People with a certain BMI are more prone to deadly infections, study reveals

Obesity may be a contributing factor in more than 10% of infection-related deaths, a new study reveals.

The researchers analyzed medical data from nearly 550,000 adults in Finland and the U.K. over a 14-year period, finding that obese patients were 70% more likely to be hospitalized with or die from an infection. 

The Finnish patients averaged 42 years of age, while the average age of U.K. patients was 57. Obesity was defined by a BMI (body mass index) of 30 or above.

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Those with "class 3 obesity," which is a BMI of 40 or higher, were three times more likely to be hospitalized for infection, death or either outcome compared to those with a healthy BMI (between 18.5 and 24.9), according to a study press release.

Based on risk calculations, the researchers estimated that obesity played a role in about 9% of infection-related deaths in 2018, about 15% in 2021 and about 11% in 2023.

The association was even seen in obese people who did not have diabetes, heart disease or metabolic syndrome. The amount of physical activity also did not appear to affect the link.

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The study, which was published in The Lancet, looked at prevalence of 925 bacterial, viral, parasitic and fungal infectious diseases, including flu, COVID-19, pneumonia, gastroenteritis, urinary tract infection and lower respiratory tract infections.

Nearly all types of infection were linked to higher risk of obesity in terms of poor outcomes, the researchers found.

"Obesity is well-known as a risk factor for metabolic syndrome, diabetes, cardiovascular disease and many other chronic conditions," said the study's lead author, Professor Mika Kivimaki of the Faculty of Brain Sciences at University College London, in a press release. 

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"Here, we have found robust evidence that obesity is also linked to worse outcomes from infectious diseases, as becoming very ill from an infection is markedly more common among people with obesity."

The conditions that were most strongly associated with obesity were skin and soft-tissue infections.

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Professor Kivimaki added, "Our findings suggest that obesity weakens the body's defenses against infections, resulting in more serious diseases. People may not get infected more easily, but recovery from infection is clearly harder."

Based on the findings, the researchers concluded that up to 11% of infection-related deaths globally could be prevented by addressing obesity.

Those who were initially obese and then lost weight reduced their risk of severe infections by around 20%, the study found.

"As obesity rates are expected to rise globally, so will the number of deaths and hospitalisations from infectious diseases linked to obesity," first author Dr. Solja Nyberg at the University of Helsinki commented in the press release.

"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."

She also emphasized the importance of obese people staying up to date with their vaccinations.

The study did have some limitations, primarily that it only shows a strong link between obesity and infection severity, but doesn't prove causation, the researchers noted.

There is also some debate surrounding the reliability of BMI as a definition of obesity.

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The study is limited to adults from Finland and the UK Biobank, so it may not apply to other populations.

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