This spring, after days of flulike symptoms and fever, a man arrived at the emergency room at the University of Vermont Medical Center. He was young—in his late 30s—and adored his wife and small children. And he had been healthy, logging endless hours running his own small business, except for one thing: He had severe obesity. Now, he had tested positive for COVID-19 and was increasingly short of breath.
People with obesity are more likely than normal-weight people to have other diseases that are independent risk factors for severe COVID-19, including heart disease, lung disease, and diabetes. They are also prone to metabolic syndrome, in which blood sugar levels, fat levels, or both are unhealthy and blood pressure may be high. A recent study from Tulane University of 287 hospitalized COVID-19 patients found that metabolic syndrome itself substantially increased the risks of ICU admission, ventilation, and death.
But on its own, “BMI [body mass index] remains a strong independent risk factor” for severe COVID-19, according to several studies that adjusted for age, sex, social class, diabetes, and heart conditions, says Naveed Sattar, an expert in cardio metabolic disease at the University of Glasgow. “And it seems to be a linear line, straight up.”
Another study captured the rate of COVID-19 hospitalizations among more than 334,000 people in England. Published last month in the Proceedings of the National Academy of Sciences, it found that although the rate peaked in people with a BMI of 35 or greater, it began to rise as soon as someone tipped into the overweight category. “Many people don’t realize they creep into that overweight category,” says first author Mark Hamer, an exercise physiologist at University College London.
The physical pathologies that render people with obesity vulnerable to severe COVID-19 begin with mechanics: Fat in the abdomen pushes up on the diaphragm, causing that large muscle, which lies below the chest cavity, to impinge on the lungs and restrict airflow. This reduced lung volume leads to collapse of airways in the lower lobes of the lungs, where more blood arrives for oxygenation than in the upper lobes. “If you are already starting [with] this mismatch, you are going to get worse faster” from COVID-19, Dixon says.
Other issues compound these mechanical problems. For starters, the blood of people with obesity has an increased tendency to clot—an especially grave risk during an infection that, when severe, independently peppers the small vessels of the lungs with clots. In healthy people, “the endothelial cells that line the blood vessels are normally saying to the surrounding blood: ‘Don’t clot,’” says Beverley Hunt, a physician-scientist who’s an expert in blood clotting at Guy’s and St. Thomas’ hospitals in London. But “we think that signaling is being changed by COVID,” Hunt says, because the virus injures endothelial cells, which respond to the insult by activating the coagulation system.
Add obesity to the mix, and the clotting risk shoots up. In COVID-19 patients with obesity, Hunt says, “You’ve got such sticky blood, oh my—the stickiest blood I have ever seen in all my years of practice.”
Immunity also weakens in people with obesity, in part because fat cells infiltrate the organs where immune cells are produced and stored, such as the spleen, bone marrow, and thymus, says Catherine Andersen, a nutritional scientist at Fairfield University. “We are losing immune tissue in exchange for adipose tissue, making the immune system less effective in either protecting the body from pathogens or responding to a vaccine,” she says.