The coronavirus has been an unpredictable foe from the start. It triggers silent or barely perceptible infections in some individuals, while in others it sets off a cascade of complications that overwhelm the body and lead to death.
Why some patients sail through the disease and others are felled by it is a question that has bedeviled doctors.
Older age and chronic health conditions like high blood pressure and heart disease are known to increase the risk of severe COVID-19. The Centers for Disease Control and Prevention also lists extreme obesity as a high risk.
But is excess weight in and of itself to blame? Or all of the health problems that accompany obesity, like metabolic disorders and breathing problems?
A new study points to obesity itself as a culprit. An analysis of thousands of patients treated at a Southern California health system identified extreme obesity as an independent risk factor for dying among COVID-19 patients — most strikingly, among younger and middle-aged adults 60 and younger, and particularly among men.
Among women with the illness, body mass index — a measure of body fat based on height and weight — does not appear to be independently associated with an increased risk of dying at any age, the authors said, possibly because women carry weight differently than do men, who tend to have more visceral and abdominal fat. The study was published in Annals of Internal Medicine on Wednesday.
“Body mass index is a really important, strong independent risk factor for death among those who are diagnosed with COVID-19,” said Sara Tartof, the study’s first author, a research scientist at Kaiser Permanente of Southern California.
But “the impact is not uniform across the population,” she added. “You don’t really see it for the older ages, and we didn’t see it as an important risk for females at any age.”
Obesity and the coronavirus are a dangerous combination for a number of reasons.
Obesity causes restricted breathing, making it more difficult to clear pneumonia and other respiratory infections. Fat is biologically active and a source of pro-inflammatory chemicals, promoting a state of chronic inflammation in the body even before COVID-19 sets in. Obesity causes metabolic changes and abnormalities, even in the absence of diabetes.
The study is not the first to finger obesity as a culprit in COVID-19 deaths in younger people. While early reports from China and Italy did not focus on obesity as an independent risk factor, physicians in other parts of the world, where obesity is more prevalent, were quick to notice that younger individuals who became very ill were often obese.
Only 6% of the Chinese population is obese, compared with 20% of the population in Italy and 24% in Spain. The United States, by contrast, has one of the highest rates of obesity in the world.
Some 42% of American adults have a body mass index of 30 or more, which classifies them as having obesity, and 9% have a BMI of 40 or more. (Someone who is 5 feet 9 inches tall and weighs 270 pounds or more has a BMI of 40, according to federal guidelines.) An individual of that height at 304 pounds has a BMI of 45.
Normal weight ranges from a BMI of 18.5 to 24.9; people with BMIs of 25 to 29.9 are considered overweight.
A report issued by Public Health England concluded that overweight and obesity increased the risk of complications and death from COVID-19. Hundreds of similar articles on the topic have been published.
Among them was a study last month from Columbia University, also published in Annals of Internal Medicine, which found that individuals with COVID-19 who were extremely obese were 60% more likely to require mechanical ventilation or to die from COVID-19. The study noted that this was the also case for people under age 65.
“Is it just that we in the United States have more obese people, so we’re seeing this?” said Dr. David Kass, a professor of medicine at Johns Hopkins University who wrote an editorial accompanying the new study.
“But this is beyond the proportion that would just be in the general population,” he said. “It’s not just that there are a lot of fat people, so we’re seeing a lot of fat people who are very sick.”
Kass wrote a letter in the Lancet in April noting that many younger COVID-19 patients admitted to American hospitals suffered from obesity and predicting that as the coronavirus spread through areas where obesity was more prevalent, more younger people would be affected.
The disparate effect on men who are obese is also understandable, he said, because of differences in fat distribution. “If you take a man and woman side by side with the same BMI, the male is much more likely to have the background problems that we think are a component for being more at risk,” Kass said.
To carry out the new study, researchers analyzed the health records of 6,916 members of the Kaiser Permanente Southern California Health System who were treated for COVID-19, both as inpatients and outpatients, from mid-February to May 2.
The median age of the patients was 49 years, and the mean BMI of the patients was 30.6; nearly half were obese.
In general, obesity rates vary by race and ethnicity, according to the CDC. The age-adjusted obesity rate among Blacks is 49.6%, compared with 45% among Hispanics, 42% among whites and 17% among Asian Americans.
Just over half of the Kaiser Permanente patients were Hispanic, about 1,000 were Asian/Asian-American, and 584 were Black. Many patients had underlying health problems that are linked to poor COVID-19 outcomes. Some 206 — or 3% — of the patients died within 21 days of receiving a COVID-19 diagnosis.
To figure out whether obesity, in and of itself, was associated with a higher death risk, the researchers tried to factor out conditions like high blood pressure and diabetes, which are known to be associated with more severe forms of COVID-19, as well as heart, kidney and chronic lung disease.
The scientists also wanted to know whether demographic factors, like age, sex and race or ethnicity, played a role.
While Black and Latino populations have been disproportionately stricken by the virus, with hospitalizations and deaths at higher rates, the study did not find race or ethnicity to be an independent risk factor.
The researchers did find extreme obesity to be a strong independent risk factor for worse outcomes. “We’re not saying the disparities don’t exist — we’re teasing apart what’s driving the disparities,” Tartof said.
“We see that racial and ethnic minorities are having more bad outcomes. They are also more likely to be obese, or to have less access to health care, and they’re more likely to have co-morbidities.”
Among COVID-19 patients in the study, those with extreme obesity — defined as a BMI of 40 or more — were nearly three times at greater risk of dying than those of normal weight. Those with a BMI of 45 were more than four times more likely to die than patients of normal weight, with the risk most striking among men and those under 60, Tartof said.
The study draws attention to the intersection of two major health concerns, Tartof said, underscoring the need for policies to tackle both.
“There is a lot of work we can do to better combat COVID, and a lot we can do to improve our strategies on obesity as well,” she said. “It is also an epidemic, and something we need to pay attention to.”