Obesity may increase chances of dying from serious infections, study finds

A new study of more than 500,000 people found that people who are obese are 70% more likely to be hospitalized or die from a serious infection than those who are not obese.

The study found that obese people account for one in 10 deaths from infectious diseases worldwide. This association persisted even in obese people without metabolic syndrome or diabetes. And it appeared regardless of socioeconomic status or physical activity level, researchers reported in the Feb. 9 issue of The Lancet.

“People who are obese may have a harder time fighting infections,” study lead author Mika Kivimäki, an epidemiologist at University College London, told Live Science. “Excess body fat can affect the immune system in a number of ways, including impaired lymphatic function, decreased lung function, and increased long-term low-grade inflammation.” The lymphatic system helps maintain fluid balance in the body and trains and transports immune cells.

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An earlier study by various researchers, published in August 2025, found that using semaglutide, the active ingredient in medicines such as Ozempic and Wigovy, reduced users’ risk of serious infections by 10%. This finding may suggest that obesity and infectious disease susceptibility are linked, and perhaps suggest that risk can be reduced.

The link between obesity and risk of infectious diseases has become more pronounced during the coronavirus pandemic, a new study has found. In 2021, 15% of all infection-related hospitalizations and deaths were associated with obesity. The link between obesity and the severity of COVID-19 infections motivated Kivimäki and his team to investigate whether the coronavirus is particularly dangerous for obese people, or whether the danger extends to all types of infections.

They looked at the UK Biobank, a large database of genetic sequences and linked medical records of adults in the UK, and two similarly large studies in Finland: the Finnish Public Sector Study and the Health and Social Support Study. These databases included more than 540,000 people in total.

In the Finnish cohort, participants were able to self-report their height and weight, which could be used to calculate body mass index (BMI). BMI is a rough estimate of body composition that is often used in large population studies. UK Biobank participants had more accurate measurements taken using a body composition measurement device, and waist circumference was also measured. (Some researchers argue that these types of indicators should replace BMI because they help predict health status.)

In this study, obese people included those with a BMI of 30 or higher. Waist circumference of at least 40 inches (102 cm) for men and 35 inches (88 cm) for women. Or the waist-to-height ratio is 0.6 or more. After adjusting for age and sex, obesity was associated with a 70% higher likelihood of hospitalization or death from an infectious disease. This relationship held across different definitions of obesity and across all bacterial, viral, fungal, and parasitic infections.

The results also showed that the greater the degree of obesity, the greater the risk.

People with a BMI of 30 to 34.9 had a 50% higher risk of infection, hospitalization, and death than those with a BMI of less than 30. However, those with a BMI of 35 to 39.9 had twice the risk, and those with a BMI of 40 or more had three times the risk. Among participants whose obesity was measured at follow-up after the initial baseline measurement, the risk of serious infection decreased or increased sequentially in those who lost or gained weight.

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Due to the nature of the study, the researchers were unable to prove causation or determine exactly how obesity increases these risks, Kivimäki said.

That said, there is evidence to suggest that adipose tissue and the immune system are intertwined. Nikhil Dhurandhar, a professor of nutritional science at Texas Tech University who was not involved in the new study, said progenitor cells that can turn into fat cells can act like immune cells, and some fat cells can also excrete pro-inflammatory substances.

Previous research has found that not only weight can influence infection risk, but also that certain pathogens are associated with the development of obesity, Durandal told Live Science. Evidence that specific bacteria cause fat gain has so far been collected primarily in laboratory animals, but human data are inconclusive.

Obesity can cause immune dysfunction, making it difficult for the body to fight infections, Dhurandar added. Because of this dysfunction, obese people may not respond to vaccination as well as people who are classified as overweight or normal weight, leaving them more vulnerable. He said obesity is associated with a lack of sensitivity to the hormone leptin, which helps regulate weight and appetite and also has an immune protective role.

Dhurandar said the one in 10 obesity-related infectious disease deaths in the study should not be assumed to be easily prevented by weight loss, as it is very difficult to lose weight and keep it off.

“Obesity is a disease,” he said. “This is a chronic disease. It’s not a matter of will. It’s not a matter of discipline.”

However, the advent of drugs like semaglutide, collectively known as GLP-1 agonists, may make weight loss easier for some people. Kivimäki said the effect of these drugs on the risk of infection remains unknown.

Although trials suggest these drugs reduce risk, people taking these drugs often lose muscle mass as well as fat, which can have a negative effect on the immune system. Muscle tissue provides an amino acid called glutamine, which provides energy to some immune cells, and also produces the anti-inflammatory compound interleukin-6.

“In future research, we hope to better understand why obesity increases the risk of serious infections and, importantly, what can be done to reduce that risk,” Kivimäki said.