You may have heard of the "obesity paradox," a phenomenon suggesting that obese and overweight people have a survival advantage over normal-weight people in certain cases. 

New research, including studies from Denmark as well as the United States, reignited the debate around the controversial paradox. The studies were presented recently at the 25th European Congress on Obesity in Vienna, Austria.

In one study, researchers looked at 18,000 patients admitted to a hospital in Denmark for an infectious disease between 2011 and 2015. To understand how body mass index (BMI) played a role, they examined the risk of death within 90 days after discharge date in association with the BMI of the patients. Compared to normal weight patients, overweight patients were 40 percent less likely to die and obese patients were 50 percent less likely to die.

"Overweight and obesity were associated with substantially reduced 90-day mortality following incident hospital admission for infection," the authors stated.

Another study analyzed patient data from over 1,000 U.S. hospitals, suggesting that the obesity paradox may hold true in the case of pneumonia. Outcomes of the hospitalized patients were examined in moderate cases (without the use of a ventilator) as well as severe cases (requiring a ventilator).

When compared to normal weight patients without a ventilator, overweight patients were 23 percent more likely to survive pneumonia and obese patients were 29 percent more likely to survive. In the cohort involving serious cases with the use of a ventilator, overweight and obese patients were more likely to survive by 21 percent and 30 percent respectively.

Similar findings were noted in other studies with patients admitted to hospitals for sepsis or blood poisoning. There were two possible reasons to explain the survival effect of obesity, according to the lead author of the first study, Sigrid Gribsholt from Aarhus University in Denmark.

First, she suggested that a stronger response from the immune system of such patients due to obesity-induced inflammation. This may help them recover from infections better than normal-weight patients. Secondly, Gribsholt said people who are obese have larger energy reserves which may have a protective effect, especially during a serious illness.

On the other hand, most researchers have debunked the obesity paradox by linking the condition to the risk of early death and questioning biases in research methods. One of the major criticisms of studies supporting the paradox is the use of BMI as a measure. Experts have referred to it as "a poor discriminator of total body fatness" lacking the inclusion of waist circumference and waist to hip ratio. 

"The obesity paradox caused a lot of confusion and potential damage because we know there are cardiovascular and non-cardiovascular risks associated with obesity," said Dr. Sadiya Khan, an assistant professor of medicine from Northwestern University. "A healthy weight promotes healthy longevity or longer healthspan in addition to lifespan so that greater years lived are also healthier years lived. It’s about having a much better quality of life."