debated what could be causing it. Several new studies presented recently at the European Congress on Obesity have added further weight to the hypothesis of an obesity paradox, finding several strange correlations between obesity and survival rates across a variety of conditions.

The first study looked generally at patients admitted to hospital for an infectious disease. Tracking more that 18,000 patients admitted to hospitals in Denmark over a four-year period, the study found that within 90 days of discharge those patients of a normal weight displayed a significantly higher chance of dying when compared to both overweight and obese patients.

Two more studies presented at the conference examined mortality rates from patients admitted to hospitals for pneumonia and sepsis. Both studies examined large banks of data tracking admissions from over 1,000 US hospitals.

The pneumonia study, which included data from 1,690,760 hospitalizations, found that obese and overweight patients were between 20 and 30 percent less likely to die from the condition than those of normal weight. The sepsis study impressively gathered data from 3.7 million hospital admissions and found obese and overweight patients were around 20 percent more likely to survive following admission than patients of normal weight.

The final study was much smaller than the others but was more interested in investigating a specific potential hypothesis that could help explain the obesity paradox. This research examined the rate of muscle wasting in critically ill patients admitted to intensive care. Only 26 patients were studied, but the results saw the obese subgroup displaying notably different muscle wasting patterns than the non-obese group.

The hypothesis is that when a critically ill body enters an acute stage of illness it moves into a phase called hyper-catabolism. This phase finds the body rapidly searching for extra fuel for energy production in vital organs. The state can often lead to muscle wasting as the body breaks down that tissue for energy.

This study found that obese patients in this context displayed a slower rate of muscle wasting when compared to non-obese patients, and also a higher muscle quality index. In conclusion, the authors of the research suggest that, "Critically ill patients with obesity seem to have higher muscle quality, as measured by ultrasound at the point of admittance to intensive care compared to non-obese patients. This might be the metabolic protective shield also described as the 'obesity paradox'."

None of this research at all suggests general recommendations for healthy body mass indexes to be changed, as an enormous body of data still exists linking obesity to increased risk of a number of illnesses. But, the strange idea of an obesity paradox still remains present in some observational studies, and any causal connection is still frustratingly unexplained.

David Carslake from the University of Bristol says this new research doesn't necessarily imply a causal link between obesity and higher survival rates for the specific conditions but instead suggests there may be a variety of unmeasured factors that are causing these paradoxical results.

"First, unmeasured factors such as smoking behavior or other diseases could cause both a reduction in BMI and increased mortality," says Carslake. "The authors took steps to adjust for these, but they can never be fully accounted for by the methods they used. Second, it's possible that overweight and obese people get admitted to hospital more readily with less severe cases of sepsis, infection or pneumonia. This is a risk whenever analyses are restricted to specific patient groups, and in this case could give a spurious impression that high BMI was protective."

Other research also suggests that by simply measuring obesity by body mass index can result in skewed and questionable results. A 2013 study suggested that when other obesity-related data points are taken into account, such as waist circumference (WC) and waist to hip ratio (WHR), any indication of the obesity paradox tends to disappear from a study's results.

Anne Peeters, from Deakin University, has gone as far as suggesting that journals should reject articles that imply the existence of an obesity paradox due to fundamental flaws in the correlation between body mass index and mortality and it may give people a false belief that obesity can be beneficial.

Despite these new studies again raising the controversial specter of an obesity paradox, no one is suggesting gaining weight improves the odds of a healthy outcome for any illness. Ultimately, any positive causal connection between obesity and survival in a specific scenario is outweighed by the overall volume of data linking obesity with a great number of terminal illnesses.

The new studies are currently unpublished and yet to be peer reviewed but were presented at the recent European Congress on Obesity in Vienna, Austria.