Obesity is usually discussed as a driver of diabetes, heart disease, and other chronic illnesses. But a major new study argues it is also tied to something many people do not connect to weight: how likely you are to end up hospitalized or dead from an infectious disease.
Researchers analyzing long term health data from Finland and the United Kingdom found what they call “robust evidence” that obesity is linked to worse outcomes from a wide range of infections. Their takeaway is blunt: “People may not get infected more easily, but recovery from infection is clearly harder.”
What BMI Is, and How the Study Defined Obesity
BMI stands for body mass index. It is a number calculated from height and weight, used as a quick screening tool to categorize weight status.
In this research, BMI was grouped like this:
- Healthy weight: 18.5 to 24.9
- Overweight: 25.0 to 29.9
- Obesity class I: 30.0 to 34.9
- Obesity class II: 35.0 to 39.9
- Obesity class III: 40 or higher
Obesity in general was defined as a BMI of 30 or above, and the highest risk category was class III obesity, BMI 40 or higher.
The work was led by Professor Mika Kivimaki of University College London’s Faculty of Brain Sciences, with major contributions from researchers including first author Dr Solja Nyberg of the University of Helsinki.
The study followed adults from two major sources:
- Two Finnish cohort studies
- The UK Biobank dataset
Altogether, the analysis included well over 540,000 participants, tracked for about 13 to 14 years on average.
Methodology: How the Researchers Tested the Link
This was a prospective multicohort study. Participants had their BMI measured at the start, then researchers followed them through national hospital and mortality registries to see who later had severe infections.
Key methodological points the researchers emphasized:
- They excluded people who already had recent infection related hospitalizations at baseline.
- Severe infection was defined as either hospitalization for infection or death from infection.
- They examined 925 infectious diseases across bacterial, viral, parasitic, and fungal categories.
- They adjusted for many baseline factors, including age and sex, and tested whether the pattern held across subgroups and alternative measures of obesity such as waist circumference and waist to height ratio.
They also modeled global impact using Global Burden of Diseases data to estimate how many infection deaths might be attributable to obesity in different years.
The Findings: Which BMI Was Most Prone to Deadly Infections
The risk rose steadily as BMI increased, but the most dangerous category was class III obesity.
- People with obesity overall (BMI 30 or higher) had a 70% higher risk of being hospitalized or dying from infection compared to people at a healthy BMI.
- People with class III obesity (BMI 40 or higher) had about a three times higher risk of severe infection outcomes compared to healthy weight individuals.
The paper also reported separate risk estimates for hospitalization and death in each cohort. In the UK Biobank, for example, class III obesity was associated with roughly 3.5 times the risk of death from infection compared with healthy weight.
For the most extreme BMI category, the increased risk is measured in multiples, not small percentages.
The study reported that compared with healthy weight individuals:
- Class III obesity was linked to about three times higher risk of infection related death, as well as about three times higher risk of infection hospitalization.
- Across all obesity classes combined, the pooled hazard ratio for severe infection was about 1.7, which matches the “70% higher risk” headline finding.
In plain terms, the heaviest obesity category carried the strongest association with deadly outcomes.
The researchers found the increased risk applied to nearly all types of infection, including flu, COVID 19, pneumonia, gastroenteritis, urinary tract infection, and lower respiratory tract infections.
They highlighted one category as especially linked:
- Skin and soft tissue infections showed the strongest association with obesity in the detailed analysis.
There were exceptions. The study found no clear association for severe HIV or tuberculosis outcomes, and the authors discussed why those patterns might differ.
The researchers called the evidence robust because it held up across multiple tests. The association appeared:
- In two independent populations, Finland and the UK Biobank
- Across different measures of body fat, not only BMI
- In people with obesity even if they did not have diabetes, metabolic syndrome, or heart disease
- After adjusting for many possible confounders
- Across many different infection categories
Professor Kivimaki summarized the finding this way: “Here we have found robust evidence that obesity is also linked to worse outcomes from infectious diseases, as becoming very ill from an infection is markedly more common among people with obesity.”
Still, the researchers also flagged limitations: the study is observational, so it shows a strong link but does not prove obesity directly causes severe infection outcomes. They also noted debates about BMI as a measurement, and that the cohorts were from Finland and the UK, so results may not perfectly generalize to all populations.
What Happens in the Body
The study did not directly test biological mechanisms, but the authors pointed to existing research. They said previous work suggests obesity may impair immune function through immune dysregulation, chronic inflammation, and metabolic disturbances.
Kivimaki put it plainly: “Our findings suggest that obesity weakens the body’s defenses against infections, resulting in more serious diseases. People may not get infected more easily, but recovery from infection is clearly harder.”
Other researchers involved in the work stressed that the issue could grow as obesity rises worldwide. Dr Solja Nyberg warned: “As obesity rates are expected to rise globally, so will the number of deaths and hospitalisations from infectious diseases linked to obesity.”
She argued for practical steps, including policies that support healthier living and access to weight loss resources. She also emphasized vaccination: if someone has obesity, “it is especially important to keep their recommended vaccinations up to date.”
The modeling work suggested the global impact is not small. Using global mortality estimates, the analysis suggested roughly one in ten infection related deaths worldwide could be linked to obesity in 2023, though a co author cautioned that global data sources can be imperfect, especially in low resource countries.
The One Encouraging Signal: Weight Loss Lowered Risk
The study found evidence that risk can move in the right direction. People with obesity who lost weight had about a 20% lower risk of severe infections than those who remained obese.
That does not erase risk entirely, but it suggests the link is not fixed, and that reducing obesity could reduce the odds of severe infection outcomes.
The study’s central message is that obesity is not only a chronic disease risk, it is also an infection survival risk. The danger climbs with BMI, and the highest risk group was class III obesity, BMI 40 or higher, where severe infection outcomes and infection related death risk were roughly tripled compared with a healthy BMI.
Or as the lead author put it, the key issue may not be catching infections more often. It is what happens after you get one: “Recovery from infection is clearly harder.”
