Obesity Linked to 1 in 4 Infectious Disease Deaths in U.S., 1 in 10 Globally

Obesity contributes to about 1 in every 4 infectious disease deaths in the United States, the most among wealthy countries, a major new study estimates. Overall, obesity is linked to about 1 in every 10 infection-related deaths worldwide, researchers concluded.

People with obesity have a higher risk of hospitalization or death from infectious diseases ranging from influenza and COVID-19 to stomach flu and urinary tract infections, researchers reported Feb. 9 in the journal The Lancet. The research used data from more than 540,000 people enrolled in large cohort studies in the UK and Finland.

Participants had their body mass index recorded at baseline and were then followed for an average of 13 to 14 years. The researchers tracked severe infectious disease outcomes, defined as hospitalization or death related to infection.

Results showed that people with obesity — defined as a BMI as 30 or more — had a 70% higher risk of hospitalization or death from any infectious disease, compared to those with a healthy BMI of 18.5 to 24.9.

The risk rises with a person's weight. This risk increased steadily as body weight rose, researchers found. People with severe obesity of 40 or more had three times the risk of people at a healthy weight.

The findings were consistent across different ways of measuring obesity, including BMI and, where available, waist-based measures such as waist circumference and waist-to-height ratio. The association was also broad across infection types.

The study covered hundreds of infectious diseases and took a closer look at common conditions such as flu, COVID-19, pneumonia, gastroenteritis, urinary tract infections, and lower respiratory tract infections. For most of these, obesity was linked with a higher likelihood of severe outcomes.

However, the study found no link between obesity and increased risk from HIV or tuberculosis.

Notably, the researchers found the association was not fully explained by obesity-related chronic conditions. The link remained even among people with obesity who did not have metabolic syndrome, diabetes, or heart disease. Lifestyle factors such as physical activity did not appear to explain away the relationship either.

There was also evidence that weight loss could reduce risk. People with obesity who lost weight showed around a 20% lower risk of severe infections compared with those who remained obese, suggesting the relationship may be modifiable.

To estimate the wider impact, the researchers combined their findings with infectious disease mortality data from the Global Burden of Disease project. Their modelling suggested that in 2023, roughly 0.6 million of 5.4 million infection-related deaths globally – about 10.8% – could be linked to obesity.

They estimated a higher proportion in some countries, including around 17% in the UK and 26% in the US, while noting that such modelling should be interpreted cautiously because underlying data quality varies by region.

There are a number of potential explanations why obesity might increase risk of infections, researchers said. The study did not aim to prove exactly why this happens, but the authors pointed to existing evidence suggesting obesity can impair immune function. Proposed mechanisms include chronic low-grade inflammation, immune dysregulation, and metabolic changes that may make it harder to mount an effective response and recover from infection.

"It is plausible that obesity weakens the immune system's ability to defend against the infectious bacteria, viruses, parasites or fungi, therefore resulting in more serious diseases," said senior researcher Mika Kivimaki, chair of social epidemiology at University College London.

"Evidence from trials of GLP-1 weight-loss drugs fits with this, as reducing obesity also appears to lower the risk of severe infections, alongside many other health benefits," Kivimaki said in a news release. "That said, additional research is required to confirm the mechanisms underlying these associations."