Being overweight doesn’t just make people more susceptible to chronic illnesses such as heart disease and diabetes—it might also increase their risk of severe influenza and other infections, a new study confirms.
The study, published today in the Lancet, suggests that people with obesity may be more susceptible to death and hospitalization from a variety of infections caused by viruses, fungi, parasites and bacteria. Researchers analyzed health data from more than 540,000 people enrolled in databases in the U.K. and Finland and found that the risk of severe infection increased as body weight increased. These findings reinforce growing evidence that high amounts of excess fat might impair the body’s ability to fight off infection.
“This was an incredibly important paper,” says Aubree Gordon, an epidemiologist at the University of Michigan, who was not involved in the new study. It “truly shows that adults with obesity have a substantially higher risk of being hospitalized or dying from infections, and we’re talking not just COVID-19 but hundreds of different types of infections.”
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The study’s authors parsed hospitalization and mortality data from time periods before, during and after the COVID pandemic’s peak. They looked at infections such as influenza, COVID and pneumonia, as well as HIV and gastrointestinal, urinary tract, and skin and soft tissue infections. In addition to body mass index, or BMI (a controversial measurement for obesity), they factored in waist circumference and waist-to-height ratio, as well as other demographic information, including age, sex, ethnicity, education and socioeconomic status. The team also determined baseline health status to control for conditions such as high blood pressure, metabolic syndrome, diabetes, heart disease, stroke and cancer.
Obesity was connected to an estimated 8.6 percent of infection-related deaths in 2018 (a time before the pandemic), 15 percent of such deaths in 2021 (a time during the pandemic) and 10.8 percent of them in 2023 (a time after the pandemic’s height). And the higher a person’s BMI was, the higher their risk of severe disease from infections became. (A BMI of 30 and above is generally classified as representing obesity.) The risk of severe infection was 1.5 times higher for those with a BMI of 30 to 35 (about 30 pounds overweight) compared with people with a lower BMI. And people with a BMI above 40 (about 80 to 100 pounds overweight) had a risk that was nearly three times higher. These findings were consistent across all three body size metrics, as well as sociodemographic and lifestyle groups, the study authors wrote. And the data from the populations in both the U.K. and Finland showed very similar results, says Samuel Klein, a physician and obesity expert at Washington University in St. Louis, who was not involved in the study. “That’s a major observation, and it’s robust,” he adds.
The findings line up with past research. Gordon’s group demonstrated in 2018 that people with obesity were more susceptible to flu infections and shed the virus for 42 percent longer than those who did not have obesity. Several studies and reports during the pandemic found that people with obesity were more likely to develop COVID and had a much greater risk of hospitalization and death from the disease.
In the new study, obesity had the strongest association with severe skin and soft tissue infections, but the data showed an elevated risk across the board for nearly every infection type.
Biologically, this makes sense. For one, in some people, being overweight can impair lung function, which can lead to worse outcomes with infections, particularly respiratory infections, Gordon says. People with obesity may also have metabolism or hormone issues that could influence their ability to fight off infections. But more recent compelling evidence has placed an emphasis on differences in immune responses.
Inflammation is a natural defense mechanism—but chronic uncontrolled inflammation can be harmful and worsen disease, Klein explains. Evidence suggests that obesity increases chronic low-grade inflammation and also impairs the immune system’s ability to fight pathogens, he says. Other preliminary research suggests that obesity may affect the quality and quantity of protective antibodies, which latch on to pathogens to signal attacker immune cells to target them.
The new findings suggest an association but do not show a clear cause and effect. Gordon would like to see more data on low- and middle-income populations, but she wouldn’t be surprised to see similar trends globally.
Gordon and Klein say the results could motivate people with obesity and their clinicians to focus on rapid treatment and prevention measures, such as vaccines. In the study, the authors also emphasized policies to help people eat a healthy diet and engage in exercise.
“To reduce the risk of severe infections, as well as other health issues linked with obesity, there is an urgent need for policies that help people stay healthy and support weight-loss, such as access to affordable healthy food and opportunities for physical activity,” said study author Solja Nyberg of the University of Helsinki in Finland in a press release.
The relationship between weight and health is nuanced—it is influenced by stigma, health care access and individual biology, as well as diet, exercise and other lifestyle choices. And growing research suggests that some people who are considered to have obesity based on their BMI are metabolically healthy.
“There are complex biological factors for sure with obesity,” Gordon says. “But there are really complex social and environmental factors that impact it, too.”