Obesity Raises Severe Infection Risk by 70%, Study Finds

People with obesity face a significantly heightened risk – a 70% increase – of hospitalization or death from severe infections, according to a new, large-scale study published in The Lancet. The research, encompassing data from over half a million individuals, also revealed that roughly 1 in 10 infection-related deaths worldwide are linked to obesity. This connection persists even in individuals without metabolic syndrome or diabetes and regardless of socioeconomic factors or activity levels.

The findings underscore a growing understanding of how excess weight impacts the body’s ability to fight off illness. While the link between obesity and severe COVID-19 outcomes became starkly apparent during the pandemic, this study suggests the increased vulnerability extends to a broad range of infectious diseases – bacterial, viral, fungal, and parasitic.

How Obesity Impacts Immune Function

The reasons behind this increased susceptibility are complex and multifaceted. As Mika Kivimäki, an epidemiologist at University College London and senior author of the study, explained to Live Science, “Extra body fat can affect the immune system in several ways, including impairing lymphatic function, reducing lung function, and increasing long-term low-grade inflammation.” The lymphatic system, crucial for fluid balance and immune cell circulation, can become compromised with increased fat tissue. Reduced lung function further hinders the body’s ability to respond to respiratory infections.

Chronic, low-grade inflammation, often associated with obesity, also plays a role. Fat tissue isn’t simply inert storage; it actively participates in the immune response, sometimes in ways that are counterproductive. Precursor cells that develop into fat cells can also behave like immune cells, and fat cells themselves release substances that promote inflammation. This constant state of alert can exhaust the immune system, making it less effective when a genuine threat arises.

Study Details and Scope

Researchers analyzed data from three large cohorts: the UK Biobank, the Finnish Public Sector study, and the Health and Social Support study. The UK Biobank provided detailed body composition measurements, while participants in the Finnish cohorts self-reported their height and weight, allowing researchers to calculate Body Mass Index (BMI). While BMI is a commonly used metric, it’s an imperfect measure of body composition, and some experts argue for the use of more precise measurements like waist circumference and waist-to-height ratio, which were also included in the analysis.

Participants were categorized as having obesity based on a BMI of 30 or higher, a waist circumference exceeding 40 inches for men or 35 inches for women, or a waist-to-height ratio of 0.6 or more. The study meticulously adjusted for age and sex, and the association between obesity and infection risk remained significant across different definitions of obesity and types of infection.

Degrees of Risk: A Dose-Response Relationship

The study revealed a clear dose-response relationship: the greater the degree of obesity, the higher the risk. Individuals with a BMI between 30 and 34.9 had a 50% increased risk of severe infection outcomes. This risk doubled for those with a BMI between 35 and 39.9, and tripled for individuals with a BMI of 40 or above. Notably, changes in weight – either gain or loss – were associated with corresponding changes in infection risk, suggesting the relationship is not static.

Beyond Correlation: Understanding Causation

While the study establishes a strong association between obesity and increased infection risk, it doesn’t prove causation. It’s possible that other factors, not fully accounted for in the analysis, contribute to both obesity and susceptibility to infection. Though, emerging research suggests a more bidirectional relationship. Nikhil Dhurandhar, a professor of nutritional sciences at Texas Tech University, points to evidence suggesting that certain pathogens may even contribute to the development of obesity, although more research is needed to confirm this link in humans.

The Role of GLP-1 Agonists and Future Research

Recent findings offer a potential avenue for mitigating this risk. A separate study, published in August 2025, found that semaglutide – the active ingredient in medications like Ozempic and Wegovy – reduced users’ risk of severe infections by 10%. This suggests that weight loss, facilitated by these medications, could potentially lower infection risk. However, Kivimäki cautions that these medications can also lead to muscle loss, which could negatively impact immune function, as muscle tissue provides glutamine and interleukin-6, both vital for immune response.

Kivimäki and his team plan to continue investigating the underlying mechanisms linking obesity and infection risk. “In our future research, we desire to better understand why obesity raises the risk of severe infections,” he said, “and, importantly, what can be done to reduce that risk.”

Implications for Public Health and Clinical Practice

The findings reinforce the importance of addressing obesity as a public health priority, not simply as a lifestyle issue. As Dhurandhar emphasizes, “Obesity is a disease. It’s a chronic disease. It’s not a matter of willpower; it’s not a matter of discipline.” This perspective shifts the focus from individual blame to systemic factors and the need for comprehensive interventions.

While the study doesn’t offer immediate clinical guidance, it highlights the need for clinicians to be aware of the increased infection risk in patients with obesity and to consider this factor when assessing and managing their care. Further research is needed to determine whether targeted interventions, such as vaccination strategies or preventative therapies, can effectively reduce infection risk in this vulnerable population.

Ongoing surveillance of infection rates and severity, stratified by BMI and other obesity-related metrics, will be crucial for monitoring the impact of this relationship and informing public health strategies. The findings also underscore the importance of continued research into the complex interplay between obesity, the immune system, and infectious diseases.