A new study published in The Lancet shows that obesity is a major factor in infection-related complications, with patients having a body mass index (BMI) of 30 or higher facing substantially higher risks of hospitalization or death.

The study analyzed medical data from nearly 550,000 adults in Finland and the United Kingdom over 14 years. Obese patients were found to be 70% more likely to be hospitalized or die from infections compared to individuals with a healthy BMI (18.5–24.9). Those with class 3 obesity (BMI of 40 or higher) were three times more likely to experience severe outcomes.

Researchers examined 925 bacterial, viral, parasitic, and fungal infections, including influenza, COVID-19, pneumonia, gastroenteritis, urinary tract infections, and lower respiratory infections. Nearly all types of infection showed worse outcomes among obese patients, even in the absence of diabetes, heart disease, or metabolic syndrome. Physical activity levels did not appear to significantly alter the risk.

Professor Mika Kivimaki, lead author of the study, said obesity “weakens the body’s defenses against infections, resulting in more serious diseases,” adding that while obese individuals may not be infected more easily, recovery is significantly harder. Skin and soft-tissue infections were most strongly associated with obesity-related complications.

The study estimated that obesity contributed to about 9% of infection-related deaths in 2018, 15% in 2021, and 11% in 2023. Patients who lost weight after being initially obese reduced their risk of severe infection by approximately 20%.

Dr. Solja Nyberg emphasized the need for public health policies that promote healthy weight and physical activity, as well as keeping obese individuals up to date on vaccinations, to mitigate severe infection outcomes.

The research was funded by Wellcome Trust, the Medical Research Council, and the Research Council of Finland. Limitations include reliance on BMI as a measure of obesity and the study’s focus on populations from Finland and the U.K., which may not generalize globally.