Large international cohort data reveal that excess body fat may increase susceptibility to severe infections and substantially contribute to global infection mortality, underscoring prevention as a major public health priority.
Obesity Significantly Increases Risk of Severe Infection, Global Study Finds
A major new study published in The Lancet has revealed a strong link between obesity and an increased risk of severe infections, as well as a substantial contribution to infection-related deaths worldwide. The research, analyzing data from over 540,000 individuals in Finland and the United Kingdom, demonstrates a clear dose-response relationship: the higher a person’s body mass index (BMI), the greater their risk of hospitalization or death from infection.
Biological Mechanisms and Previous Observations
The findings build upon observations made during the COVID-19 pandemic, where individuals with obesity consistently experienced more severe illness and higher mortality rates. Researchers have long suspected that obesity impacts the immune system, creating a state of chronic inflammation and disrupting metabolic processes, potentially increasing vulnerability to infection. This study provides compelling evidence supporting that biological plausibility across a broad spectrum of infectious diseases.
Study Design and Methodology
The study utilized a multicohort design, pooling data from two Finnish cohorts – the Finnish Public Sector study and the Health and Social Support study – and replicating the analysis in the UK Biobank. Participants had their BMI assessed upon entering the studies, with follow-up lasting an average of 13-14 years. Researchers categorized participants into weight classes: healthy weight (BMI 18.5-24.9 kg/m²), overweight (BMI 25.0-29.9 kg/m²), and obese (BMI ≥30.0 kg/m²). Within the obese group, further classifications were made based on BMI levels (class I, II, and III). The study tracked first-time hospitalizations and deaths due to over 925 different infectious diseases.
Sophisticated statistical modeling, including Cox proportional hazards regression, was used to adjust for a range of confounding factors such as age, sex, socioeconomic status, lifestyle choices, and pre-existing medical conditions. These adjusted hazard ratios were then combined with global obesity prevalence and mortality data to estimate the population attributable fraction – the proportion of infection-related deaths that could be attributed to obesity.
Key Findings: A Clear Dose-Response Relationship
The study revealed a consistent and concerning trend. Compared to individuals with a healthy weight, those with class III obesity (BMI ≥40.0 kg/m²) had nearly three times the risk of infection-related hospitalization or death. Even those in class I obesity (BMI 30.0-34.9 kg/m²) experienced a significantly elevated risk. The risk increased steadily with increasing body weight, demonstrating a clear dose-response relationship.
The increased risk was observed across a wide range of infection types, including viral and bacterial infections. Notably, skin and soft tissue infections showed the strongest association, with nearly a threefold increase in risk among individuals with obesity. While no increased risk was observed for HIV or tuberculosis, researchers suggest this may be due to weight loss often associated with those conditions.
Analyses of weight changes over time further supported the findings. Individuals who transitioned from overweight to obese experienced an increased infection risk, while those who lost weight showed modest risk reductions.
Global Impact: One in Ten Infection Deaths Linked to Obesity
Applying the study’s findings to global data, researchers estimated that approximately 8.6% of infection-related deaths in 2018, 15.0% in 2021, and 10.8% in 2023 were attributable to adult obesity. This translates to roughly 0.6 million infection-related deaths worldwide in 2023 being linked to obesity. The highest attributable fractions were observed in North Africa and the Middle East, while the lowest were in South Asia.
Public Health Implications and Future Directions
These findings underscore the critical importance of addressing obesity as a major public health priority. Adult obesity is not merely a risk factor for chronic diseases like diabetes and heart disease; it also significantly compromises the body’s ability to fight off infections, leading to more severe illness and increased mortality.
“Adult obesity is a significant and consistent risk factor for severe infections across diverse pathogens and populations,” the study authors noted. Preventing obesity, implementing effective weight-management programs, and developing vaccination and infection-control strategies that consider the unique vulnerabilities of individuals with obesity are crucial steps in protecting public health. Addressing obesity could substantially reduce hospitalizations, deaths, and the overall burden on healthcare systems, both during routine infectious disease seasons and in the face of future pandemics.
The researchers acknowledge that the study is observational and does not prove causation. The study populations may not be fully representative of the global population, and residual confounding cannot be entirely ruled out. However, the consistent findings across multiple cohorts and the robust statistical analysis provide strong evidence for a causal link between obesity and increased infection risk.
