- A large study found that people with obesity had a much higher risk of being hospitalised or dying from infectious diseases compared with people at a healthy weight.
- The risk rose steadily with higher BMI, reaching around three times higher for people with very severe obesity.
- Researchers also estimated that roughly one in 10 infection-related deaths globally could be linked with obesity.
Obesity is widely recognised as a driver of long-term conditions such as type 2 diabetes, cardiovascular disease, and metabolic syndrome.
A new major study adds another piece to the picture: obesity also appears to be strongly linked with worse outcomes from infectious diseases, ranging from respiratory infections to stomach bugs and urinary tract infections.
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 13-14 years.
The researchers tracked severe infectious disease outcomes, defined as hospitalisation or death related to infection.
Compared with people in the healthy BMI range (18.5-24.9), people with obesity (BMI 30 or above) had around a 70% higher risk of being hospitalised or dying from an infectious disease during follow-up.
The relationship was dose-related: as BMI rose, so did risk.
For people with a BMI of 40 or higher, the risk of severe infection was around three times higher than in the healthy-weight group.
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.
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.
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.
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.
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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.
For people living with diabetes, this matters because diabetes and obesity often overlap and can compound risk.
The practical message is not panic – it is prevention: staying up to date with vaccinations, acting early when infections develop, and addressing weight and metabolic health where possible.
The study reinforces that excess weight can affect more than chronic disease risk – it may also shape how resilient the body is when it faces infection.
