"The risk factors we identified for hospitalization in COVID-19 are largely similar to those associated with any type of severe disease requiring hospitalization or ICU level care," they wrote.

However, the researchers added that what was "most striking" were the importance of inflammatory markers that appear to show which patients would need critical care. This, they say, could explain the risk posed by obesity.

"It is notable that the chronic condition with the strongest association with critical illness was obesity, with a substantially higher odds ratio than any cardiovascular or pulmonary disease," they wrote. "Obesity is well-recognized to be a pro-inflammatory condition."

The researchers note there are limitations to their work. For example, patients were all from New York and were treated in the same health system, so results may not be applicable to people in different parts of the U.S. and different countries. They also did not have information on inflammatory markers for people who were not hospitalized: "It is possible that these would have been strong predictors for hospitalization risk as well if available," they wrote.

Study author Leora Horwitz, from NYU Langone, told Newsweek they were surprised to see obesity was such a big factor in risk. "None of the initial reports from China examined obesity as a risk factor, so we didn't have existing evidence already showing it was important."

She explained that in severe cases of COVID-19, the body overreacts to the infection with inflammation, causing irritation to the lungs. "It turns out that obesity itself actually increases the amount of inflammation in the body. So it's possible that obesity also directly contributes to worse outcomes in that way."

Ian Hall, Director of the Centre for Biomolecular Sciences at the U.K.'s University of Nottingham, told Newsweek the study provides "a valuable analysis" of patients with COVID-19, showing obesity is a "major risk factor" in hospital admission.

He said that while obesity may be associated with other conditions placing the patient at greater risk, such as diabetes and heart failure, the researchers tried to control for this: "It looks as if obesity is by itself an independent risk factor for admission," he said.

Hall said one possible explanation is that people with severe obesity have lower oxygen levels than someone of a healthy weight. "Because having a low oxygen level is one of the main reasons for hospital admission this could result in more of the obese patients being admitted," he said. "However, there is a further possible explanation. It is well established that obesity is itself associated with an increased inflammatory response in the body, and so it is possible that the inflammatory response to SARS-CoV-2 infection may be increased in obese individuals when compared with individuals with a normal BMI."

"Whatever the explanation, given that in the U.S. and many other countries obesity is common it is useful for planning clinical care pathways to know that obesity appears to be an important independent risk factor for hospital admission," Hall said.

Joshua Bell, an epidemiologist at University of Bristol, U.K., said the results were "striking," but added they point to factors that mark poorer outcomes: "Not necessarily to factors which cause those poorer outcomes," he told Newsweek.

"This health threat is new, and it will take time to gather and link all the data needed to robustly identify factors which mark, and factors which cause, poorer outcomes. It would also be important to consider socioeconomic and occupational exposures in such studies.

"Obesity is a strong cause of diabetes and heart disease, largely through its effect on metabolic and inflammatory pathways. Knowing if any of those same pathways alter the course of COVID-19 may help reduce its impact and improve survival."

David Hamer, from the Boston University Schools of Public Health and Medicine, told Newsweek the findings mirrored preliminary findings from his own hospital. "It would not surprise me if subsequent studies show that this is a risk factor for COVID-19-related mortality," he said. "Given the 'growing' obesity epidemic in the US, this is definitely an association worth highlighting as obesity may increase both the severity of COVID-19 and potentially mortality."

The World Obesity Federation highlights a number of other issues relating to the hospitalization of obese patients. "Given the extremely high rates of obesity around the globe we expect that a high percentage of the population who will contract coronavirus will also have a BMI over 25," it said.

Problems treating obese people generally include not being able to intubate as easily, problems running diagnostic tests because of machine weight limits, and challenges moving them.

"Special beds and positioning/transport equipment are available in specialized surgery units, but may not be widely available elsewhere in hospitals and certainly not in all countries," World Obesity Federation said. "In general health systems are already not well set up to manage patients with obesity...and the current crisis will expose their limitations even more."