Overweight Issues The Risk of Severe Infections May Increase in Obese People

A new study has found that obesity may increase the risk of severe infections such as influenza, Covid-19, pneumonia, and gastrointestinal and urinary tract infections in people.

Helsinki/Finland – A recent study examined the link between obesity and severe infectious diseases. The researchers found that obesity significantly increases the risk of hospitalization or death from common infections, including influenza, Covid-19, pneumonia, and gastrointestinal and urinary tract infections. For people with morbid obesity, the risk was found to be three times higher.

“During the pandemic, obesity was widely linked to a higher likelihood of severe Covid-19. We set out to investigate how broadly this link applies across different types of infections and whether any underlying factors contribute to it. Our findings extend beyond any single pathogen, with similar associations observed for bacterial, viral, parasitic and fungal infections,” says one of the article’s lead authors, Solja Nyberg of the University of Helsinki and the Finnish Institute of Occupational Health.

HIV and tuberculosis were exceptions, where obesity did not appear to increase the risk of severe disease. In addition, comorbidities, social status and lifestyle factors such as alcohol use and physical activity did not influence the increased disease risk associated with obesity.

Weight matters

Participants were tracked for an average of 13–14 years, with their body mass index (BMI) recorded at the start of the study. People with obesity, that is, a BMI of 30 kg/m² or higher, had a 70 % greater risk of hospitalization or death from any infectious disease than those of healthy weight, meaning a BMI of 18.5–24.9. This risk rose steadily with body weight. Individuals with morbid obesity (a BMI of 40 kg/m² or higher) faced a risk three times that of people of healthy weight.

The study also showed that changes in body weight affected the risk of severe infection. Those who lost weight and moved from obesity to overweight or healthy weight had 20 % fewer severe infections than those with persistent obesity. By contrast, weight gain from overweight to obesity was associated with a 30 % higher risk of infection.

Possible links with immunodeficiency

“Obesity is a well-known risk factor for diabetes and other chronic diseases. The links now identified indicate that severe infectious diseases should be added to the same list,” says Mika Kivimäki of the University of Helsinki and University College London, who led the study.

“Obesity seems to weaken the immune system’s ability to manage infections, raising the risk of severe disease,” he explains.

He also points out that experimental evidence of weight-loss drugs supports an association with the immune system, as reducing obesity appears to lower the risk of severe infections, alongside offering other health benefits. However, Kivimäki stresses that more research is needed to confirm the mechanisms underlying these associations.

Obesity linked to an estimated one in ten global deaths from infectious disease

The study used a large Finnish and British dataset, monitoring participants through national registers. The researchers also incorporated infectious disease mortality data from the Global Burden of Disease (GBD) study to examine how obesity affects infectious disease deaths in different countries, regions and worldwide. The analysis suggests that 0.6 million of the 5.4 million infectious disease deaths worldwide in 2023 (11 % or one in ten) were linked to obesity.

In the Nordic countries, the shares of infectious disease deaths associated with obesity are estimated to be:

Finland: 19 %

Sweden: 13 %

Norway: 11 %

Denmark: 12 %

In 2023 the United States recorded the largest share among high-income countries, at 26 %.

Keep vaccinations up to date

The researchers emphasize that adults with obesity should keep their vaccinations current and take any booster doses offered to at-risk groups.

The study has some limitations: it relies on observational data and thus cannot prove any causality. Moreover, because the study participants in Finland and the UK Biobank are not fully representative of the general population, the generalizability of the results should be approached with caution.

Original article: Nyberg ST, Frank P, Ahmadi-Abhari S, Pentti J, Vahtera J, Ervasti J, Suominen SB, Strandberg TE, Sipilä PN, Meri S, Sattar N, Kivimäki M. Adult obesity and risk of severe infections: a multicohort study with global burden estimates

Doi: 10.1016/S0140-6736(25)02474-2, Lancet 2026.

Funders

The study was funded by the Research Council of Finland, the Wellcome Trust and the Medical Research Council.

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