At 5-foot-9 and 248 pounds before covid-19 struck, John Place knew he needed to work on his health. In the scramble to run a small business and help raise four children, he ate high-calorie restaurant food every day. He never exercised. He was often fatigued and urinated frequently — warning signs of diabetes that he ignored.

When Place, 43, landed in a Florida intensive care unit in June, infected with the coronavirus and unable to breathe on his own, a brutally frank doctor put his survival chances at 20 percent.

“Your husband is morbidly obese, he’s diabetic, he has sleep apnea and the only thing he has going for him is he’s still young,” the physician told Place’s wife, Michelle Zymet.

Place survived 18 days on a ventilator and returned home, but his weight complicated his illness and care, and now is influencing his painful, laborious recovery.

Eight months into the pandemic, obesity has turned out to be one of the clearest predictors of a difficult battle against covid-19, for reasons that may vary from person to person. Some experts say they consider obesity to have contributed to the stunning coronavirus death and morbidity rate in the United States, which has one of the highest obesity rates in the world. And there is some evidence it is particularly harmful for people under 60, who generally fare better than the elderly against the disease caused by the novel coronavirus.

A constellation of factors can influence a patient’s outcome: Fat can physically compress parts of the lungs, impeding respiration. In the hospital, it can make calculating medication doses, inserting intravenous tubes and moving patients more difficult. It can stimulate parts of the body’s hormonal system, worsening covid-19, a disease that often provokes a powerful inflammatory response itself. And it is associated with a wide range of comorbidities, from heart disease to diabetes, that increase vulnerability to the worst impacts of the infection.

“It changes how you evaluate all kinds of considerations, from vascular issues to medications,” said Lewis Kaplan, a critical care doctor for Veterans Affairs and the University of Pennsylvania. “We have an [obesity] epidemic in the midst of a pandemic.”

The pandemic already has killed more than 183,000 Americans and infected more than 6.1 million, according to a tally kept by The Washington Post. The Centers for Disease Control and Prevention lists 5,614 covid-19 deaths where obesity was a contributing factor, but this is probably a sharp undercount, said Robert N. Anderson, chief of the Mortality Statistics Branch at the National Center for Health Statistics. Clinicians and researchers in all parts of the United States have noted the condition’s influence on patients.

More than 42 percent of U.S. residents are obese, defined as a body mass index of 30 or greater, and more than 9 percent are severely obese, with BMIs of 40 or more. A 5-9 person weighing 203 pounds has a BMI of 30. The same person would weigh 271 pounds if his BMI were 40.

For some groups, conditions are worse: 56 percent of African American women, for example, are obese, according to data from the CDC. It lists a BMI over 30 as a risk factor for severe covid-19.

Britain is urging its citizens to lose weight to protect themselves against covid-19. When University of North Carolina researchers reviewed the academic literature on obesity and covid-19 for an analysis released this week, they found that people with BMIs over 30 were at much greater risk for hospitalization, intensive care and death.

Early analyses point to obesity itself — rather than the comorbidities it creates — as a separate precursor to poor outcomes.

When researchers at Kaiser Permanente of Southern California looked at nearly 7,000 people with covid-19, they found an association between a BMI over 40 and higher death rates, particularly among men and people younger than 60, when they controlled for other weight-related conditions. The latter finding is surprising, because deaths from covid-19 are overwhelmingly concentrated among older people.

Sara Tartof, a Kaiser Permanente research scientist who led the analysis, speculated that large amounts of visceral fat — the fat stored in the abdomen around body organs — may play a role in producing severe covid-19. Fat is not inert; it secretes chemicals that can influence bodily systems. It may affect the angiotensin system that helps regulate blood pressure and blood flow, leading to more severe symptoms, Tartof said.

“We almost think this is akin to throwing a match on a tinderbox,” she said.

Jennifer Lighter, an NYU Langone hospital epidemiologist, has been studying a link between covid-19 and fat cells. Lighter said people with obesity seem to have more ACE2 receptors, the gateway the virus uses to invade cells. “So there are more opportunities to attack,” she said.

Lighter, who published a study showing that people under age 60 are two to three times more likely to be admitted to the hospital for covid-19 if they are obese, also said it is possible a mechanism of the disease is hormonally regulated. That could explain why men are doing worse, she said, and why prepubescent children are faring well in the pandemic.

“Obese people have more androgens and male hormones. Maybe that’s impacting the virus affecting the cells,” she hypothesized.

Another study published in the International Journal of Obesity by Candida Rebello, a pharmacology researcher at Louisiana State University, found that the hormone leptin, which regulates metabolism and appetite and is found in greater amounts in obese people, may make them more vulnerable to covid-19. High levels of leptin have been associated with a type of systemic inflammatory state.

“We propose that leptin may be the link between obesity and its high prevalence as a comorbidity of the SARS-CoV-2 infection,” she wrote.

In addition to metabolic issues, “obesity in itself is a complex issue, because it usually leads to so many comorbidities” and can affect lung mechanics, said Maria Plataki, an assistant professor at Weill Cornell Medicine who studies the impact of obesity on lung cells. A recent study of 1,687 hospitalized adults by a team that included Plataki found that obese patients were more likely to have respiratory failure but not any more likely to die.

Place, the Florida patient, had a BMI of 37 when the disease sent him to Westside Regional Medical Center in Plantation two months ago. His temperature had reached 103.9 degrees and pneumonia clogged both his lungs.

When doctors discovered Place’s uncontrolled diabetes, they put him on an insulin drip. As the oxygen level in his blood plummeted, he was sent to the ICU, where he was sedated and intubated.

At times, there was not enough staff for the delicate task of turning him onto his stomach — a procedure called “proning” that helps open airways — or returning him to his back, Zymet said. Five people were needed to accomplish the task because of Place’s weight and the medical devices he was attached to. Heavier people also are more likely to develop bed sores.

“We prone obese people all the time,” said Russell Buhr, a pulmonary and critical care physician at Ronald Reagan UCLA Medical Center in Los Angeles. “Instead of needing four people to do it, you might need six or eight people to do it.”

Intubating very obese people also can be more complex because fat deposits around the neck can make proper positioning more difficult, Buhr said. When patients are sedated and their muscles relax, fat can compress the airway itself, he said.

Standard doses for drugs — such as pain killers, blood thinners and other critical medications — usually stop at certain weights. That requires caretakers to calculate appropriate amounts for the severely obese, and often there is not a simple linear relationship between weight and dosage.

Opioids, for example, are stored in fatty tissue, Kaplan said, which can lead to accumulation and complicate doses.

Place lost 49 pounds during his hospital stay while he was being fed through a tube and on a ventilator, leaving at 199 pounds. But the physical cost was enormous. When he awoke, the only part of his body he could move was his left arm, from the elbow to his fingers.

“All my muscle was gone,” he said. “My calf muscles were just flopping around. They just hung. My ribs were sticking out.” He could not sit up without becoming dizzy and short of breath. Standing was out of the question.

Gradually regaining his strength with the aid of physical therapy, Place is able to walk for five minutes at a time now, though parts of his body remain numb and he is still in considerable pain. He and his family haven’t eaten restaurant food since the coronavirus lockdown began in March and he is off insulin.

Now at 208 pounds, Place wants to keep his weight around 180 once he fully regains his health. He has started a new business selling T-shirts, hats and masks with the logo “I’m alive.”

“One silver lining is at least he’s going to take care of his health,” Zymet said. “It puts everything into perspective.”