A large study following nearly 550,000 adults over 14 years links excess body fat to worse outcomes from infections, showing higher hospitalization and death rates for people with obesity and estimating that a notable share of infection-related deaths could be tied to weight issues. The research compared BMI groups across populations in Finland and the U.K., tracked many kinds of infections, and found that losing weight was associated with a measurable drop in severe infection risk. It also highlighted stubborn gaps, like limits of BMI as a measure and the fact that the results do not prove cause and effect.
The analysis pulled together long-term medical records from adults in Finland and the U.K., with average ages around the 40s and 50s depending on the cohort. Obesity was defined using a BMI of 30 or above, with class 3 obesity marked at a BMI of 40 or higher. Researchers examined outcomes like hospitalization or death tied to infections over a 14-year window to estimate the scale of the association.
Across the pooled data, patients with obesity were about 70% more likely to end up hospitalized with an infection or to die from one compared with people at a healthy BMI. Those with class 3 obesity faced roughly three times the risk of severe infection outcomes relative to healthy-weight peers. Using these risk estimates, the team suggested obesity may have contributed to roughly 9% of infection-related deaths in 2018, 15% in 2021 and about 11% in 2023.
The study looked for links across a broad range of pathogens and syndromes, tallying 925 bacterial, viral, parasitic and fungal infections in the dataset. That list included flu, COVID-19, pneumonia, gastroenteritis, urinary tract infections and lower respiratory infections. Almost all types showed higher rates of poor outcomes when obesity was present, with skin and soft tissue infections showing some of the strongest associations.
“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. He added context about the infectious disease angle, noting that the evidence tied obesity to more severe infection outcomes. “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 pattern held even for people without diabetes, heart disease or what’s typically called metabolic syndrome, and self-reported physical activity did not erase the association. Those who started out obese and later lost weight appeared to cut their risk of severe infection by roughly one fifth, a signal that change can matter even after years of excess weight. That finding suggests risk is at least partially modifiable, though the study design does not prove causation.
“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. She urged policy responses to lower population risk and protect vulnerable people, arguing that individual choices are shaped by broader food and activity environments. “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.”
The researchers were careful to note the study’s limits: it shows a strong association but cannot prove that obesity directly causes worse outcomes from infection. Debate remains over how well BMI captures body fat and health risks, and the sample drew from Finland and the U.K. Biobank so the findings may not generalize worldwide. Funding sources included Wellcome, the Medical Research Council and the Research Council of Finland, and the paper calls for more work to test mechanisms and evaluate public health strategies.
