Obese Americans are more likely to become dangerously ill if they are infected with the new coronavirus. Now public health officials are warning that a much broader segment of the population also may be at risk: Even moderately excess weight may increase the odds of severe disease.

The warning, reported by the Centers for Disease Control and Prevention this past week, may have serious implications for Americans. While about 40% of U.S. adults are obese, another 32% are simply overweight, among the highest rates of obesity and overweight in the world.

By the new calculus, nearly three-quarters of Americans may be at increased risk of severe COVID-19 if infected with the coronavirus.

“It’s important to make sure the public and individuals are aware of this potential risk,” said Dr. Brook Belay, a medical officer at the CDC.

“The message is to strive to make healthy changes on a daily basis, through healthy food choices, choices about physical activity, and getting sufficient sleep.”

Other medical conditions for which there is limited or mixed evidence of increased COVID-19 severity include asthma, cerebrovascular disease and cystic fibrosis, the CDC said. Medical conditions clearly shown to increase the risk of COVID-19 include cancer, chronic kidney disease, heart disease and sickle cell disease, among others.

Overweight and obesity are defined by a person’s body mass index, a ratio of an individual’s weight and height. People with a BMI between 18.5 and 24.9 are considered to be of healthy weight; the overweight zone ranges from a BMI of 25 through 29, and obesity starts at a BMI of 30.

Someone who is 5 feet, 9 inches and weighs 125 to 168 pounds is in the healthy range, for example; above that, the individual is overweight, and at 203 pounds or higher, is obese.

“This greatly expands the risk to a pretty big chunk of the U.S. population,” Barry M. Popkin, a professor of nutrition at the University of North Carolina at Chapel Hill, said of the CDC’s new advice.

In a recent review of 75 studies published in August, Popkin found that obese people were twice as likely to be hospitalized with COVID-19, compared with those who were overweight or of healthy weight, and nearly twice as likely to wind up in intensive care.

Popkin and his colleagues were unable to pinpoint the risk of being merely overweight, because so few studies have examined that variable.

Doctors observed early on in the pandemic that excess weight appeared to pose an extra risk to patients. But since obesity is often accompanied by other medical problems, it took some time for researchers to learn whether excess fat, in and of itself, was the culprit. Many studies now indicate that it may be, at least in some patients.

Adipose tissue — the fat accumulated by the body — is itself biologically active, causing metabolic changes and abnormalities. Adipose promotes a state of chronic low-grade inflammation in the body, even without an infection.

In addition, abdominal obesity — which is more common in men — may cause compression of the diaphragm, lungs and chest cavity, restricting breathing and making it more difficult to clear pneumonia and other respiratory infections.

The CDC based its warning on a small number of studies that successfully differentiated between overweight and obesity, including a paper on risk factors for severe COVID-19 among patients in the United Kingdom and a report analyzing the outcomes of more than 500 patients hospitalized in March and April at Downstate Health Sciences University in Brooklyn.

Among those patients, 43% were obese, 30% overweight and 27% of healthy weight. After adjusting for age, diabetes and other such factors, the researchers found that patients who were overweight or obese were at increased risk for requiring mechanical assistance with breathing and were more likely to die. The paper was published in July in the International Journal of Obesity.

Surprisingly, the risk of being overweight was even greater than that linked to obesity. Overweight patients were 40% more likely to die than healthy-weight patients, while obese patients were at 30% greater risk, compared with the healthy-weight patients.

The findings clearly demonstrated an increased risk of severe COVID-19 in anyone with a BMI of 25 or above, according to the authors of the study, Dr. Mohamed Rami Nakeshbandi, an assistant professor of infectious diseases at SUNY Downstate Health Science University, and Rohan Maini, a medical student.

But while obesity increased the risk of death for men, it did not do so for women, they noted. (Other studies have also reported this disparity.)

The British study examined lifestyle risk factors among 387,109 men and women, 760 of whom had COVID-19. People with the virus who were overweight were roughly 30% more likely to be hospitalized than those of healthy weight; those who were obese were about twice as likely, compared with healthy-weight individuals.

The study, published in July in the journal Brain, Behavior and Immunity, also looked at factors like smoking, alcohol consumption and levels of physical activity, and concluded that moderate exercise reduced the odds that an infected person would be hospitalized.

“Socially distanced physical activity may be a good intervention,” Mark Hamer, a professor of sports and exercise medicine at University College London and an author of the paper, said in an email. “It gives immune protection, and also helps with weight loss.”

Physical activity, which has been extensively studied, can reduce the risk of developing chronic conditions linked to excess weight, like diabetes and high blood pressure. But it won’t entirely eliminate the risk of impaired immune function and heightened inflammation, Popkin cautioned.

“Overweight is quite different than other diseases in terms of inflammation,” he said. “The fat tissue are inflamed for a long period of time, and it affects immune function more over time. It is a continuous insult.”

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